Commonly Asked Questions Notes BISE Lahore
THE CHEMISTRY MUST BE RESPECTED
Engr. Zaeem Yousaf
Email: zaeem3.zy@gmail.com
Contact: 0321 7218070
Paper Scheme 2024
Q.1: MCQs Chap (1 to 6) 2 from each and Chap (7 to 11) 1 from each
Q.2: Short Questions Chap 1=3Qs, Chap 2=3Qs, Chap 3=3Qs, Chap 8=3Qs
Q.3: Short Questions Chap 4=4Qs, Chap 5=4Qs, Chap 9=2Qs, Chap 11=2Qs
Q.4: Short Questions, Chap 6=4Qs, Chap 7=3Qs, Chap 10=2Qs
Longs Questions
Q.5: Chap 1+4 Q.6: Chap 3+5 Q.7: Chap 6+8 Q.8: 7+10 Q.9: 9+11
Numerical from Chap 3 & 8
List of Some Elements mostly used in textbook
Table of Contents
Unit #2 Experimental Techniques in Chemistry 7
Unit #8 Chemical Equilibrium 45
Unit #1 Basic Concepts
Multiple Concepts Questions or Multiple Choice Questions.
Short Questions
Prove that one mole of each N2, CO2 and H2 contain equal number of molecules?
As we know, 1 mole of each substance contains equal Avogadro’s number of molecules. So here,
1 mole of N2 = 28g of nitrogen = 6.02×1023 molecules of N2
Similarly,
1 mole of CO2 = 44g of carbon dioxide = 6.02×1023 molecules of CO2
1 mole of H2 = 2.016g of hydrogen = 6.02×1023 molecules of H2
Hence, one mole of each N2, CO2 and H2 contain equal number of molecules.
The atomic masses may be in fractions. Why?
The atomic masses depend upon the number of possible isotopes and their natural abundance. The atomic masses of most elements are fractional because they exist as a mixture of isotopes of different masses. The overall atomic mass, which is an ordinary isotopic mixture, is the average of determined atomic masses of individual isotopes.
Why the experimental/actual yield is mostly less than the theoretical yield?
In most chemical reactions, the experimental yield is less than the theoretical yield. There are various reasons for that which are as follows:
A practically inexperienced worker has many shortcomings and cannot get the expected yield.
The processes if not properly carried out like filtration, separation by distillation, separation by separating funnel, washing, drying and crystallization etc.
A competing side reaction in which some of reactants might take part.
Give two assumptions of stoichiometry.
The two assumptions to be considered while performing stoichiometric calculations are as follows:
All the reactants are completely converted into the products.
No side reaction occurs.
While doing calculations, the law of conservation of mass and the law of definite proportions are obeyed.
Define stoichiometry.
Stoichiometry:
The branch of chemistry which tells us the quantitative relationship between reactants and products in a balanced chemical equation is called stoichiometry.
While doing calculations, the law of conservation of mass and the law of definite proportions are obeyed.
Calculate the mass in grams of 10-3 moles of water/2.74 mole of KMnO4.
Solution:
No. of moles = 10-3
Molar mass of water = 18g
Mass in grams is given by;
Mass = no. of moles × molar mass
= 10-3 × 18 g
= 0.018g
So, the mass in grams of 10-3 moles of water is 0.018g.
How can the efficiency of a chemical reaction be expressed? Or What is percentage yield?
The efficiency of a chemical reaction is expressed by comparing the actual and theoretical yields in the form of percentage (%) yield.
Efficiency = % yield = Actual yieldTheoretical yield ×100
Mg atom is twice heavier than carbon atom. Justify.
Since, the atomic mass of Magnesium (24) is twice than that of Carbon (12). Therefore, magnesium atom is twice heavier than that of carbon atom.
Molar mass of Mg = 24g
Mass of 1 atom of Mg = 246.02 × 1023 = 3.99 × 10-23g
Molar mass of C = 12g
Mass of 1 atom of C = 126.02 × 1023=1.99 × 10-23g
Mg : C
3.99 × 10-231.99 × 10-23 : 1.99 × 10-231.99 × 10-23
2 : 1
Define actual yield. Write formula for the calculation of percentage yield.
Actual yield:
The amount of the products obtained in a chemical reaction is called the actual yield of that reaction.
Percentage yield:
Percentage yield is calculated as:
% yield = Actual yieldTheoretical yield ×100
One mg of K2CrO4 has thrice the no. of ions than the number of formula units when ionized in water. Justify.
When K2CrO4 ionizes in water, its one formula unit gives three ions i.e. two K+ and one CrO42- (chromate) ions. i.e.
K2CrO4 ⇌ 2K+ + CrO42-
Thus, each formula unit of K2CrO4 produces three ions in the solution. Hence, one mg of K2CrO4 has thrice the no. of ions than the number of formula units when ionized in water.
23g of Na and 238g of U have equal no. of atoms in them, justify it.
As, 1 mole of each element contains Avogadro’s number of atoms in it. So, here;
23g of sodium = 1 mole of Na = 6.02×1023 atoms of Na
Similarly,
238g of uranium = 1 mole of U = 6.02×1023 atoms of U
Hence, 23g of Na and 238g of U have equal no. of atoms in them which is Avogadro’s number of atoms.
180g of glucose and 342g of sucrose have the same no. of molecules but different number of atoms present in them. Give reason.
180 grams of glucose (C6H12O6) and 342 grams of sucrose (C12H22O11) are their molar masses indicating one mole of each (glucose and sucrose). And 1 mole of each substance contains Avogadro’s number of molecules. But the number of atoms is different as both contains different number of C, H and O atoms. So here,
180g of glucose = 1 mole of C6H12O6 = 6.02×1023 molecules of C6H12O6 = 24NA atoms
342g of sucrose = 1 mole of C12H22O11 = 6.02×1023 molecules of C12H22O11 = 45NA atoms
Define limiting reactant. Give an example.
Limiting Reactant:
The limiting reactant is a reactant that controls the amount of the product formed in a chemical reaction due to its lesser amount.
Example:
In the reaction between hydrogen and oxygen below to form water, hydrogen is a limiting reactant since less hydrogen is present as compared to oxygen.
2H2g + O2g → 2H2Ol
What are molecular ions? Write their uses.
Molecular Ions:
When molecules gain or lose electrons, they form molecular ions, e.g., CH4+, CO+, N2+.Cationic molecular ions are more abundant than anionic ones.
Uses:
The breakdown of molecular ions obtained from the natural products can give important information about their structure.
Differentiate between empirical formula and molecular formula.
Calculate the no. of molecules present in 34g of H3PO4.
Solution:
Mass of H3PO4 given = 34g
Molar mass of H3PO4 = 3(1) + 31 + 4(16)
= 98g/mol
No. of molecules = 3498 × 6.02 × 1023
= 2.08 × 1023 molecules
So, the no. of molecules present in 34g of H3PO4 is 2.08 × 1023.
Write functions of Mg (ClO4)2 and 50% KOH in combustion analysis.
The function of Mg (ClO4) is to absorb H2O vapors produced during the burning of organic compound in combustion analysis.
While 50% KOH is used in the CO2 absorber chamber in combustion analysis to absorb CO2 gas produced.
Many chemical reactions taking place in our surroundings involve the limiting reactants. Explain.
In our surroundings, many chemical reactions taking place involve limiting reactants. Some of these are as follow:
Burning of coal to form CO2. Here, coal is limiting reactant
C + O2 → CO2
Burning of sui gas to form CO2 and H2O, Here, sui gas is limiting reactant
CH4 + 2O2 → CO2 + 2H2O
Rusting of iron, in which iron is limiting reactant.
In above reactions oxygen is always in excess, while other reactants are consumed earlier. So other reactants are limiting reactants.
Why relative atomic mass unit scale is used?
The masses of the atoms are extremely small. We don’t have any balance to weigh such an extremely small mass that is why we use the relative atomic mass unit scale.
The unit used to express the relative atomic mass is called atomic mass unit (amu) and it is 1/12th of the mass of one carbon atom. 1 amu = 1.1667 x 10-27 kg
How is the law of conservation of mass obeyed during stoichiometric calculations?
The law of Conservation of mass states that mass is neither created nor destroyed. This applies to chemical equations because chemical equations can neither create nor destroy mass or energy. (Equations must be balanced) And Stoichiometry deals with the quantitative relationship between reactants and products in a balanced chemical equation. Therefore, law of conservation of mass is obeyed during stoichiometric calculations.
How no individual Ne atom in sample of the element do has mass of 20.18 a.m.u.?
Neon has three isotopes of atomic masses 20, 21 and 22. And the relative atomic mass of neon comes out to be 20.18 a.m.u. which is the average mass of all the three isotopes and hence, no individual Ne atom in the sample of the element has mass of 20.18 a.m.u.
Define relative atomic mass. Give two examples.
Relative atomic mass is the mass of an atom of an element as compared to the mass of an atom of carbon taken as 12. The unit used to express it is called atomic mass unit (a.m.u).
Examples: The relative atomic mass of H is 1.008 a.m.u.
The relative atomic mass of O is 15.9994 a.m.u.
Calculate the percentage of nitrogen in NH2CONH2 (Atomic masses of C = 12, N = 14, O = 16 and H = 1).
Molecular mass of NH2CONH2 = 14+2(1) +12+16+14+2(1) = 60g/mol
So,
%age of N in NH2CONH2 = 14+1460 ×100 = 46.67%
Define gram formula giving one example
The formula mass of an ionic compound expressed in grams is called gram formula of the substance.
Example: 1 gram formula of NaCl is 58.50g. It is also called gram mole or simply mole.
Number of gram formulas or moles of a substance = Mass of the ionic substance in gramFormula mass of the ionic substance
Define isotopes. Why they have same chemical properties but different physical properties?
Isotopes are different kind of atoms of the same element having same atomic number but different atomic masses. They have equal number of protons and electrons and differ only in number of neutrons. They have same chemical properties because these properties depend upon atomic number and atomic numbers of all the isotopes of an element are same. But physical properties depend upon atomic masses which are different for different isotopes of same element.
What is mass spectrum?
Mass spectrum is the plot of data in such a way that m/e is plotted at x-axis and the relative number of ions at y-axis.
Define gram atom and gram molecule.
Gram Atom
When the substance at our disposal is an element then the atomic mass of that element expressed in grams is called one gram atom. It is also called one gram mole or simply a mole of that element.
1 gram atom of carbon = 12.000 g
1 gram atom of uranium = 238.0 g
Number of gram atoms or moles of an element = Mass of an element in gramsMolar mass of an element
Gram Molecule
The molecular mass of a substance expressed in grams is called gram molecule or gram mole or simply the mole of a substance.
1 gram molecule of water = 18.0 g
1 gram molecule of H2SO4 = 98.0 g
No. of gram molecules or moles of a molecular substance = Mass of molecular substance in gramsMolar mass of the substance
How many moles of CO2 can be produced from burning one mole of octane?
The octane burns according to the following equation.
2C8H18 + 25O2 16CO2 + 18H2O
According to balanced chemical equation,
Molecular formula is nth multiple of empirical formula. Explain with an example.
Molecular formula is the nth multiple of empirical formula. For example, in the case of glucose the empirical formula is CH2O and if it is multiplied by a positive integer say ‘n’ it gives the molecular formula, where
n = molecular massempirical formula mass
For glucose, n = 6 Hence, molecular formula = n (empirical formula)
Molecular formula of glucose = 6 (CH2O) = C6H12O6
Define the term atomicity. Give example.
The number of atoms present in a molecule determines its atomicity. Thus molecules can be monoatomic, diatomic and triatomic, etc., if they contain one, two and three atoms respectively. For example each molecule of haemoglobin is made up of nearly 10,000 atoms so its atomicity is 10,000.
N2 and CO have same number of electrons, protons and neutrons, justify?
What is importance of limiting reactant?
The reactant which is consumed earlier is called a limiting reactant. Often, in experimental work, one or more reactants is/are deliberately used in excess quantity. This strategy is used to ensure that all of the expensive reactant is completely used up in the chemical reaction. Sometimes, it is employed to make reactions occur faster. Once this reactant is consumed, the reaction stops and no additional product is formed.
Long Questions
What is mass spectrometer? How is it used to determine the relative atomic masses of isotopes?
What are isotopes? How do you deduce the fractional atomic masses of element from the relative isotopic abundance? Give one example in support of your answer.
Mg metal reacts with HCl to give H2 gas. What is the minimum volume of HCl solution
(27% by weight) required to produce 12.1 g of H2? The density of HCl solution is 1.14 g/cm3.
Mg(s) + 2HCl(aq) → MgCl2 + H2
Define limiting reactant. How it is helpful to control the chemical reaction?
Calculate the no. of grams of K2SO4 and water produced when 14g of KOH are reacted with excess of H2SO4. Also calculate the no. of molecules of water produced.
2KOH(aq) + H2SO4(aq) → K2SO4(aq) + 2H2O(l) (At mass K=39, S=32, O=16)What is stoichiometry? Give its assumptions. Mention two important laws which helps to perform the stoichiometric calculations.
Serotenin (Molar mass = 176g mol-1) is a compound that conduct nerve impulse in brain and muscle. It contains 68.2% C, 6.86% H, 15.09% N and 9.08% O. What is its molecular formula?
NH3 gas can be prepared by heating together two solids NH4Cl and Ca(OH)2. If a mixture containing 100 g of each solid is heated then
Calculate the number of grams of NH3 produced.
Calculate the excess amount of reagent left unreacted.
Describe the combustion analysis method for the determination of percentage composition of organic compound
Define types of yield. How do we calculate the percentage yield of a chemical reaction?
Write all the steps involved in determination of empirical formula.
Explain evidence of atom in detail.
Write detailed note on (i) Avogadro’s no. (ii) Molar volume
Unit #2 Experimental Techniques in Chemistry
Multiple Concepts Questions or Multiple Choice Questions.
Short Questions
What is solvent extraction?
Solvent extraction technique involves the separation of a solute from a solution by shaking it with an immiscible solvent in which the solute is more soluble and the added solvent does not mix with the solution.
The most common example of solvent extraction is ether extraction.
Define sublimation with examples. Give the importance of sublimation.
Sublimation:
It is a process in which a solid, when heated, vaporizes directly without passing through the liquid phase and these vapors can be condensed to form the solid again.
Example:
The technique is frequently used to purify a solid. Examples of such solids are ammonium chloride, iodine, naphthalene, benzoic acid, etc.
Give the main characteristics of the solvent used for crystallization.
An ideal solvent for crystallization should have following features:
It should not react chemically with the solute.
It should not dissolve the impurities or the impurities should not crystallize from it along with the solute.
It should be inexpensive.
It should be safe to use and should be easily removable.
What is Rf value? Why it has no units?
Rf stands for retardation factor. The Rf value of a component is related to its distribution coefficient and is given by:
Rf = Distance travelled by a component from the original spot
Distance travelled by solvent from the original spot
Since, it is a ratio of two distances, therefore, it has no units.
Mention the major steps involved in crystallization.
The steps involved in crystallization are as follows:
Mention various experimental techniques which are used for the purification of substances.
The experimental techniques which are used for the purification of substances are as follows:
Define distribution law and how it is helpful in solvent extraction.
Distribution Law
This law states that a solute distributes itself between two immiscible liquids in a constant ratio of concentrations irrespective of the amount of solute added.
Solvent extraction is an equilibrium process and follows this distribution or partition law.
Define chromatography. Give its two uses.
Chromatography:
The word chromatography originates from the Greek word “Khromatos” meaning color writing. Chromatography is a method used primarily for the separation of a sample of mixture. It involves the distribution of a solute between a stationary phase and a mobile phase.
Uses:
The techniques of chromatography are very useful in organic synthesis for separation, isolation and purification of the products.
Chromatography is important in qualitative and quantitative analysis and for the determination of the purity of a substance.
How the de-colorization of undesired colors is carried out for the freshly prepared crystalline substances?
The de-colorization of undesired colors for the freshly prepared crystalline substance is carried out by boiling the substance in the solvent with the sufficient quantity of finely powdered animal charcoal and then filtering the hot solution. The colored impurities are adsorbed by animal charcoal and the pure decolorized substance crystallizes out from the filtrate on cooling.
Differentiate between adsorption and partition chromatography.
In solvent extraction technique, repeated extractions using small portions of solvent are more efficient than using the single extraction but large volume of solvent. Comment.
It has been observed that repeated extractions using small portions of solvent are more efficient than using a single but larger volume of solvent. Because more product is extracted with more extractions using small portions of solvent. In repeated extractions, we can get maximum amount of solute from the other solvent.
What do you know about Gooch crucible and sintered glass crucible? Why sintered glass crucible is superior to Gooch crucible.
Gooch Crucible:
It is made of porcelain having a perforated bottom which is covered with paper pulp or a filter paper cut to its size. Quick filtration can be done by placing the Gooch crucible in a suction filtering apparatus. It is useful for the filtration of precipitates, which need to be ignited at high temperature.
Sintered glass crucible:
Sintered glass crucible is a glass crucible with a porous glass disc sealed into the bottom. It is very convenient to use because no preparation is needed as with the Gooch crucible.
How crystallized substances are dried by reliable method?
A safe and reliable method of drying crystals is through a vacuum desiccator. In this process the crystals are spread over a watch glass and kept in a vacuum desiccator for several hours. The drying agents used in a desiccator are CaCl2, silica gel or phosphorus pentoxide.
Concentrated HCl and KMnO4 solutions cannot be filtered by Gooch Crucible. Give reason.
Concentrated HCl and KMnO4 are the reagents that react with paper, therefore, these solutions cannot be filtered with Gooch crucible having perforated bottom covered with paper pulp or a filter paper cut to its size. However, if its perforations are covered with asbestos mat then it may be used to filter these solutions.
Define sublimand and sublimate.
The compound which is sublimed is called sublimand. While the pure product obtained after sublimation is known as the sublimate. e.g., in the mixture of benzoic acid in sand, benzoic acid is sublimand.
Why is there a need to crystallize the crude product?
When a compound is prepared in laboratory, it may contain impurities. This impure and un-refined compound is called crude product. It is necessary to purify the crude product. So, there is a need to crystallize the crude product.
How the mixture of amino acids can be separated?
A mixture of unknown amino acids can be separated and identified by means of paper chromatography. The positions of amino acids in chromatogram can be detected by spraying with ninhydrin, which reacts with amino acids to yield highly colored products (purple).
How solvent extraction is related with paper chromatography?
Solvent Extraction
It is a technique, in which a solute can be separated from a solution by shaking the solution with a solvent in which solute is more soluble and added solvent does not mix with the solution. Solvent extraction is mostly applied to separate organic compounds from water.
Paper Chromatography
It is a technique of partition chromatography in which the stationary phase is water adsorbed on a paper. The mobile phase is usually an organic liquid.
Both of these techniques are related to each other because both are based on distribution law and used to separate the organic liquids.
Define qualitative and quantitative analysis of a compound.
Qualitative Analysis:
The analysis which deals with the detection or identification of the elements present in a compound is called qualitative analysis. It includes salt analysis and detection of functional groups.
Quantitative analysis:
The analysis in which the relative amounts of constituents are estimated is called quantitative analysis. For example, combustion analysis.
Write two disadvantages of drying the crystals in the folds of filter paper.
When crystals are dried between several folds of filter paper, the crystals may get crushed into fine powder.
The fibers of filter paper contaminate the product making it impure.
How can rate of filtration be increased by fluted filter paper? Or how the fluted filter paper is prepared?
The rate of filtration can be increased using fluted filter paper for preparation of such filter paper, an ordinary filter paper is folded in such a way that a fan like arrangement with alternate elevations and depressions at various folds is obtained. In such a way, the surface area for filtration is increased.
Define ether extraction.
In ether extraction, aqueous solution containing the organic product is shaken up with ether in a separating funnel and allowed to separate. The inorganic impurities remain in aqueous phase whereas the organic compound goes to the ether layer. Ether extraction follows the distribution law which states that the solute distributes itself between two immiscible solvents on the basis of their relative solubilities.
Unit #3 Gases
Multiple Concepts Questions or Multiple Choice Questions.
Short Questions
Gases deviate more from ideal behavior at 0°C than at 100°C. Why?
As temperature increases, the properties of gas more nearly approach that of the ideal gas due to the intermolecular forces becoming insignificant at high temperature. The deviations from ideal behavior increase as temperature decreases, becoming significant near the temperature at which the gas is converted into a liquid. Therefore, gases deviate more from ideal behavior at 0oC.
Calculate the value of gas constant R in SI units.
From the general equation PV = nRT we get: R = PVnT
Where P is pressure, V is volume, n is number of moles of a given substance, and T is temperature.
In SI units, P is in Nm-2, V is in m3, T is in K
So,
R = 101325 Nm-2 × 0.02241 m3 1 mol× 273.16 K = 8.3143 Nm K-1 mol-1 = 8.3143 J K-1 mol-1
Give two important scales of thermometry. How are these related?
The two important scales of thermometry are as follows:
Centigrade Scale: It has a zero mark for the temperature of ice at one atmospheric pressure. The mark 100℃ indicates the temperature of boiling water at 1 atm. The space between these marks is divided into 100 equal parts and each part is 10C.
Kelvin scale: The melting point of ice at 1 atm pressure is 273K. The water boils at 373K or more precisely at 373.16K.
These two scales are related by: K = ℃ + 273.16
Define pressure. Give its SI units.
Pressure:
The force exerted on a surface per unit area is called pressure.
SI unit: SI unit of pressure is Nm-2.
What are the faulty points in the Kinetic molecular theory of gases? Or what are the causes of deviation from ideality?
Those two points/causes are as follows:
There are no forces of attraction among the molecules of the gas.
The actual volume of gas molecules is negligible as compared to the volume of the vessel.
Define Dalton’s law of partial pressure. Give an example. Or why pilots feel uncomfortable in breathing at higher altitudes?
Dalton’s law:
According to this law, the total pressure exerted by a mixture of non-reacting gases is equal to the sum of their individual partial pressures.
Example: At higher altitudes, the pilot feels uncomfortable breathing because the partial pressure of oxygen in the un-pressurized cabin is low, as compared to 159 torr, where one feels comfortable breathing.
Greater the temperature of gas, closer the straight line between P and PV to the pressure axis. Justify it.
Greater the temperature of gas, the volume of the gas increases, the gas spreads over a larger region of space. Consequently, it exerts less pressure on the container. It means increase in column volume decreases the pressure of the gas. The product PV remains constant, and a straight line closer to pressure axis is obtained.
How the various scales of thermometry be interconvertible?
Following relationships show that the various scales of temperature are interconvertible:
Joule Thomson effect is operative in the Linde`s method of liquefaction of air. How?
The basic principle of the Linde’s method of liquefaction of gases is the Joule-Thomson effect. For liquefaction of air, pure dry air is compressed. The air expands into the chamber where the pressure of the air falls down from 200 atm to 1atm. As a result of joule-Thomson effect a considerable drop of temperature occurs. The process of compression and expansion is repeated again and again till the air is cooled to such an extent that it liquefies. The liquefied air collects at the bottom of the expansion chamber from where it is drawn off. All gases except H2 and He can be liquefied by this method.
State the Joule Thomson effect. Write its application.
Joule Thomson Effect:
When sudden expansion of gases occurs, cooling takes place. This effect is called Joule Thomson Effect.
Application:
The basic principle of the Linde’s method of liquefaction of gases is the Joule-Thomson effect.
Why regular air cannot be used in diver`s tank?
Deep sea divers take oxygen mixed with an inert gas (He) and adjust the partial pressure of oxygen according to the requirement because in sea, after every 100feet depth, the diver experiences about 3 atm pressure. Therefore, normal air cannot be breathed in depth of sea. Moreover, the pressure of N2 increases in depth of sea and it diffuses in the blood.
Calculate the density of methane at STP.
The density of methane is given by:
d=PMRT
At STP,
MCH4 = 12+4 = 16g mol-1
P = 1 atm
R = 0.0821dm3 atm K-1 mol-1
T = 273+0=273K
So, d=1×160.0821×273 = 0.7138 g dm-3 So, the density of methane at STP is 0.7138 g dm-3
Define absolute zero.
The temperature of −273.16°C (0K), the hypothetical point at which all molecular activity ceases, is called absolute zero.
It is the temperature (absolute temperature) as measured on a scale in which the hypothetical lowest limit of physical temperatures is assigned the value zero (absolute zero), as the Kelvin scale.
Define plasma state. Give its one application.
Plasma:
“A high temperature ionized gas mixture consisting of free electrons, positive ions and neutral atoms is called plasma.”
Plasma contains a significant number of electrically charged particles which are free electrons and positive ions.
Applications:
Plasmas are used to light up offices and homes, make computers and electronic equipment to work.
They drive lasers and particle accelerators, help to clean up the environment, pasteurize foods, and make tools corrosion-resistant.
SO2 is not ideal at 273 K but behaves like an ideal gas at 327°C. Justify the statement.
At low temperature, the molecules of SO2 possess low kinetic energy. They come close to each other. The intermolecular attractive forces become very high. So, it behaves non-ideally at 273K. At high temperature, the molecules of SO2 have high kinetic energy. The molecules are at larger distances from one other another. The intermolecular attractive forces become very weak. So, it behaves ideally at 327℃.
Write down the properties of solids.
The particles of solid substances are very close to each other and they are tightly packed. Due to tight packing of particles solids are non-compressible and non-diffusible.
There are strong attractive forces in solids which hold the particles together firmly. Due to this reason solids have definite shape and volume.
The solid particles possess only vibrational motion.
Define Avogadro`s law.
The law is stated as:
“Equal volumes of ideal gases at the same temperature and pressure contain equal number of molecules.” V ∝ n
22.414 dm3 of an ideal gas at STP has number of molecules = 6.02x1023
1 dm3 of an ideal gas at STP has number of molecules =molecules
=2.68x1022 molecules
So, if we have one dm3 of each of H2, O2, N2 and CH4 in separate vessels at STP, then the number of molecules in each will be 2.68x1022.
Prove that pi = Pt Xi
According to Dalton’s law, the partial pressure of any gas in a mixture of gases can be calculated provided we know the mass or number of moles of the gas, the total pressure and
the total number of moles present in the mixture of gases.
PtV = ntRT …… (equation for mixture of gases)
piV = niRT …… (equation for gas i)
Dividing both equations
piVPtV = niRTntRT
piPt = nint
Pi = nint Pt
Pi = Xi Pt … (Xi is mole fraction of gas i)
Hence proved, Pi = Pt Xi
Calculate the fraction of total pressure exerted by oxygen when equal masses of CH4 and O2 are mixed in an empty container at 25°C.
In an empty container, the partial pressure of gas is directly proportional to the mole-fraction of the gas. The molar mass of oxygen gas (32 g/mol) is double than that of methane (16 g/mol). When equal masses of CH4 and O2 are mixed in an empty container at 25°C, the fraction of total pressure exerted by oxygen is given by the mole fraction of oxygen.
X=11+2
So, X=13 will be the fraction of total pressure exerted by oxygen.
Justify that volume of gas becomes theoretically zero at -273°C.
According to quantitative statement of Charles’s law;
“At constant pressure, the volume of a given mass of gas increases or decreases by 1273 of its original volume at 0℃ for every 1℃ rise or fall in temperature respectively.”
Mathematically;
Vt= Vo(1+t273)
For t = -273oC
Vt= Vo(1+-273273)
Vt= 0
Hence proved, volume of gas becomes theoretically zero at -273°C.
Write four possible units of general gas constant ‘R’.
The four possible units of general gas constant ‘R’ along with values are as follows:
Why gases deviate from ideal behavior? Or Gas show non ideal behavior at low T and high P, Give reason.
In reality, gases have intermolecular forces. Also, the actual volume of gas molecules is not negligible as compared to the volume of the vessel. Gases show some ideal behavior at high temperatures and low pressures but when the temperature of gases is low, attractive forces become dominant and they deviate from ideal behavior and do not follow the gas laws. Similarly, at high pressure, collisions become more frequent and forces of attraction are created and they behave non-ideally.
Throw some light on the factor 1/273 in Charles’s law. Or Write quantitative statement of Charles`s law.
The factor is very important in the quantitative definition of Charles law according to which:
“At constant pressure, the volume of a given mass of gas increases or decreases by 1273 of its original volume at 0℃ for every 1℃ rise or fall in temperature respectively.”
Mathematically; Vt= Vo(1+t273)
This equation helps us to calculate the volume of gas at any temperature.
Derive expression of molecular mass of a gas by using general gas equation.
According to general gas equation;
PV = n RT
Where,
n = no. of moles and is given by:
n = massmolecular mass = m M
Putting in general gas equation;
PV=mMRT
Rearranging, M=mRTPV
Where do natural plasma and artificial plasma exist?
Natural plasma exists only at very high temperatures or low temperature vacuums. It does not break down or react rapidly. It is extremely hot having a temperature over 20,000oC.
Artificial Plasma can be created by using charges on a gas, as in neon signs. Plasma at low temperatures is hard to maintain. This is because outside a vacuum, low temperature plasma reacts rapidly with any molecule it meets.
Define Graham’s law of diffusion. Derive/Define Graham`s law from kinetic equation.
Graham’s law states that the rate of diffusion or effusion of a gas is inversely proportional to square root of its density at constant temperature and pressure.
Rate of diffusion (r) ∝ 1√d
Graham`s law from kinetic equation
Applying the kinetic equation
If we take one mole of a gas having Avogadro’s number of molecules (N = NA) then the equation (13) can be written as:
Where M is the molecular mass of the gas. Taking square root
'V' is the molar volume of gas at given conditions. Since the root mean square velocity of the gas is proportional to the rate of diffusion of the gas.
At constant pressure which is Graham’s law of diffusion.
Write down two characteristics of plasma.
In plasma the number of electrons and ions are equal, so macroscopically it is neutral.
Plasma have charged particles so as a whole, it exhibits collective response to electric and magnetic fields.
Derive the SI units of Van der Waal’s constant ‘a’.
As, a = PV2n2
In SI units:
Units of ‘P’ are Nm-2 and ‘v’ are m3
Hence,
a = Nm-2X (m3)2(mol)2 = Nm4 mol-2
Why do we get straight line, when pressure is plotted against inverse of volume?
When pressure is plotted against inverse of volume, a straight line is obtained which means pressure and inverse of volume are directly proportional to each other, it means when pressure is high, the volume is very close to zero and its inverse is very high.
Why lighter gases diffuse more rapidly than heavier gases?
Lighter gases have low density and so according to Graham’s law of diffusion, it diffuses more rapidly than heavier gases having more density. For example, helium is lighter than neon gas, so helium diffuses more rapidly than neon.
How density of an ideal gas can be calculated from ideal gas equation?
For calculating the density of an ideal gas, we substitute the value of number of moles (n) of the gas in terms of the mass (m), and the molar mass (M) of the gas.
PV = nRT
How do you differentiate between diffusion and effusion?
What is compressibility factor? Write it`s value for an ideal gas.
The factor PV/nRT is called the compressibility factor. It represents the ratio of the actual molar volume of a gas to the molar volume predicted by the ideal gas law under the same conditions of pressure and temperature. Its value is unity under all conditions for an ideal gas.
Derive Charles`s law from kinetic equation of gases.
According to one of the postulates of kinetic theory of gases, the kinetic energy is directly proportional to the absolute temperature of the gas. The kinetic energy of N molecules is 12mNc2. So
Long Questions
Calculate the mass of 1 dm3 of NH3 gas at 30°C and 1000 mmHg pressure considering that NH3 behaving ideally.
What pressure is exerted by mixture of 2.0g H2 and 8.0 g N2 at 273 K in 10 dm3 vessel?
Calculate the masses of 1020 molecules of each of H2, O2 and CO2 at STP. What will happen to the masses of these gases when the temperatures of these gases are increased by 100°C and pressure is decreased by 100 mmHg?
Describe Dalton`s law of partial pressure. Write its three applications.
Give postulates of kinetic molecular theory.
State and explain Graham’s law of diffusion of gases.
Derive Boyle’s and Charles’s law from kinetic equation.
One mole of methane gas is maintained at 300K, its volume is 250 cm3. Calculate the pressure exerted by the gas, when the gas is ideal.
250 cm3 of hydrogen is cooled from 127oC to -27oC by maintaining the pressure constant Calculate the new volume of the gas at low temperature
A sample of nitrogen gas is enclosed in vessel of volume 380 cm3 at 1200C and pressure of 101325 Nm-2. This gas is transferred to a 10 dm3 flask and cooled by 270C. Calculate the pressure in Nm-2 exerted by the gas at 270C.
Unit #4 Liquids and Solids
Multiple Concepts Questions or Multiple-Choice Questions.
Short Questions
Ionic solids do not conduct electricity in solid state. Give reason.
Ionic solids do not conduct electricity in solid state, because on account of the electrostatic force existing between them, the cations and anions remain tightly held together and hence occupy fixed positions restricting all movement and preventing electrical current from forming. Consequently, ionic solids do not conduct electricity. However, in solution or in the molten state, they conduct electricity because ions become free.
Diamond is hard and electrical insulator. Why?
Diamond is hard because it is a giant covalent molecule with strong covalent bonds. Diamond crystal is a three-dimensional network of carbon atoms. All carbon atoms in the network are strongly bonded by carbon-carbon covalent bonds. The diamond crystal has a highly symmetric cubic structure, which does not allow electrons to move around freely, hence it is an insulator.
Define lattice energy and give example.
Lattice Energy:
The energy released when one mole of the ionic crystal is formed from the gaseous ions. It is also defined as the energy required to break one mole of solid into isolated ions in the gas phase. It is expressed in kJ mole-1.
Example: Na+g + Cl-g → NaCls ∆H= -787 kJ mole-1
The crystals showing isomorphism mostly have same atomic ratio, justify.
Isomorphic substances are the substances having same crystalline structures. The crystals showing isomorphism mostly have same atomic ratio, because it is dependent upon the way of combination of atoms in isomorphs. For example, NaNO3 and KNO3 have same atomic ratios. Because both Na and K belong to 1st group and give only one electron for bond formation.
How the rate of evaporation depends upon surface area?
The rate of evaporation is directly proportional to the surface area. Since evaporation occurs from the liquid surface, so if surface area is increased then more molecules are able to escape and liquid evaporates more quickly. Hence, the rate of evaporation increases with a greater surface area.
Water is liquid at room temperature but H2S is a gas, give reason.
The large electronegativity difference between oxygen and hydrogen (about 1.3) and the strong hydrogen bonding in water causes the water molecules to have strong intermolecular forces, whereas in the case of hydrogen sulfide, these forces of attraction are much weaker due to the relatively smaller electronegativity difference between sulfur and hydrogen (about 0.3). So, lesser energy is required to overcome the forces of interaction between the hydrogen sulfide molecules than those between water molecules. This energy is available at room temperature and hence, hydrogen sulfide is a gas, while water is still a liquid.
Justify HF is weaker acid than HCl. or HF is the weakest acid among all halogens. Why?
The low acidic strength of HF molecule as compared to HCl is due to strong hydrogen bonding in HF. The acidic strength is ability to lose H, while in HF, due to strong hydrogen bonding, the partial positively charged hydrogen is entrapped between two highly electronegative F atoms. Therefore, HF is weaker acid than HCl.
Why ice floats on the surface of liquid H2O? Explain.
A substance floats if it is less dense, or has less mass per unit volume, than other components in a mixture. When the temperature of water is decreased and ice is formed then the molecules become more regular and this regularity extends throughout the whole structure. Empty spaces are created in the structure. That is why when water freezes it occupies 9% more space and its density decreases. The result is that the less dense ice than liquid H2O floats on surface of water.
Define isomorphism. Give two examples.
Isomorphism:
Isomorphism is the phenomenon in which two different substances exist in the same
crystalline form. These different substances are called isomorphs of each other.
Examples:
NaNO3 and KNO3 are isomorphs of each other as both exists in rhombohedral form
NaF and MgO are also isomorphs of each other as both exists in cubic form
How liquid crystals can act as temperature sensors?
Like solid crystals, liquid crystals can diffract light. When one of the wavelengths of white light is reflected, from a liquid crystal it appears colored. As the temperature changes, the distances between the layers of the molecules of liquid crystals change. Therefore, the color of the reflected light changes accordingly. Thus, liquid crystals can be used as temperature sensors.
Lower alcohols are water soluble but corresponding alkanes are insoluble in water, why?
Alcohols are polar substances and thus dissolve in water following the principle “Like Dissolves Like”. Also, Lower alcohols are water soluble as they have the tendency to form hydrogen bonds. For example, ethyl alcohol can dissolve in water because both can form hydrogen bonds with each other. While the corresponding alkanes are insoluble in water as they are hydrocarbons which are non-polar compounds and there are no chances of hydrogen bonding between hydrocarbon and water molecules.
Why melting and boiling points of alkanes increase with increase in molecular mass?
The increase in the melting and boiling points of alkanes with the increase in molecular mass is due to the strength of London forces. Greater the number of atoms in a non-polar molecule, greater is the polarizability. The alkane molecule with a greater molecular mass and a large chain length experiences stronger London dispersion forces. The reason is that longer molecules have more places along its length where they can be attracted to other molecules. Therefore, with increase in molecular mass of alkanes, the melting and boiling points increase.
Explain electron gas theory.
The first theory of metallic bonding is electron pool or electron gas theory. This theory was proposed by Drude and extended by Loren (1923). According to this theory, each atom in a metal crystal loses all of its valence electrons. These valence electrons form a pool or a gas. The positively charged metal ions are believed to be held together by electron pool or gas.
Define transition temperature with two examples.
Transition temperature:
It is that temperature at which two crystalline forms of the same substance can co-exist in equilibrium with each other. At this temperature, one crystalline form of a substance changes to another. Examples:
Grey tin (cubic) 13.2℃ ⇔ White tin (Tetragonal)
Sulphur S8 (rhombic) 95.5℃ ⇔ Sulphur S8 (monoclinic)
Evaporation causes cooling effect. Why?
Evaporation causes cooling. The reason is that during evaporation first of all high energy molecules leave the liquid and low energy molecules are left behind. So, temperature of the liquid falls and cooling is produced. To continue the evaporation heat moves from surrounding to the liquid. Thus, temperature of surrounding also falls. For example, when we put spirit on our hands then spirit evaporates and hand feels cooling.
Boiling needs a constant supply of heat, why?
When we supply heat to a liquid, then K.E of molecules and rate of evaporation increase. At boiling point the K.E of molecules becomes maximum. Thus, heat supplied at boiling point is used to break intermolecular forces and to change the phase of liquid into vapor. So, there is no increase in K.E of molecules. Hence temperature remains constant at boiling point. It is the reason that boiling needs a constant supply of heat.
Sodium is a good conductor of electricity but NaCl is not. Give reason.
Sodium is a metal which have free electrons due to which it is a good conductor of electricity. On the other hand, NaCl is an ionic compound and it does not conduct electricity, because on account of the electrostatic force existing between them, the cations and anions remain tightly held together and hence occupy fixed positions restricting all movement. Consequently, NaCl does not conduct electricity. Therefore, Sodium is a good conductor of electricity but NaCl is not.
Give two important uses of liquid crystals. How are liquid crystals used to locate veins, arteries, infections and tumors?
Liquid crystals are used in the display of digital watches, calculators and laptop computers.
Liquid crystals are used as temperature sensors. When white light is reflected from liquid crystal then its color changes. When temperature changes, the color of reflected light also changes. Thus, temperature is accurately measured.
Liquid crystals are used to locate infection, tumors and breast cancer. They are also used to detect blockage of veins and arteries. These parts are warmer than other tissues. When liquid crystal is painted on them, they produce blue colored spots. Thus, indication of any problem is easy.
In chromatographic separations, liquid crystals are used as solvents
Write a brief note on solubility of hydrogen bonded molecules.
Water is the best example of H-bonded system. Similarly, ethyl alcohol also has the tendency to form hydrogen bonds. So, ethyl alcohol can dissolve in water because both can form hydrogen bonds with each other. Similarly, carboxylic acids are also soluble in water, if their sizes are small. Hydrocarbons are not soluble in water at all as they are non-polar compounds and there are no chances of hydrogen bonding between hydrocarbon and water molecules.
Why vapor pressure increases with temperature?
The most important parameter that controls the vapor pressure of a liquid is its temperature. Vapor pressure increase with increase in temperature because at an elevated temperature, the kinetic energy of molecules is enhanced and capability to leave the surface increases.
Ionic crystals are highly brittle. Why?
Ionic solids are composed of parallel layers of cations and anions in alternate positions such that opposite ions lie over each other. When an external force is applied, these layers slip pass each other along a plane. In this way, similar ions come in front of each other and they start repelling. So, the application of a little external force develops repulsion between two layers causing brittleness. Therefore, Ionic crystals are highly brittle.
Cleavage is an anisotropic behavior. Explain it.
Whenever the crystalline solids are broken they do so along definite planes. These planes are called the cleavage planes and they are inclined to one another at a particular angle for a given crystalline solid. Anisotropy is the property of a crystal to obey a certain property better in one direction. Since cleavage can take place only in particular direction, so cleavage itself is an anisotropic behavior.
Why electrical conductivity of the metals is decreased by increasing temperature.
With the increase in temperature the positive ions of metals also begin to vibrate and the motion hinders the free movement of mobile electrons between the positive ions. Due to this hindrance, electrical conductivity also decreases. Therefore, electrical conductivity of the metals decreases by increasing temperature.
Why boiling point of H2O is greater than HF?
The boiling point of water is more affected by hydrogen bonding than that of HF. The boiling point of HF is lower than H2O. The reason is that the fluorine atom can make only one hydrogen bond with electropositive hydrogen of a neighboring molecule. Water can form two hydrogen bonds per molecule, as it has two hydrogen atoms and two lone pairs on oxygen atom. Therefore, boiling point of H2O is greater than HF.
Justify the following order ∆Hf < ∆Hv < ∆Hs.
When a solid substance melts then atoms, molecules or ions undergo relatively small changes in intermolecular distances and the potential energy also undergoes a small change. But when a liquid evaporates, then large changes in intermolecular distance and in potential energy takes place. So ∆H of vaporization of a substance is greater than ∆H of fusion. The values of ∆Hs are even larger than ∆Hv because attractive forces in solids are stronger than those in liquids.
Why transition temperature of a crystalline solid is always less than its melting point?
It is that temperature at which two crystalline forms of the same substance can coexist in equilibrium with each other. At this temperature one crystalline form of substance changes to another. As crystalline forms exist only at temperature lesser than the melting point. Therefore transition temperature of a crystalline solid is always less than its melting point.
Give reason how earthenware vessels keep water cool?
Earthenware vessels have pores in them. Water evaporates from these pores and causes cooling. During evaporation, the escaping molecules get energy from neighboring molecules to overcome intermolecular forces. Thus, temperature of remaining water decreases. In this way, earthenware vessels keep water cool. In the old earthenware, pores are blocked with dust. So, water cannot evaporate. Thus, they do not keep water cool.
Define isomorphism and polymorphism.
Isomorphism:
Isomorphism is the phenomenon in which two different substances exist in the same crystalline form. These different substances are called isomorphs of each other. For Example, NaNO3 and KNO3 are isomorphs of each other as both exists in rhombohedral form.
Polymorphism:
Polymorphism is a phenomenon in which a compound exists in more than one crystalline forms. The substance which exists in more than one crystalline forms is called polymorphic, and these forms are called polymorphs of each other. For example, polymorphs of CaCO3 are trigonal and orthorhombic.
Why heat of sublimation of I2 is very high than other halogens?
I2 has greatest size among all the halogens, so its polarizability is high. Due to high polarizability the additional forces develop and hence it has high heat of sublimation among all the halogens.
What is symmetry of a crystal?
The repetition of faces, angles or edges when a crystal is rotated by 360° along its axis is called symmetry. This an important property of the crystal and there are various types of symmetry elements found in crystals like, center of symmetry, plane of symmetry and axis of symmetry, etc.
Define anisotropy with an example?
Some of the crystals show variation in physical properties depending upon the direction. Such properties are called anisotropic properties and the phenomenon is referred to as anisotropy. For example, electrical conductivity of graphite is greater in one direction than in another. Actually electrons in graphite are mobile for electrical conduction parallel to the layers only. Therefore, its conductivity in this direction is far better than perpendicular to other direction.
Define unit cell. Give its crystallographic elements.
The smallest part of the crystal lattice has all the characteristic features of the entire crystal and is called a unit cell. These six parameters of the unit cell are its three edges or axes, a, b, c and three angles between the axes α, β, γ. These are called unit cell dimensions or crystallographic elements.
What is polarizability? Give its relation with London dispersion forces.
Polarizability is the quantitative measurement of the extent to which the electronic cloud can be polarized or distorted. This increased distortion of electronic cloud creates stronger London forces.
The boiling point of water is different at Murree hills and at Mount Everest. Give reason.
A liquid can be made to boil at any temperature by changing the external pressure. When the external pressure is high the liquid requires greater amount of heat to equalize its vapour pressure to external pressure. In this way boiling point is raised. Similarly, at a lower external pressure a liquid absorbs less amount of heat and it boils at a lower temperature. Water boils at 98°C at Murree hills due to external pressure of 700 torr while at the top of Mount Everest water boils at only 69oC 323 torr.
What are intermolecular forces of attraction? Give two examples.
The attractive forces which exist between individual particles i.e. atoms, ions and molecules are called intermolecular forces or van der Waal`s forces. Four types of such forces are dipole-dipole forces, ion-dipole forces, dipole-induced dipole forces and Instantaneous dipole-induced dipole forces or London dispersion forces
Define crystal lattice with an example.
A crystal lattice is an array of points representing atoms, ions or molecules of a crystal, arranged at different sites in three dimensional space. For example, cubic crystal lattice of NaCl in which sodium ions are surrounded by chloride ions and vice versa.
Differentiate between amorphous and crystalline solids.
Evaporation takes place at all temperatures. Explain with reason.
At room or low temperature, there are always some molecules whose K.E is greater than average K.E of molecules. These molecules can overcome the intermolecular forces and escape from the surface in the form of vapors. At high temperature, the rate of evaporation increase so evaporation continues at all temperatures but rate of evaporation will be different at different temperatures.
What is habit of a crystal? How is it changed?
The shape of a crystal in which it usually grows is called habit of a crystal. If the conditions for growing a crystal are changed the shape of the crystal may change. For example, a cubic crystal of NaCl becomes needle like when 10% urea is present in its solution as an impurity.
Long Questions
What are molecular solids/metallic solids/covalent solids/ionic solids? Give their properties.
Define Debye forces, amorphous solids and Hydrogen bonding.
Define hydrogen bonding. How does it explain the indicated properties of the following substances? (Solubility of Hydrogen bonded molecules, Structure of ice).
Explain hydrogen bonding in NH3, H2O and HF.
Describe four properties of crystalline solids. Or Write a detailed note on anisotropy, allotropy, isomorphism and polymorphism.
What is boiling point? Why temperature does not change at boiling point? Explain the effect of external pressure on boiling point?
Define evaporation. On what factors it depends? Discuss.
Define metallic solids. Discuss metallic solids in terms of electron gas theory and molecular orbital theory.
Define vapor pressure. Write down the manometric method for its determination with diagram.
Unit #5 Atomic Structure
Multiple Concepts Questions or Multiple Choice Questions.
Short Questions
Energy of an electron is inversely proportional to ‘n’ but energies of higher orbits always greater than those of lower orbits. Justify it.
The energy of an electron revolving in any orbit is given by the formula:
En = -2.178 × 10-18 1n2 J
Greater the value of n, greater is the energy because energy is negative inverse of n. It becomes more and more less negative. The value of Energy approaches zero when n=. Therefore, energies of higher orbits are always greater than those of lower orbits.
Why is it necessary to decrease the pressure in discharge tube to get cathode rays?
The current does not flow through the gas at ordinary pressure even at high voltage about 5000 volts. However, when the pressure inside the tube is decreased to about 0.01 torr, the gas in the tube begins to conduct electricity at low pressure. Therefore, it is necessary to decrease the pressure in the discharge tube to get the cathode rays.
How do you come to know that velocities of electrons in higher orbits are less than those in lower orbits of H atom?
According to the relation; r=Ze24πomv2
The radius of the moving electron is inversely proportional to the square of its velocity. It conveys that electron should move faster nearer to the nucleus in an orbit of smaller radius. Therefore, we say that velocities of electrons in higher orbits with greater radii are less than those in lower orbits of H atom with smaller radius.
State Moseley`s law.
Mosely’s law states that:
The frequency of spectral line in X–ray spectrum varies as the square of atomic number of an
element emitting it.
Mathematically; v = aZ-b
This law convinces us that it is the atomic number and not the atomic mass of the element which determines its characteristic properties, both physical and chemical.
Give two defects in Rutherford`s atomic model.
Rutherford’s planet-like picture was defective because the moving electron must be accelerated towards the nucleus.
The radius of the orbiting electron should become smaller and smaller and electron should fall into the nucleus. But this doesn’t happens. Thus, an atomic structure as proposed by Rutherford would collapse.
Write names of spectral series of hydrogen spectrum.
Define Pauli-exclusion principle and wavelengths.
Pauli-exclusion principle:
This principle can be stated as follows:
Two electrons in the same orbital should have opposite spins.
Wavelength:
Wavelength is the distance between the two adjacent crests or troughs. It is expressed in Ao, nm or pm.
Explain orbital.
“The volume of space within an atom in which there is 95% chance of finding an electron is called orbital.”
It may be spherical, dumbbell or double dumbbell in shape. It represents that an electron can move around the nucleus in three-dimensional space. The maximum number of electrons in an orbital is two.
State Auf-bau principle and Pauli-exclusion.
Auf-bau principle:
This principle can be stated as follows:
The electrons should be filled in energy subshells in order of increasing energy values. The electrons are first placed in 1s, 2s, 2p and so on.
Pauli-exclusion principle:
This principle can be stated as follows:
Two electrons in the same orbital should have opposite spins.
Why e/m value of positive rays for different gases are different but those for cathode rays the e/m values are same? Justify it.
The e/m value for positive rays are different for different gases because they differ in mass. The mass of the positive particles is the same as that of the atom or molecule from which it is created. Heavier the gas, smaller the e/m value. While cathode rays are composed of negatively charged particles (electrons). They are constituents of all gases. So, cathode rays are independent of the nature of the gas in the discharge tube.
How we come to know that cathode rays are material particles with negative charge?
Cathode rays drive a small paddle, wheel which shows that these rays possess momentum. From this observation, it is inferred that cathode rays are not rays but particles having a definite mass and velocity. Therefore, cathode rays are material particles. And the charge was discovered by Thomson in 1897 in his experiment when these were deflected by positive plates. Hence, cathode rays are material particles with negative charge.
The energy associated with violet color is greater than red color in visible spectrum, why?
The photons of violet color bend to maximum extent after passing through the prism and
their range of wavelength is from 400-420 nm. They have high energy and have shorter wavelengths. The photons of the red color have the longer wavelength ranging from 630 to 800 nm. These photons bend in prism to lesser extent.
The e/m values of positive rays obtained from the hydrogen gas is 1836 times less than that of cathode rays, justify.
The e/m value for positive rays obtained from hydrogen gas is 1836 times less than that of cathode rays. This is because the mass of proton which is created from H-atom is 1836 times more than that of an electron (cathode rays particle).
Why e/m value of cathode rays is same for all gases?
A cathode ray consists of beam of electrons and electrons are constituents of all matter. So, cathode rays do not depend upon the nature of the gas. Whichever gas is used in the discharge tube, as the nature of cathode rays remains the same, therefore, e/m value of cathode rays is same for all gases.
Define spectrum. Give its two types.
Spectrum:
The dispersion of the components of white light, when it is passed through prism is called spectrum. The distribution among various wavelengths of the radiant energy emitted or absorbed by an object is also called spectrum.
Types:
Continuous spectrum: A spectrum containing light of all wavelengths is called continuous spectrum. In this type of spectrum, the boundary line between the colors cannot be marked. For example rainbow.
Line spectrum: When an element or its compound is volatilized on a flame and the light emitted is seen through, a spectrometer. We see distinct lines separated by dark spaces. This type of spectrum is called line spectrum. For example the atomic spectrum of Hydrogen.
State Heisenberg`s uncertainty principle and represent its formula.
Heisenberg`s uncertainty principle:
It is impossible to determine simultaneously both the position and momentum of an electron.
Suppose that Δx is the uncertainty in the measurement of the position and Δp is the uncertainty in the measurement of momentum of an electron.
Δx Δp ≥ h4π
This relationship is called uncertainty principle.
Write the electronic configuration of Cu = 29 and K= 19.
The electronic configuration of Cu = 29 is as follows:
29Cu → 1s2 2s2 2p6 3s2 3p6 4s1 3d10
The electronic configuration of K = 19 is as follows:
19K → 1s2 2s2 2p6 3s2 3p6 4s1
What is stark effect?
When the excited hydrogen atoms are placed in an electric field, its spectral lines are further split up into closely spaced lines. This type of splitting of spectral lines is called stark effect.
Calculate the mass of an electron when its e/m value is 1.7588 x 1011 Ckg-1.
Mass of Electron:
The value of charge on electron is 1.602 × 10-19 coulombs, while e/m of electron is 1.7588× 1011 coulombs kg-1. So,
em = 1.6022 × 10-19 coulombs Mass of electron = 1.7588 × 1011coulombs kg-1
Rearranging,
Mass of electron = 1.6022 × 10-19 coulombs 1.7588 × 1011coulombs kg-1
Mass of electron = 9.1095 × 10-31kg
Write two points of importance of Moseley`s law.
Moseley arranged K and Ar, Ni and Co in a proper way in Mendeleev’s periodic table.
This law has led to the discovery of many new elements like Tc(43), Pr(59), Rh(45).
The atomic number of rare earths have been determined by this law.
What particles are formed by the decay of free neutron? Write nuclear reaction for the decay of neutron.
Free neutron decays into a proton (+1P ) with the emission of an electron (-10e ) and a neutrino ( 00n ).
01n → +11P + -10e + 00n
So, an electron, proton and a neutrino is formed by the decay of free neutron.
Why P.E of bounded electrons is negative?
According to Bohr, the potential energy of electron is given by:
Epotential = -Ze24πor
The minus sign indicates that the P.E of electron decreases, when it is brought from infinity to a point at a distance ‘r’ from the nucleus. At infinity, the electron is not being attracted by anything and P.E of the system is zero. Whereas at a point nearer the nucleus, it will be attracted by the nucleus and the P.E becomes less than zero. The quantity less than zero is negative. For this reason, the P.E of bounded electron is negative as electron is bound by the nucleus.
Justify that Bohr`s atomic model is based on Plank`s quantum theory.
Bohr proposed that electrons move around the nucleus in the fixed orbits with definite energies. Whenever they change the orbits they emit or absorb the energy in terms of photons which was suggested by Planck in 1900. Bohr made an extensive use of the quantum theory of Planck and proposed that the electron, in the hydrogen atom, can only exist in certain permitted quantized energy levels.
Why e/m values of positive rays for different gases is different?
The e/m value for positive rays are different for different gases because they differ in mass. The mass of the positive particles is the same as that of the atom or molecule from which it is created. Heavier the gas, smaller the e/m value.
Justify that the distance gaps between different orbits go on increasing from lower to the higher orbits.
As we know,
r=(oh2πme2)n2
After putting values of different parameters for H-atom, we get;
r=0.529 Ao(n2)
Putting values of n as 1, 2, 3, 4……, the radii of orbits of hydrogen atom are
n=1 r1 = 0.529 Ao n=2 r2 = 2.11 Ao
n=3 r3 = 4.75 Ao n=4 r4 = 8.4 Ao
n=5 r5 = 13.22 Ao
The comparison of radii shows that the distance gaps between different orbits of H-atom goes on increasing as we move from 1st orbit to the higher orbits. The orbits are not equally spaced.
r2 – r1 < r3 – r2 < r4 – r3 < ………………...
What is Zeeman effect?
When the excited atoms of hydrogen are placed in a magnetic field, its spectral lines are further split up into closely spaced lines. This type of splitting of spectral lines is called Zeeman effect.
Distribute electrons in orbitals of : (a) 24Cr (b) 35Br
(a) 24Cr : 1s2 2s2 2p6 3s2 3p6 4s1 3d5
(b) 35Br : 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5
State Hund’s rule.
If degenerated orbitals are available and more than one electron are to be placed in them, they should be placed in separate orbitals with the same spin rather than putting them in same orbital with opposite spin.
According to rule, two electrons in 2p orbital of carbon will be distributed as follows:
Three orbitals of 2p subshell are called degenerate orbitals.
Write nuclear reaction for the production of neutrons.
The nuclear reaction for the production of neutron is as follows:
In above reaction α-particles and the nuclei of Be are rearranged and extra neutron is emitted.
Why positive rays are called canal rays?
Positive rays after passing through the perforated cathode produce a glow on the wall opposite to the anode. Since these rays pass through the canals or the holes of cathode, they are called canal rays.
What are slow and fast neutrons?
When neutrons travel with an energy 1.2 MeV, they are called fast neutrons but with energy below 1eV are called slow neutrons. Slow neutrons are usually more effective than fast ones for the fission purposes.
Write any two points of plank’s quantum theory.
Main points of plank’s quantum theory are:
Energy is not emitted or absorbed continuously. Rather, it is emitted or absorbed in a discontinuous manner and in the form of wave packets. Each wave packet or quantum is associated with definite amount of energy is often called photon.
The amount of energy associated with a quantum of radiation is proportional to the frequency (ν) of radiation. Frequency (ν) is the number of waves passing through a point per second.
E α ν
E = hν
Where ‘h’ is constant known as plank’s constant and its value is 6.626 x 10-34 Js. It is in fact, the ration of energy and frequency of photon.
What is origin of X-rays?
X-rays are produced when rapidly moving electrons collide with heavy metal anode in the discharge tube. Energy is released in the form of electromagnetic waves when the electrons are suddenly stopped. The wavelength of X-rays produced depends upon the nature of the target metal.
State (n + l) rule. Give its importance.
This rule says that subshells are arranged in the increasing order of (n + l) values and if any two subshells have the same (n + l) values, then that subshell is placed first whose n value is smaller. The arrangement of subshells in ascending order of their energy may be as follows: 1s, 2s, 2p, 3s, 3p, 4s, 3d, and so on.
What is atomic emission spectrum? Explain.
When solids are volatilized or elements in their gaseous states are heated to high temperature or subjected to an electrical discharge, radiation of certain wavelengths are emitted. The spectrum of this radiation contained bright lines against a dark background. This is called atomic emission spectrum.
Write any two defects of Bohr’s atomic model.
Defects of Bohr’s atomic model are as under:
Bohr’s theory can successfully explain the origin of the spectrum of H-atom and ions like He+1, Li+2, Be+3 etc. These are all one electron system. But this theory is not able to explain the origin of spectrum of multi electron or poly electron system like He, Li and Be etc.
Bohr suggested circular orbits of electrons around the nucleus of hydrogen atom but researches have shown that the motion of electron is not in a single plane, but takes place in three-dimensional space. Actually, the atomic model is not flat.
Long Questions
Give defects of Bohr`s atomic model.
Explain dual nature of matter (de-Broglie`s equation).
Define orbital. Discuss shape of its two types.
Explain experiment which helped us to understand discovery of protons/electrons/ neutrons.
What are cathode rays? How are they produced? Write their properties.
Give postulates of Bohr`s atomic model.
Enlist quantum numbers. Write a detailed note on magnetic and azimuthal quantum number.
Derive equation of radius using Bohr`s atomic model.
Derive the formula to calculate the energy of an electron in nth orbit using Bohr`s model.
Describe J.J Thomson’s experiment of determining the e/m value of an electron.
Write a detailed note on Millikan`s oil drop method.
Give properties of neutron in detail.
Unit #6 Chemical Bonding
Multiple Concepts Questions or Multiple Choice Questions.
Short Questions
Define coordinate covalent bond with an example.
A coordinate covalent bond is formed between two atoms when the shared pair of electrons
is donated by one of the bonded atoms.
The radius of an anion is always larger than parent atom. Why?
When electrons are added to the gaseous atom, the number of electrons increase but protons remain same (nuclear charge remains same). As a result the effective nuclear charge decrease i.e., hold of nucleus on valence electron decrease. As a result electrons move away from nucleus resulting in the expansion of the electron cloud. Thus, an anion is larger than its
parent atom. For example, radius of Cl increase from 99 pm to 181pm.
Why atomic radius is greater than cationic radius?
The atomic radius of the element is greater than the radius of its cation. This is due to the reason that the cationic radius decreases with the increase in the effective nuclear charge on the ion. With the successive loss of electron, the nuclear charge attracts the remaining electrons with a greater force. For example, radius of Na reduces from 186 pm to 95 pm.
Why 2nd ionization energy of an element is always greater than 1st ionization energy?
The ionization energy values undergo an increase with the increase in the number of electrons to be removed. This is due to the reason that second electron is removed from a positively charged ion rather than a neutral atom. The dominant positive charge holds the electron more tightly and thus further removal of electrons become more difficult. For example, the 1st ionization energy of Mg is 738 kJmol-1 and its 2nd ionization energy is 1450 kJmol-1.
Differentiate between bonding molecular orbitals and anti-bonding molecular orbital.
Two atomic orbitals after overlapping form two molecular orbitals which differ in energy. One of them, having lower energy, is called bonding molecular orbital while the other having higher energy is called anti-bonding molecular orbital. The bonding molecular orbital is symmetrical about the axis joining the nuclei of the bonded atoms. It is designated at sigma (σ) bonding molecular orbital while the anti-bonding molecular orbital is called as (σ*).
The bond angles between H2O and NH3 are not 109.5° like that of CH4 although oxygen and nitrogen atoms are sp3 hybridized, why?
In NH3 and H2O, there is sp3 hybridization like CH4. Four sp3 hybrid orbitals are formed. The angle between sp3 orbitals should be 109.5o. But we know that NH3 has one lone pair and H2O has two lone pairs of electrons. There is repulsion between lone pairs and bonding pairs of electrons. Due to lone pair-bond pair repulsion the bond angles in NH3 and H2O are not 109.5o like CH4 which has no lone pair of electrons.
Explain that π-bonds are more diffused than sigma bond. Or Why sigma bond is stronger than pi bond?
A sigma bond (σ) lies along the internuclear axis and is under the strong hold of the two bonded nuclei, while a pi bond (π) has its electron density located away from the internuclear axis (above and below it) and is not strongly attracted by the two nuclei. That is why pi bonds are more diffused and weaker than sigma bonds.
Define atomic radius. How does it vary in groups and periods?
The atomic radius means the average distance between the nucleus of the atom and its outermost electronic shell. In general, atomic radii decrease from left to right in a period and increase from top to bottom in a group. The decreasing trend in a period is due to increase in the nuclear charge. The increase in atomic radii in a group is due to increase in the number of shells and screening effect.
Why the lone pair of electrons on an atom occupy more space than bond pair?
A lone pair of electrons occupies more space than bond pair because lone pair is attracted by only one nucleus while bond pair is attracted by two nuclei. Due to less nuclear attraction to lone pair its electronic charge is spread out more in space than that of bond pair.
The abnormality in bond length in HI is less prominent than HCl, why?
Chlorine has smaller size and higher electronegativity than Iodine. Thus, HCl has more polarity than HI. Due to this reason abnormality of bond length and bond strength in HI is less prominent than that of HCl. The electronegativity of chlorine is 3.2 and iodine is 2.5.
What is octet rule?
The tendency of atoms to attain a maximum of eight electrons in the valence shell is known as ‘octet rule’. For example, Neon has eight electrons its valence shell and other atoms tend
to attain eight electrons to get stable.
How does the electronegativity difference decide the nature of chemical bond?
Criteria of electronegativity helps to understand the nature of bond. So, in order to decide the % of ionic nature in a compound, it is better to note the difference between electronegativity between the bonded atoms. If the difference is 1.7 or more than that, then the bond is said to be ionic. Keeping this aspect in view, NaCl has 72% ionic character. CsF has 92% ionic character. There is no bond with 100% ionic character.
Explain the term bond order.
The number of bonds formed between two atoms after the atomic orbital overlap is called the bond order and is taken as half of the difference between the number of bonding electrons and anti-bonding electrons. The number of bonds formed between H-atoms in hydrogen molecule may be calculate as follow:
Bond order = 2-02=1
Why ionic compounds do not show the phenomenon of isomerism but covalent compounds do. Or Why ionic bonds are non-directional?
The ionic compounds involve electrostatic lines of forces between oppositely charged ions. Therefore, such bonds are non-rigid and non-directional. Because of this, ionic compound do not exhibit the phenomenon of isomerism. On the other hand, covalent compounds are rigid and directional. This leads to the possibility of a variety of isomerism. For example, the compounds, C2H6O, shows structural isomerism.
Write down two postulates of VSEPR theory.
The electrons pairs of lone pairs occupy more space than the bond pairs.
Both the lone pairs as well as the bond pairs participate in determining the geometry of the molecules.
What factors influence the ionization energy?
It is observed that the ionization energies of atoms depend upon the following factors.
Atomic radius of atom
Nuclear Charge or Proton number of the atom
Shielding effect of inner electrons
Nature of orbital
The distinction between coordinate covalent bond and covalent bond vanishes after bond formation in NH4+ and H3O+. How?
In a covalent bond, two atoms provide shared pair of electrons. In a coordinate covalent bonds shared pair of electrons is donated by one atom only. There is no difference between their bond length and bond energy. In ions, all four bonds are taken equally. It is the reason that distinction between covalent and coordinate covalent bond vanishes after their formation.
Helium is diamagnetic in nature, Justify.
Molecules which have paired electrons in their anti-bonding molecular orbitals are not attracted by magnetic field and called as diamagnetic in nature. Helium is diamagnetic in nature because it has paired electrons in its anti-bonding molecular orbital.
Define ionic bond with example.
According to the Lewis theory, ionic bond is formed by the complete transfer of electron or electrons from an atom with low ionization energy to another atom with high electron affinity. The bond formed by complete transfer of electron between Potassium and Chlorine to from Potassium Chloride is ionic bond.
Why dipole moments of CO2 and CS2 are zero but that of SO2 is 1.61D? or Why dipole moment of CO2 is zero but that of CO is 0.12D?
CO2 and CS2 have linear structures. The dipoles being equal and opposite, cancel out each other`s effect. In CO, there is a single dipole directed from carbon to oxygen and it is not cancelled. So, CO has a dipole moment 0.12D. While SO2 has a V-shaped angular structure. Therefore, the dipole moment of SO2 is 1.61D.
Define bond energy. On what factors strength of bond/bond energy depends upon?
The bond energy is the average amount of energy required to break all bonds of a particular type in one mole of the substance. The bond energy for H-H bond is 436 KJmol-1. The bond energy depends upon the following factors:
Electronegativity difference of bonded atoms
Sizes of the atoms
Bond length
Differentiate between covalent and coordinate covalent bond.
In a covalent bond, both atoms are contributing same number of electrons to the bond, but in a coordinate covalent bond, two electrons are donated by a single atom.
In a covalent bond, the electronegativity difference between the two atoms can be zero or a very low value, but in coordinate covalent bond, type of a polar covalent bond is forming.
For a coordinate covalent bond to form, an atom in the molecule should have a lone pair.
Why the radius of an atom cannot be determined precisely?
The radius of atom cannot be determined precisely due to following reasons:
The electronic probability distribution is affected by neighboring atoms. For this reason, the size of an atom may change from one compound to another.
There is no sharp boundary of an atom. The probability of finding an electron never becomes equal to zero even at larger distances from the nucleus.
Define ionization energy. Give its trend in periods and groups of periodic table.
The ionization energy of an element is the minimum amount of energy required to remove electron from its gaseous atom to form an ion. This process is called ionization.
Mg → Mg+ + 1e- ΔH = 738 KJmol-1
Trend in periodic table:
Ionization energy increases from left to right in a period with the increase in the nuclear charge. Due to increase in nuclear charge, the force of attraction between nucleus and outermost shell also increases and more energy in required to remove electron from valence shell of an atom.
Ionization energy decreases when we move from top to bottom in a group due to increase in the shielding effect of inner shells. Due to shielding effect, the force of attraction between nucleus and outermost shell becomes weak.
How electronegativity changes in group?
When we move from top to bottom in a group the electronegativity decreases. This is due to the increase in the size of an atom and increasing shielding effect.
Why oxygen molecule is paramagnetic in nature?
Molecules which have unpaired electrons in their anti-bonding molecular orbitals are attracted by magnetic field and called as paramagnetic in nature. Liquid oxygen molecule is paramagnetic in nature because it has unpaired electrons in its anti-bonding molecular orbital and is attracted by magnet.
Draw shape and write bond angle in NH3 and BF3 molecules w.r.t VSEPR theory.
Define electron affinity. Name two factors affecting electron affinity.
The electron affinity of an atom is the energy released when an electron adds to an empty or partially filled orbital of an isolated gaseous atom in its valence energy level to form an anion having a unit negative charge. For example
The electron affinities are affected by the factors such as atomic radius, the nuclear charge and the shielding effect of inner electrons.
Bond distance is a compromised distance between two atoms. Justify the statement.
The distance at which the attractive forces dominate the repulsive forces. The potential energy of the system is minimum and the atoms are said to be bonded to form a stable molecule is called bond distance or bond length or compromise distance of two atoms. When the atoms approach the distance of minimum energy, then the system of two atoms is stabilized to maximum extent.
Sketch the hybrid orbitals of (i) PCl3 (ii) NH4+
Why σ2px is higher in energy in B2, C2, and N2 and lower in energy in O2 and F2 in energy level diagram?
This reversal is due to mixing of 2s and 2px atomic orbitals. In case B2, C2 and N2, the energy difference of 2s and 2p atomic orbitals is small. There is a possibility of mixing of these orbitals as a result of which σ2s and σ*2s MOs do not retain pure s-character. Similarly, σ2px and σ*2px MOs do not have pure p-character. All the four MOs acquire sp-character. Due to this mixing, their energies change in such a way that MOs σ2s and σ*2s become more stable and are lowered in energy MOs as σ2px and σ*2px become less stable and are raised in energy.
Anyhow, O2 and F2 do not do so. The reason is high energy difference of their 2s and 2p.
Explain the geometry of H2S molecule on the basis of VSEPR theory.
H2S exhibits the geometry of AB4 type with two lone pairs and two bond pairs. It has bent or angular geometry. Two of the corners of tetrahedron are occupied by each of the two lone pairs. The angle is about 92o.
Long Questions
Define ionization energy, write factors affecting ionization energy and explains its trend along period/group.
Explain, what do you understand by the term electronegativity? Discuss its variations in the periodic table. How does it affect the bond strength?
Define dipole moment. Give its SI unit. How does it explain the geometry of CO2 and BF3 molecules? Or define dipole moment. Give its various units. How it is used to determine the geometry of molecule by an example. Find relationship between Debye and mC.
Describe the VSEPR theory of covalent bond.
Define bond energy. Discuss two factors affecting the bond energy.
Define hybridization process and explain the structure of ethyne/ethane/methane/H2O/NH3 molecule on the basis of it.
Draw the molecular orbital picture of O2 molecule.
Unit #7 Thermochemistry
Multiple Concepts Questions or Multiple Choice Questions.
Short Questions
Define thermochemistry.
The study of heat changes accompanying a chemical reaction is known as thermochemistry.
Define state and state functions
State
The state of a system is the condition of a system having certain temperature and volume. The initial condition of a system may be called the initial state. The final condition of the system may be called the final state of the system.
State Function
State function is a macroscopic property of a system which has some definite values for initial and final states, and which is independent of the path adopted to bring about a change. By convention, we use capital letters as symbol for a state function. For example pressure (P), temperature (T), volume (V) etc.
Comment that enthalpy of neutralization is merely the heat of formation of one mole of liquid water.
The standard enthalpy of neutralization is the amount of heat evolved when one mole of hydrogen ions (H+) from an acid, react with one mole of hydroxide ions (OH-) from a base to form one mole of water. During the process of neutralization, the only change that actually occurs is the formation of water molecule leaving the ions of acids and bases as free ions in the solution. Thus, the enthalpy of neutralization is merely the heat of formation of one mole of liquid water from its components. For example, the enthalpy of neutralization of NaOH by HCl is -57.4 kJmol-1.
H+aq+Cl-aq+Na+aq+OH-aq⇌Na+aq+Cl-aq+H2Ol
H+aq+ OH-aq⇌ H2Ol ∆Hno=-57.6+KJmol-1
Explain the burning of a candle is a spontaneous process. Or what is spontaneous process? Give two examples.
A process which takes place on its own without any outside assistance and move from non-equilibrium state towards an equilibrium state is termed as spontaneous process or natural process. A reaction will also be called spontaneous process if it needs energy to start with, but once it is started, then it proceeds on its own. Burning of candle is an example of such spontaneous process. Candle does not burn on its own rather the reaction is initiated by spark and once it starts burning, the reaction goes spontaneously to completion.
What is first law of thermodynamics? Justify that heat of formation of compound is sum of all the other enthalpies.
First Law of Thermodynamics
The first law of thermodynamics, also called the law of conservation of energy, states that energy can neither be created nor destroyed, but can be changed from one form to another. System can exchange energy with its surroundings in the form of heat and work. Mathematically it can be written as
𝛥E = q + w
Where ‘q’ represents the amount of heat absorbed by the system and ‘w’ is the work done by the system.
Heat of Formation
Standard enthalpy of formation of a compound is the amount of heat absorbed or evolved when one mole of the compound is formed from its elements. For example when carbon reacts with oxygen to form CO2, the energy released in the formation is sum of all the other enthalpies.
Cs+ O2g→ CO2g ∆H =-393.7 KJmol-1
What are exothermic and endothermic reactions? Give one example of each.
Exothermic Reactions
The reactions in which heat is given out by the system and the temperature of the system rises above room temperature are called exothermic reactions. Eventually the temperature of the system falls to room temperature again as the heat produced is lost to the surrounding. Negative sign shows the enthalpy of exothermic reactions. For example, formation of CO2.
Cs+ O2g→ CO2g ∆H =-393.7 KJmol-1
Endothermic reactions
The reactions in which the heat required for the reaction is taken from the reacting materials (system) and the temperature of the system falls below the initial temperature are called endothermic reactions. Eventually, the temperature of the system rises to room temperature again as heat is absorbed from the surrounding. For example formation of NO. Positive sign shows the enthalpy of endothermic reactions.
N2g+ O2g→2NOg ∆H =+180.51 KJmol-1
Is it true that ∆H and ∆E have the same values for the reaction taking place in solution state? Explain it.
For the reaction taking place in the solution state, since the change in volume is insignificant. i.e 𝛥V=0
So, according to enthalpy equation
∆H= ∆E+P∆V
∆H= ∆E+P × 0
∆H= ∆E
Hence, ∆H and ∆E have the same values for the reaction taking place in solution state.
What is thermochemical equation? What information does it convey? Explain with an example.
A thermochemical equation is a balanced stoichiometric chemical equation that gives information about a reaction in which heat is being evolved or absorbed.
Cs+ O2g→ CO2g ∆H =-393.7 KJmol-1
It tells whether the reaction is exothermic or endothermic. It gives information about the physical states of reactants and products. It tells about the amount of energy absorbed or evolved.
Define enthalpy of a solution. Give examples.
Enthalpy of Solution:
The standard enthalpy of a solution is the amount of heat absorbed or evolved when one mole of a substance is dissolved in so much solvent that further dilution results in no detectable heat change.
Examples:
Enthalpy of solution of ammonium chloride is +16.2 KJmol-1
Enthalpy of solution of sodium carbonate is -25.0 KJmol-1
Describe system and surroundings.
System
Anything under observation or consideration or test in the laboratory is called as system
Surroundings
The remaining portion of the universe is known as surrounding of the system.
Consider the reaction between Zn and CuSO4 solution. This can be called as system under observation. The flask, the air etc. are the surroundings.
Define internal energy and enthalpy.
Internal Energy
The total of all the possible kinds of energies of the system is called its internal energy, E. The change in internal energy of the system ∆E is a state function.
Enthalpy
The total heat content of a system is termed as enthalpy of a system. It is a state function and represented by H. In general, enthalpy is equal to the sum of internal energy, E and the product of pressure and volume (PV). H = E + PV
Why enthalpy of combustion of some compounds cannot be measured directly?
The reason is, that some compounds like boron oxide (B2O3) and aluminium oxide (Al2O3) provide problems for the measurement of standard enthalpies of their formation. In these cases, it is difficult to burn these elements completely in oxygen, because a protective layer of oxides covers the surface of the unreacted element. Similarly, heat of formation of CO cannot be measured directly due to the formation of CO2 with it.
Define enthalpy of reaction. Give one example.
The standard enthalpy of a reaction ∆Ho is the enthalpy change which occurs when the certain number of moles of reactants as indicated by the balanced chemical equation, react together completely to give the products under standard conditions, i.e 25°C (298K) and one atmosphere pressure.
-285.8 kjmol-1 is the standard enthalpy of reaction.
How do you determine the ∆H for food and fuel in laboratory?
A bomb calorimeter is usually used for the accurate determination of the enthalpy of combustion for food, fuel and other compounds.
What is enthalpy of combustion? Give example.
The standard enthalpy of combustion of the substance is the amount of heat evolved when one mole of the substance is completely burnt in excess of oxygen under standard conditions. It is denoted by ∆Hoc. For example, standard enthalpy of combustion of ethanol ∆Hoc is -1368KJ mol-1. The reaction is represented by the following equation.
Long Questions
Explain enthalpy and prove that qp = ∆H.
Write a note on construction and working of bomb calorimeter.
State and explain first law of thermodynamics. Also prove that ∆E = qv.
Hydrazine burns in oxygen to give N2 and H2O as follows
N2H4(l)+ O2(g) → N2(g)+2H2O(l)
1.00 g of N2H4 is burnt in a bomb calorimeter. An increase of temperature 3.51°C is noted. The heat capacity of calorimeter is 5.5 kJ K-1g-1. Calculate the amount of heat evolved.
Define the following with suitable examples. (i) Enthalpy of Neutralization (ii) Enthalpy of formation.
Explain spontaneous and non-spontaneous reactions. Describe four points which differentiate them.
Define and explain Hess`s Law of constant summation and give its applications.
“This law states that overall enthalpy change is same if a chemical reaction takes place in one step or several steps.”
Or
“The overall enthalpy change depends upon initial and final states of a reaction not upon path followed by reaction”
Mathematically it can be written as,
ΣH (cycle) = 0
From the above expression it can be said that “The sum of all enthalpy changes in a cyclic process is equal to zero”
For a general reaction in which A is converted to D, the reaction takes place in one step or three steps.
According to Hess`s Law ∆H=∆H1 +∆H2+∆H3
The formation of carbon dioxide takes place by two ways.
The first one is single step process C + O2 CO2 ∆H= -393.7 KJmol-1
Second ways is two-step process C + 1/2O2 CO ∆H1= -110 KJmol-1
CO + 1/2O2 CO2 ∆H2= -283 KJmol-1
The enthalpy change is same in both cases
∆H=∆H1 +∆H2
-393.7 = -110.7 + (-283)
-393.7 KJmol-1 = -393.7 KJmol-1
Applications of Hess`s Law
There are many reactions which are very slow. Their ∆H cannot be measured directly. Therefore ∆H is measured indirectly using Hess`s law.
Carbon tetrachloride (CCl4) cannot be directly prepared by reaction of carbon and chlorine. Therefor its heat of formation is measured indirectly using Hess`s law.
During the combustion of Aluminum and Boron a protective layer is formed on the surface of element. So Al and B do not burn completely. Hence heat of formation of Al2O3 and B2O3 are measured indirectly by Hess`s law.
The heat of formation of CO cannot be measured directly due to formation of CO2 with it. Therefore, it is indirectly measured by Hess`s law.
C + O2 → CO2 ∆H= -393.7KJ mol-1
C + 12 O2 → CO ∆H1= ?
CO + 12 O2 → CO2 ∆H2= -283KJ mol-1
Applying Hess’s law;
∆H= ∆H1 + ∆H2
∆H1= ∆H - ∆H2
= -393.7--283
∆H1= -110.7 KJ mol-1
What is lattice energy? How does Born-Haber cycle help to calculate the lattice energy NaCl?
“The enthalpy change when on mole of an ionic crystal is formed from its gaseous ion is called lattice energy.”
e.g. Na(g)++Cl(g)- NaCl(s) ∆H= -787 KJmol-1
Born-Haber Cycle
This is a special application of Hess`s law which is used to determine the lattice energy of an ionic compound. According to this cycle the sum of all the enthalpies in a cyclic process is equal to zero.
The energy triangle of NaCl crystal is shown in figure.
Where Hf =Enthalpy of formation
∆Hlatt = Lattice Energy
∆Hx = Total energy involved in changing sodium and chlorine into gaseous ions
When we extend this triangle, we get a complete energy cycle which is called Born-Haber cycle.
Nas + 12 Cl2g ∆Hf → NaCls
Clg ∆He → Cl-g
Nag ∆Hi → Na+g
The formation of NaCl takes place by two routes.
Direct route
Indirect route
Direct Route
Na and Cl2 react directly and form NaCl. It is an exothermic process. The enthalpy is called enthalpy of formation.
Nas + 12 Cl2g → NaCls ∆Hf= -411KJmol-1
Indirect Route
This route consists of four steps
Solid Na changes in gaseous Na. It is an endothermic process. The enthalpy is called enthalpy of atomization
Nas → Nag ∆Hat=+108KJmol-1
Na atom loses an electron to form Na+ ion. It is an endothermic process. The enthalpy is called enthalpy of ionization.
Nag → Na+ + e- ∆Hi=+496KJmol-1
Cl2 molecule changes into Cl-atoms. It is an endothermic process. The enthalpy is called enthalpy of atomization or dissociation.
12 Cl2g → Clg ∆Hat =+121KJmol-1
The chlorine atom gains an electron to form chloride ion (Cl-). It is an exothermic process. The enthalpy is called electron affinity.
Clg + e- → Cl-g ∆He= -349KJmol-1
Na+ and Cl- ions combine and form solid NaCl. The process is called lattice formation and the enthalpy is called lattice energy.
Na+g + Cl-g → NaCls ∆Hlatt= ?
According to Hess`s law enthalpy change involved in directed route or indirect route is same.
∆Hf = (∆Hat+ ∆Hi)+(∆Hat + ∆He)+ ∆Hlatt
for sodium for chlorine
- 411 = +108 + 496 + 121 – 349 + ∆Hlatt
∆Hlatt= -411-376 = -787KJmol-1
Unit #8 Chemical Equilibrium
Multiple Concepts Questions or Multiple Choice Questions.
Short Questions
Define pH and pOH.
pH is a measure of the concentration of hydrogen ion in a solution. It usually is calculated by taking negative log of the concentration of hydrogen ion.
pH = -log[H+]
pH=0 (Strong acids), pH = 7 (Neutral Solutions), pH=14 (Strong bases)
pOH is a measure of the concentration of hydroxide ion in a solution. It usually is calculated by taking negative log of the concentration of hydroxide ion.
pOH = -log[OH-]
When the concentration of H+ is increased in own acidic buffer then the pH of the solution increases, why?
Acidic buffer is prepared by mixing weak acid and a salt of it with strong bases with pH less than 7. Mixture of acetic acid and sodium acetate is one of the best examples of such a buffer. According to expression
pH = pKa + log [salt][acid]
When the concentration of acid is increased, the concentration of H+ ions will be increased which results in increase in pH of the buffer.
What will be the effect of change in pressure on NH3 synthesis?
N2 + 3H2 ⇌ 2NH3
Kc = [NH3]2[N2] [H2]3
Kc is inversely proportional to pressure.
Increase the pressure to decrease the volume of the reaction vessel. Four moles of the reactant combine to give two moles of the products. High pressure will shift the equilibrium position to right to give more and more ammonia while low pressure will shift the equilibrium position to the left to give less ammonia.
Derive Kc expression for the reaction.
CH3COOHl+CH3CH2OHl⇌ CH3COOCH2CH3l+H2Ol
Kc = [CH3COOCH2CH3] [H2O] [CH3COOH] [CH3CH2OH]
What is an ionic product of water? Give its value at room temperature.
The product of hydrogen ion concentration [H+] and hydroxyl ion concentration [OH-] at a particular temperature is known as ionic product of water. It is given as
Kw = [H+] [OH-]
The value of Kw at 25oC is 10-14. Since pure water is neutral in nature, [H+] ion concentration must be equal to [OH-] ion concentration.
What is basic buffer solution?
Basic buffer solution is prepared by mixing a weak base and a salt of it with a strong acid. Such solutions will give basic buffers with pH more than 7. Mixture of NH4OH and NH4Cl is one of the best examples of such a basic buffer.
Give statement of Le-Chatelier`s principle.
Le-Chatelier studied the effect of concentration, pressure and temperature on equilibria.
“This principle states that if a stress is applied to a system at equilibrium, the system acts in such a way so as to nullify, as far as possible, the effect of that stress.”
The system cannot completely cancel the effect of change, but will minimize it.
What is the effect of presence of common ion on solubility? Give example.
The presence of a common ion decreases the solubility of a slightly soluble ionic compound.
In order to explain it, consider a saturated solution of PbCrO4, which is a sparingly soluble ionic salt.
PbCrO4(aq) ⇌ Pb2+(aq) + CrO42-(aq)
Now add Na2CrO4, which is a soluble salt. CrO42- is the common ion. It combines with Pb2+ to from more insoluble PbCrO4. So equilibrium is shifted to the left to keep Ksp constant.
Explain the term buffer capacity.
The buffer capacity of a solution is the capability of a buffer to resist the change of pH. It can be measured quantitatively that how much extra acid or base, the solution can absorb before the buffer essentially destroyed.
What will be the effect on the position of equilibrium on following system if
(The T is increase, Cl is added) PCl5→ PCl3+ Cl2 ∆H = +90 KJmol-1
As it is an endothermic reaction, so the increase in temperature shifts the equilibrium to the forward direction according to Le-Chatelier`s principle.
Increase in concentration of products shifts the equilibrium to backward direction. Therefore when chlorine is added, it shifts the equilibrium to backward direction.
Solubility of glucose in water is increased by increasing T. Explain it with reason.
According to Le-Chatelier`s principle, a temperature increase favors the endothermic reactions. Solution of glucose in water is an endothermic process. Therefore, solubility of glucose in water is increased by increasing temperature.
Write two uses of buffer solution.
It is a common experience that the pH of the human blood is maintained at pH 7.35, if it goes to 7.00 or 8.00 a person may die.
They are used when a reaction is needed to be carried out under conditions that would suffer any associated change in the pH of the reaction mixture. Buffers are important in many areas of chemistry and allied sciences like molecular biology cell biology, soil sciences etc.
The change of temperature disturbs both equilibrium position and constant of a reaction. Explain it with reason.
The change of temperature disturbs both equilibrium position and constant of a reaction because change of temperature changes the energy contents of reactants and products without any change in their concentration. So we can conclude that the change of temperature disturbs both equilibrium position and constant (Kc).
How do the buffer act? Give example.
Buffer act on the principle of common ion effect. For example, the buffer consisting of NH4Cl and NH4OH can resist the change of pH and pOH, when acid or base is added from outside. When a base or OH- ions are added in it, they will react with H3O+ to give back H2O and the pH of the solution again will remain almost unchanged.
How the buffer solutions are prepared.
Acidic buffers are prepared by mixing weak acid and salt of it with strong base. Mixture of acetic acid and sodium acetate is one of the best examples of such a buffer. Basic buffers are prepared by mixing weak base and salt of it with strong acid. Mixture of NH4OH and NH4Cl is one of the best examples of such a basic buffer.
Calculate pH of 10-4 mole dm-3 solution of HCl.
HCI ionizes as: HCl → H+ + Cl-
Since HCI is a strong acid, and it is 100% dissociated. Hence 10-4 mol/dm3 of HCI produces 10-4 mol/dm3 of H+ ions. Thus;
[H+] = 10-4 mol/dm3
So, pH = - log [H+]
pH = - log [10-4]
pH = 4
pH of 10-4 mole dm-3 solution of HCl is 4.
How does catalyst affect the equilibrium constant?
A catalyst lowers the energy of activation of both forward and reverse steps by giving a new path to reaction. A catalyst does not affect equilibrium position of reaction. It increases the rates of both forward and backward reactions and this reduces the time to attain the state of equilibrium.
Write down the relationship of Kp and Kc.
Kp and Kc are related by: Kp = Kc(RT)∆n
Where, ∆n = no. of moles of products – no. of moles of reactants
R is general gas constant and T is absolute temperature at which the reaction is being carried out.
Differentiate between reversible and irreversible reactions.
Reversible reaction
The reaction in which conditions are favorable for the forward as well as for a reverse reaction to occur to a measurable extent is called as reversible reaction.
N2 + 3H2 ⇌ 2NH3
Irreversible reaction
The reaction, proceeds only in one direction i.e. forward or reverse is called irreversible reactions. For example, formation of water and production of NaOH.
Why solid ice at 0oC can be melted by applying pressure without supply of heat from outside?
When pressure is applied to the broken pieces of ice at 0oC, then according to Le- Chatelier’s principle, ice moves to that direction where its volume should decrease i.e., towards liquid water as solid ice occupies more volume than liquid water. Therefore, solid ice at 0oC can be melted by applying pressure without supply of heat from outside.
Give ways to maximize the yield of ammonia.
These conditions are:
Continual removal of ammonia after intervals.
Increase the pressure to decrease the volume of vessel.
Decreasing the temperature will shift it to the forward direction.
What is Henderson equation and for which purpose it is used?
This relationship is called Henderson’s equation. This equation is used to find the pH of a buffer solution.
State law of mass action.
It states that the rate at which the reaction proceeds is directly proportional to the product of the active masses of the reactants. For a general reaction
A + B C + D
Rate of forward reaction (R) ∝ [A] [B] or R = k [A] [B]
What is meant by percentage ionization of acids?
The percent ionization of a weak acid is the ratio of the concentration of the ionized acid to the initial acid concentration multiplied by 100. For example 1.3% of CH3COOH means out of 1000 molecules of acetic acid only 13 are dissociated into ions, when 0.1 molar solution is prepared.
%ionization = Amount of acid ionizedAmount of acid initially available x 100
How does the equilibrium constant of a reaction tell us the direction of a chemical reaction?
We know that Kc = ProductsReactants
The ratio is less than Kc. This implies that more of the product is required to attain the equilibrium, therefore, the reaction will proceed in the forward direction.
The ratio is greater than Kc. It means that the reverse reaction will occur to attain the equilibrium.
When the ratio is equal to Kc, then the reaction is at equilibrium
How NaCl can be purified by common ion effect?
In saturated brine, sodium chloride is fully ionized in the solution. Equilibrium constant expression for this process can be written as follows:
NaCl Na+ + Cl- HCl H+ + Cl-
Kc = Na+[Cl-][NaCl] Kc = H+[Cl-][HCl]
On passing HCl gas, concentration of Cl- ions is increased, therefore NaCl crystallizes out of the solution to maintain the constant value of the equilibrium constant.
What are pKa and pKb. How do they show the acidic and basic strength?
Ka is the strength of acid, while Kb is the strength of base. The values of Ka and Kb for weak acids and bases are small numbers usually expressed in exponential form. It is convenient to convert them into whole numbers by taking their negative log. Thus we obtain pKa and pKb values of acids and bases. pKa = -log Ka and pKb = -log Kb
Larger the pKa, weaker is the acid and vice versa. Similarly, larger the pKb, weaker is the base.
Discuss the effect of change in temperature on Kw.
Kw is called ionic product of water or dissociation constant of water. The value of Kw increases almost 75 times when temperature is increased from 0°C to 100°C. Anyhow, the increase in K is not regular.
Justify that chemical equilibrium is dynamic in nature.
Chemical equilibrium is dynamic in nature because, reactants change into products and products change into reactants even after equilibrium is achieved. But rate of forward and backward reactions are same.
Differentiate between equilibrium constant “Kc” and Chemical equilibrium.
Derive Kc expression for NH3 synthesis by Haber process.
Give applications of common ion effect.
It suppresses the ionization of weak acids.
It acts as a group reagent for the basic radicals.
It also finds applications in qualitative analysis and preparation of buffer.
Long Questions
Calculate the pH of a buffer solution in which 0.11 M CH3COONa and 0.09M acetic acid solutions are present. Ka for CH3COOH is 1.85 x 10-5.
N2 and H2 combine to give NH3 gas. The value of Kc in the reaction of 500°C is 6 x 10-2. Calculate the value of Kp for the reaction.
Ca(OH)2 is a sparingly soluble compound. It`s solubility product is 6.5 x 10-6. Calculate the solubility of Ca(OH)2.
The solubility of PbF2 at 25°C is 0.64 gdm-3. Calculate Ksp of PbF2.
Molecular mass of PbF2 = 245.2 gmol-1.
The solubility of CaF2 in water at 25°C is found to be 2.05 x 10-4 moldm-3. What is the value of Ksp at this temperature?
State Le-Chatelier`s principle. Describe the effect of change in temperature of a system of equilibrium by this principle.
OR State Le-Chatelier`s principle. How is this principle used to explain effect of change in concentration on a reaction at equilibrium state?
Unit #9 Solutions
Multiple Concepts Questions or Multiple Choice Questions.
Short Questions
What are conjugate solutions? Give an example.
As the mutual solubilities are limited, the liquids are only partially miscible on shaking equal volumes of water and ether, two layers are formed. Each liquid layer is a saturated solution of the other liquid. Such solutions are called conjugate solutions. The mutual solubility of these conjugate solutions is affected by temperature changes. Typical examples of such solutions is Phenol-water system.
Differentiate between hydration and hydrolysis with one example each.
Hydration
The process in which water molecules surround and interact with solute ions or molecules is called hydration. The ions which are surrounded by water molecules are called hydrated ions. Dissolving Potassium Iodide (KI) is water is an example of hydration.
Hydrolysis
The decomposition of the compound by dissolving in water, in which water itself is decomposed is called hydrolysis. The hydrolysis of salts like NH4Cl is are examples of hydrolysis. The hydrolysis of NH4Cl produced NH4OH, H+ and Cl-.
Define molarity and molality. Give their mathematical expression.
Molarity
Molarity is the number of moles of solute dissolved per dm3 of the solution. To prepare one molar solution of glucose in water, we take 180g of glucose and add sufficient water to make the total volume of 1 dm3.
Molarity (M) = Mass of SoluteMolar mass of solute ×1Volume of Solution (dm3)
Molality
Molality is the number of moles of solute in 1000g (1 kg) of the solvent. In order to prepare molal solutions, 180g of glucose is dissolved in 1000g of water.
Molality (m) = Mass of SoluteMolar mass of solute ×1Mass of Solvent in kg
Define parts per million. Give its mathematical expression.
It is defined as the number of parts (by weight or volume) of the solute per million parts (by weight or volume) of the solution. This unit is used for very low concentrations of solutions, e.g. to express the impurities of substances in water.
Parts per million (ppm) = Mass of soluteMass of solution × 106
What are colligative properties? Why these are called so?
The colligative properties are the properties of solution that depend upon number of solute and solvent molecules or ions. Following are the colligative properties of dilute solutions.
Lowering of vapor pressure
Elevation of boiling point
Depression of freezing point
Osmotic pressure
Because the colligative properties of solution depend only upon number of solute and solvent particles present in solution not upon the chemical nature of solute particle. For this reason these are called as.
What are hydrates? How they are formed.
Those substances which have some water of crystallization in them are called hydrates. For example MgCl2.6H2O. Those water molecules combine with substances as they are crystallized from aqueous solutions.
Give any two points which show the ideality of a solution.
If the forces of interaction between the molecules of different components are same as when they were in the pure state, they are ideal solutions.
Ideal solutions have zero enthalpy change as their heat of solution.
Freezing point of solvents are depressed due to presence of solutes. Justify.
“Freezing point is defined as that temperature at which it’s solid and liquid phases have same vapor pressures”.
When a non-volatile and non-electrolyte solute is added to solvent, its vapour pressure is decreased. Because of this decrease in the vapour pressure of solvents the freezing point is also decreased.
What is positive deviation from Raoult`s law.
If a graph is plotted between composition and vapour pressure of a solution shows positive deviation from Raoult’s law, the total vapour pressure curve rises to maximum. The vapour pressure of some of solutions are above the vapour pressure of either of the pure components. Ethanol-water mixture is an example of this type. It boils at 78.1oC with 4.5% water and 95.5% ethanol. 78.1oC is lower than the boiling point of alcohol (78.5oC) and water (100oC).
What is water of crystallization? Give two examples.
Those water molecules, which combine with substances as they are crystallized from aqueous solutions, are called water molecules of crystallization or water of hydration. For example,
Na2CO3.10H2O (washing soda), CaSO4.2H2O (gypsum).
Differentiate between azeotropic and zeotropic mixtures.
Azeotropic Mixture:
Azeotropic mixtures also called as non-ideal solutions are those which boil at constant temperature and distil over without change in composition at any temperature like a pure chemical compound.
For example, ethanol-water mixture.
Zeotropic mixture:
Zeotropic mixture also called as ideal solutions are those which distil with a change in composition. For example, methanol-water mixture.
The sum of mole fraction of all the components is always equal to unity for any solution. Explain with reason.
Suppose there are two components A and B making a solution. The number of mole are nA and nB respectively. Then,
XA = nAnA+ nB ; XB = nBnA+ nB
XA + XB = nAnA+ nB+ nBnA+ nB
= nB + nAnA+ nB
XA + XB = 1
Hence, the sum of mole fraction of all the components is always equal to unity for any solution.
The concentration in terms of molality is independent of temperature but molarity depends upon temperature. Explain with reason.
Molality is based upon the mass of solvent. The mass of the solvent does not vary with temperature, so molality is independent of temperature. Molarity is based on the volume of solution. Since the volume of solution varies with temperature, so molarity depends upon temperature. The molarity of the solution decreases with the increase in temperature of the solution.
Differentiate between continuous and discontinuous curves.
A graphical representation between temperature and solubility of its solution is called solubility curves. There are two types of solubility curves.
Continuous Solubility Curves: Continuous solubility curves don`t show sharp breaks anywhere. For examples solubility curves of NaCl, KCl and NaNO3 are continuous.
Discontinuous Solubility Curves: Sometimes, the solubility curves show sudden changes of solubilities and these curves called discontinuous solubility curves. The best example in this reference are Na2SO4.10H2O. CaCl2.6H2O.
Dissolution of KCl and Na2SO4 in water give neutral solution. Why?
Dissolution of KCl and Na2SO4 in water give neutral solutions. Because these salts are not hydrolyzed in water. Their positive ions K+ and Na+ are not hydrolyzed by water. Similarly, their negative ions Cl-, SO42- are not hydrolyzed. It means that the salts of strong bases and acids are not hydrolyzed by water.
2KCl+Na2SO4+ H2O → 2NaCl+ K2SO4+ H2O
Relative lowering of vapor pressure is independent of temperature. Justify it.
According to Raoult’s law, the relative lowering of vapor pressure is equal to the mole fraction of solute and is given as
x2= ∆ppo
From equation, we get that relative lowering does not depend upon the temperature but depends upon the concentration of solute.
Why NaCl and KNO3 are used to lower melting points of ice?
It is a common observation that the freezing point of solution is always lower than the freezing point of the pure solvent. The lowering in freezing point depends upon the number of solute particles (molecules or ions). A mixture of NaCl and KNO3 salts is used as a freezing mixture to lower melting points of ice. These salts dissociate in ice water. They split up into a number of ions each of which as a particle due to which the freezing point of water, i.e. the melting point of ice is lowered.
Depression in freezing point is a colligative property, Justify.
‘The colligative properties are the properties of solution that depend upon number of solute and solvent molecules or ions’. The depression in freezing point is a colligative property because the lowering in freezing point depends upon the number of solute particles (molecules or ions)
One molal solution of glucose is dilute as compared to one molar solution of glucose. Justify it.
To prepare one molal solution 180 g of glucose is dissolved in 1000 g of solvent. Whereas to prepare one molar solution 180 g of glucose is dissolved in 1 dm3 of solution. In case of molal solution, the amount of solvent is increased. Therefore, one molal solution of glucose is dilute as compared to one molar solution of glucose.
Why Beckmann`s thermometer is used to measure the depression in freezing point?
Beckmann’s thermometer is used to read temperatures up to 0.01K over a range of about 0.5K. Since, freezing point depressions are small, no more than a degree or two. Therefore, to measure a small difference in temperature Beckmann’s thermometer is used. It is more sensitive than ordinary thermometer because one degree is further divided into hundred divisions.
What is fractional crystallization?
Fractional crystallization is a technique for the separation of impurities from the chemical products.
In this technique, the impure solute is dissolved in a hot solvent in which the desired solute is less soluble than impurities. As the hot solution is cooled, the desired solute being comparatively less soluble, separates out first from the mixture, leaving impurities behind. In this way, pure desired product crystallizes out from the solution.
Sea water has 5.65 x 10-3 g of dissolved oxygen in one kg of water. Calculate the concentration of oxygen in sea water in parts per million (ppm).
ppm of oxygen in sea water = 5.65 x 10-31000 X 106 = 5.65
Define molal boiling point constant. Give one example.
Elevation of boiling point ΔTb is directly proportional to the molality of the solution.
ΔTb = Kb m
Where Kb is the ebullioscopic constant or molal boiling point constant. It is the ratio of elevation of boiling point and molality of the solution.
How will you prepare 10% w/v glucose solution in water?
10g of glucose dissolved in 100 cm3 of solution is 10% w/v solution of glucose. The quantity of solvent is not exactly known. In such solutions, the total volume of the solution is under consideration.
Na2SO4.10H2O shows discontinuous solubility curve. Give reason.
Solubility curves which show sudden changes of solubilities with change in temperature are called discontinuous solubility curves. Actually, these curves are combination of two or more solubility curves. Na2SO4.10H2O shows discontinuous curve because at the removal of water molecules, a new solid phase appears and show another solubility curve.
Justify that the boiling point of one molal urea solution is 100.52OC but the boiling point of two molal urea solutions is less than 101.04 OC.
Two molal urea solution is more concentrated than one molal solution. So the number of molecules of urea in two molal urea solution are twice the Avogadro`s number, but the number of particles which are free to move in the solution are less than twice the Avogadro`s number. So the elevation of boiling point is not doubled.
Define mole fraction in solutions by giving one example.
The mole fraction of any component in a mixture is the ratio of the number of moles of it to the total number of moles of all the components present. It is represent by X. For example, solution having 92 g of ethyl alcohol, 96 g of methyl alcohol and 90 g of water has mole fraction 0.2, 0.3 and 0.5 respectively.
One molal solution of urea is dilute as compared to one molar solution. Justify.
In one molar solution of urea we take one mole of urea as a solute and dissolve in the solvent to make the solution 1000 cm3. But in one molal solution of urea, we take one mole of urea and dissolve in 1 kg of solvent. In this way the amount of solvent is more than that in the molar solution. So, one molal solution of urea is more dilute.
Long Questions
Give statement of Raoult`s law when one component is volatile other is non-volatile. Also differentiate between ideal and non-ideal solutions.
Give graphical explanation for elevation of boiling point of a solution.
Define and classify colligative properties. How boiling point elevation is measured by Landsberger’s method?
Give statement of Raoult`s law when both components are volatile.
Define solubility curve. Explain different types of solubility curves with the help of graphs.
Describe the freezing point depression method for the determination of molecular mass of an organic compound.
Describe Beckmann`s method for the measurement of freezing point depression with the help of diagram.
Discuss two types of solutions of liquids in liquids.
The freezing point of pure camphor is 178.4oC. Find the freezing point of a solution containing 2.0 g of a non-volatile compound, having molecular mass 140, in 40 g of camphor. The molal freezing point constant of camphor is 37.7 oC kg mol-1.
Unit #10 Electrochemistry
Multiple Concepts Questions or Multiple Choice Questions.
Short Questions
Differentiate between oxidation and reduction.
Oxidation:
Oxidation is defined as the reaction in which oxygen is added or hydrogen or electron is removed.
Reduction:
Reduction is defined as the reaction in which oxygen is removed or hydrogen or electron is added.
H2S + Cl2 2HCl + S
In the above reaction, Cl2 is undergoing reduction while S is undergoing oxidation.
The standard oxidation potential of Zn is + 0.76 V and its reduction potential is - 0.76 V. Justify it.
According to the law of conservation of energy, energy can neither be created nor destroyed. Therefore, if standard reduction potential of Zn is - 0.76 V, then its potential for reverse process, i.e. standard oxidation potential will also be same but with positive sign. Thus
Zn2+ + 2e- Zn E0 = - 0.76 volts (Reduction)
Zn Zn2+ + 2e- E0 = + 0.76 volts (Oxidation)
What is the function of salt bridge in galvanic cell?
The purpose of the salt bridge is to prevent any net charge accumulation in either beaker by allowing negative ions to leave the right beaker, diffuse through the bridge and enter the left beaker.
Define Electrolytic conduction and Electrolytic cell.
Electrolytic conduction
It is the passage of electric current through electrolytes present in the fused state or in the solution form. It can also be defined as the movement of ionic charges through liquid brought by the application of electricity is called electrolytic conduction.
Electrolytic cell
The type of electrochemical cell in which electrical energy is converted into chemical energy is called electrolytic cell. For example, Nelson cell.
A porous plate or a salt bridge is not required in lead storage cell. Give reason.
A salt bridge has two main functions. It joins solution of two half cells and thus complete the circuit. It maintains electrical neutrality of the two half cells as ions can pass through it. In lead storage battery, both cathode and anode are dipped in the same solution. Therefore, excess positive or negative ions are not produced in the solution. Hence, there is no need of salt bridge.
How can we say that a voltaic cell is reversible cell?
If the external circuit is replaced by a source of electricity that opposes the voltaic cell, the electrode reactions can be reversed. Now, the external source pushes the electrons in the opposite direction and supplies energy or work to the cell so that the reverse non-spontaneous reaction occurs. Such, a cell is called a reversible cell.
What are electrode reactions of a dry cell?
The reactions in the dry alkaline cell battery are shown as follows:
Zn + 2OH- Zn(OH)2 + 2e- (anode)
2MnO2 + H2O + 2e- Mn2O3 + 2OH- (Cathode)
The overall reaction is
Zn + 2MnO2 + H2O Zn(OH)2 + Mn2O3
The voltage of the cell is 1.5V.
What is electrochemical series?
When elements are arranged in the order of their standard electrode potentials on the hydrogen scale, the resulting list is known as electrochemical series.
Explain through equations how lead storage battery is recharged?
The reactions for recharging of lead storage battery are as follows:
At anode PbSO4 + 2e- Pb + SO42- (reduction)
At cathode PbSO4 + 2H2O PbO2 + 4H+ + SO42- + 2e- (oxidation)
The overall reaction is
2PbSO4 + 2H2O PbO2 + 4H+ + 2SO42- +Pb
Differentiate between electrolytic and voltaic cell.
Give some advantages of fuel cell.
The fuel cell is operated at high temperature so that the water formed as product of the cell reaction evaporates and may be condensed and used as drinking water for astronaut.
The fuel cell produce electricity and pure water during space flights.
Fuel cell are light, portable and sources of electricity.
Many fuel cells do not produce pollutants.
Lead accumulators is a chargeable battery, justify it.
The anode and the cathode of the external source are connected to the anode and the cathode of the cell respectively. The redox reactions at the respective electrodes are then reversed. The reactions are summarized as follows:
At anode PbSO4 + 2e- Pb + SO42- (Reduction)
At cathode PbSO4 + 2H2O PbO2 + 4H+ + SO42- + 2e- (Oxidation)
The overall reaction is 2PbSO4 + 2H2O PbO2 + 4H+ + 2SO42- + Pb
Write down reactions taking place at the electrodes on the discharging of Ni+Cd cell.
The reactions are as follows:
At anode Cd + 2OH- Cd(OH)2 + 2e- (oxidation)
At cathode NiO2 + 2H2O + 2e- Ni(OH)2 + 2OH- (reduction)
The net reaction during the discharge is
Cd + NiO2 + 2H2O Cd(OH)2 + Ni(OH)2
What is the difference between metallic conduction and electrolytic conduction?
Electrolytic conduction
It is the passage of electric current through electrolytes present in the fused state or in the solution form. It can also be defined as the movement of ionic charges through liquid brought by the application of electricity is called electrolytic conduction.
Metallic Conduction
Most metals are conductors of electricity because of the relatively free movement of their electrons throughout the metallic lattice. This electronic conduction is simply called metallic conduction.
A salt bridge maintains the electrical neutrality in the cell, give reasons to support your answer.
Salt bridge is a U-shaped glass tube having a saturated solution of some strong electrolyte like KCl, K2SO4 or KNO3. Two half cells are electrically connected by salt bridge. It prevents the physical contact between the two electrolytic solutions. Thus, from negative half-cell, negative ions diffuse through the salt bridge into the positive half-cell. In this way, salt bridge maintains the two solution, electrically neutral.
Calculate the oxidation numbers of the elements underlined in the following compounds:
(i) K2MnO4 (ii) Ca(ClO3)2 (iii) K2Cr2O7 (iv) Cr2O3 (v) Na2PO4 (vi) KMnO4
Oxidation number of Mn in K2MnO4 is:
0 = 2(+1) + Mn + 4(-2)
0 = +2 + Mn – 8
-2 + 8 = Mn
+6 = Mn
Oxidation number of Cl in Ca(ClO3)2 is:
0 = +2 + 2Cl + 6(-2)
0 = 2Cl -10
+10 = 2Cl
+5 = Cl
Oxidation state of Cr in K2Cr2O7
0 = 2(+1) + 2Cr + 7(-2)
0 = 2 + 2Cr -14
12 = 2Cr
+6 = Cr
Oxidation state of Cr in Cr2O3
0 = 2Cr + 3(-2)
0 = 2Cr -6
6 = 2Cr
+3 = Cr
SHE acts as an anode when connected with copper electrode and acts as a cathode when connected with Zn electrode, give reasons in support of your answer.
The electrodes above SHE in electrochemical series have negative reduction potential. They have greater tendency to deliver the electrons to SHE.
Zn → Zn+2+2e- (oxidation at anode)
2H++2e- → H2 (reduction at cathode)
The electrodes below SHE in electrochemical series have negative oxidation potential. They accept the electrons from SHE.
H2 → 2H++2e-
Cu+2+2e- → Cu
Hence, SHE acts as an anode when connected with copper electrode and acts as a cathode when connected with Zn electrode.
Write reactions taking place at anode and cathode in silver oxide battery.
The reactions take place in silver oxide battery are
At anode:
Zn + 2OH- → Zn(OH)2 + 2e- (Oxidation)
At cathode:
Ag2O + H2O 2e- → 2Ag + 2OH- (Reduction)
Overall reaction:
Zn + Ag2O + H2O → Zn(OH)2 + 2Ag
How aluminum anodized in an electrolytic cell?
Anodized aluminum is prepared by making it an anode in electrolytic cell containing dilute sulphuric acid or chromic acid, which coats a thin layer of oxide on it. The aluminum oxide layer resists attack of corrosive agents. The freshly anodized aluminum is hydrated and can absorb dyes.
What are secondary cells? Name any two such cells.
Those cells which can be recharged are called secondary cells.
Examples: Lead-acid battery, Ni-Cd cattery, fuel cells.
How impure copper can be purified?
Electrolytic cell can also be used for the purification of copper. Impure copper is made the anode and a thin sheet of pure copper is made the cathode. Copper sulphate solution is used as an electrolyte. The atoms of Cu from impure Cu-anode are converted to Cu2+ ions and migrate to cathode which is made up of pure Cu. In this way Cu anode is purified. Impurities are left at anode.
What is standard hydrogen electrode?
It consists of a piece of platinum foil, which is coated electrolytically with finely divided platinum black, to give it a large surface area and suspended in one molar solution of HCl. Pure hydrogen gas at one atmosphere pressure is continuously bubbled into 1M HCl solution. The platinum acts as an electrical conductor and also facilitates the attainment of equilibrium between the gas and its ions in solution. The potential of this electrode is arbitrarily taken as zero.
Na & K can displace hydrogen from acids but Pt, Pd and Cu cannot. Why?
Greater the value of standard reduction potential of a metal, lesser is its tendency to lose electrons to form metal ions and so weaker is its tendency to displace H+ ions from acids as H2 gas. For example, metals like Au, Pt, Ag and Cu which have sufficiently high positive values of reduction potentials, do not liberate hydrogen gas from acids. While, metals like Na, K, Zn, Mg and Ca which are close to the top of the series and have very low reduction potentials, liberate hydrogen gas, when they react with acids.
Define electrode potential.
The potential set up when an electrode is in contact with one molar solution of its own ions at 298K is known as standard electrode potential. It is represented by Eo. Standard electrode potential of hydrogen has zero value.
Long Questions
What is standard electrode potential? Explain the measurement of electrode potential.
Define electrode potential. Describe the construction of voltaic cell and reaction occurring in the cell.
Describe the construction and working of standard hydrogen electrode.
What is electrochemical series? Give its four applications.
Describe the electrolysis of molten NaCl and a concentrated solution of NaCl.
Describe Ni+Cd cell (rechargeable).
Describe the electrolysis of concentrated solution of NaNO3 in aqueous solution.
What is lead accumulator battery? Discuss its discharging process.
Define oxidation number. Also write rules for assigning oxidation number.
How electrochemical series is helpful in the prediction of feasibility of chemical reaction and relative chemical reactivity of metals?
Balance the following equations using oxidation number method.
K2Cr2O7 + HCl KCl + CrCl3 + Cl2 + H2O
Cu + HNO3 Cu(NO3) + NO + H2O
MnO2 + HCl MnCl2 + H2O + Cl2
These steps are followed to balance redox equations using the oxidation number method.
Write down the skeleton equation of the redox reaction under consideration.
Identify the elements, which undergo a change in their oxidation number during the reaction. When an element exhibits both a change in oxidation number and no change, the substance containing that element should be written separately for each case. One instance represents the unchanged oxidation state, while the other accounts for the change in oxidation number.
Record the oxidation number above the symbols of the element, which have undergone a change in the oxidation number. Indicate the change in oxidation number by arrows joining the atoms on both sides of the equation. It shows number of electrons gained or lost.
Equate the increase or decrease in the oxidation number, i.e. electrons gained or lost by multiplying with a suitable digit.
Balance the rest of the equation by inspection method
This is the final balanced equation.
Unit #11 Reaction Kinetics
Multiple Concepts Questions or Multiple Choice Questions.
Short Questions
What is catalysis? Give an example.
A process, which takes place in the presence of a catalyst, is called catalysis. For example, the reaction between H2 and O2 to form water is very slow at ordinary temperature. But it proceeds more rapidly in the presence of platinum. This reaction is called as catalysis.
Differentiate between average and instantaneous rate of reaction.
At first instantaneous rate is higher than the average rate. At the end of the interval the instantaneous rate becomes lower than the average rate. The average rate becomes equal to instantaneous rate when time interval approaches to zero.
A catalyst is specific in its action, justify.
When a particular catalyst works for one reaction, it may not necessarily work for any other reaction. If different catalysts are used for same reactant then the products may change. For example,
Formic acid is decomposed by Al2O3 to H2O and CO while Cu causes its decomposition to H2 and CO2.
HCOOH H2O + CO HCOOH H2 + CO2
A finely divided catalyst may prove more effective. Give reason.
A catalyst is more effective, when it is present in a finely divided form. For example, a lump of platinum will have much less catalytic activity than colloidal platinum. In hydrogenation of vegetable oils finely divided nickel is used. It is because it provides larger surface area and more active sites in finely divided form.
Radioactive decay is always a first order reaction, justify.
Radioactive decay is always considered a first-order reaction because its half-life is constant and independent of the initial amount of radioactive substance. Since the half-life of a first order reaction is constant and is independent of initial concentration of the reaction, so the radioactive decay is always a first order reaction.
How does a catalyst affect the reversible chemical reaction?
A catalyst cannot affect the equilibrium constant of a reaction but it helps the equilibrium to be established earlier. The rates of forward and backwards steps are increased equally.
What is half-life period? What is half-life method for the determination of order of a reaction?
Half-life period of a reaction is the time required to convert 50% of the reactants into products. For example, the half-life period for decomposition of N2O5 at 45°C is 24 minutes.
Half-life of a reaction is inversely proportional to the initial concentration of reactants raised to the power one less than the order of reaction. If we know the two initial concentrations and two half-life values, we can calculate the order of reaction using the equation,
n=1+ log[t1t2]log[a2a1]
What are enzymes? Give two examples in which enzymes act as catalyst.
Enzymes are the complex protein molecules and catalyze the organic reactions in the living cells.
Urea undergoes hydrolysis into NH3 and CO2 in the presence of enzyme urease, urease is present in soya bean.
Glucose is converted into ethanol by enzyme zymase present in yeast.
Define activation energy and activated complex. What is the effect of temperature on the activation energy of a reaction?
The minimum amount of energy required for an effective collision is called activation energy.
Activated complex is an unstable combination of all the atoms involved in the reaction for which the energy is maximum. It is a short-lived species and decomposes into product immediately. It has a transient existence that is why it is also called a transition state.
Effect of temperature
The temperature does not affect the activation energy of a reaction. But the increase in temperature increases the rate of reaction by increasing the kinetic energy of reacting substances.
Define specific rate constant. Give equation to support your answer.
The specific rate constant of a chemical reaction is the rate of reaction when the concentration of the reactants is unity. It is also called as velocity constant.
By applying law of mass action to a general reaction.
aA + bB → cC + dD
Rate of reaction = k [A]a [B]b Where, k is specific rate constant
Let [A] = 1 mol dm-3 and [B] = 1 mol dm-3
Rate of reaction = k × 1a × 1b = k
What happens to the rate of chemical reaction with the passage of time?
The reaction rate decreases with time. It never remains uniform during different time periods. It decreases continuously till the reaction ceases.
Define order of reaction and give an example.
The order of reaction is given by the sum of all the exponents to which the concentrations in the rate equation are raised. The order of reaction may also be defined as the number of reacting molecules, whose concentration alters as a result of the chemical change.
For example, decomposition of nitrogen pentoxide is first order with respect to the concentration of N2O5.
Define autocatalysis, give equation to support your answer.
In some of the reactions, a product formed acts as a catalyst. This phenomenon is known as auto-catalysis. For example, the reaction of oxalic acid with acidified KMnO4 is slow at the beginning, but after sometime, MnSO4 produced in the reaction makes it faster.
2KMnO4+3H2SO4+5(COOH)2 Mn2+ → K2SO4+2MnSO4+10CO2+8H2O
Justify that rate of chemical reaction is an ever-changing parameter under the given condition.
Rate of reaction is the rate of change of concentration of reactants or products with respect to the time. Thus, concentration of reactants and products change throughout the reaction, so rate of reaction is ever changing parameter under the given conditions.
What is meant by homogeneous catalysis? Give one example.
In this process, the catalyst and the reactants are in the same phase and the reacting system
is homogeneous throughout. The catalyst is distributed uniformly throughout the system. For example: Esters are hydrolysed in the presence of H2SO4. Both the reactants and the catalyst are in the solution state.
What do you mean by rate determining step? Give example.
If a reaction occurs in several steps, one of the steps is the slowest. The rate of this step determines the overall rate of reaction. This slowest step is called the rate determining or rate limiting step.
The first step is the rate determining step.
Define zero order reaction. Give an example.
A reaction is said to be zero order if it is entirely independent of the concentration of reactant molecules. Photochemical reactions are usually zero order.
What do you mean by the term activation of a catalyst? Give example.
Such a substance which promotes the activity of a catalyst is called a promotor or activator. It is also called “catalyst for a catalyst”. For example: Hydrogenation of vegetable oils is accelerated by nickel. The catalytic activity of nickel can be increased by using copper and tellurium.
How surface area of reactants affect the rate of reaction?
The increased surface area of reactants, increases the possibilities of atoms and molecules of reactants to come in contact with each other and the rates enhance. For example, Al foil reacts with NaOH moderately when warmed, but powdered Al reacts rapidly with cold NaOH and H2 is evolved with frothing.
Long Questions
Explain the effect of temperature/Nature of reactants/surface area on the rate of reaction.
Define order of reaction. Describe it with three examples.
Distinguish between homogeneous and heterogeneous catalysis with suitable examples.
Name the various methods for finding order of reaction. Explain half-life method and method of large excess for finding the order of reaction.
What is enzyme catalysis? Give one example. Write any four characteristics of enzyme catalysis.
What is Arrhenius equation? How it can be used to calculate energy of a reaction?
Explain energy of activation.
Define the following with examples. (i) Autocatalysis (ii) Negative Catalysis
(iii) Homogeneous Catalysis (iv) Enzyme Catalysis
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