IGNOU| PHYSICAL CHEMISTRY (CHE - 04)| SOLVED PAPER – (DEC - 2022)| (B. Sc.)| ENGLISH MEDIUM
BACHELOR OF SCIENCE (B. Sc.)
Term-End Examination
December - 2022
CHE-04
PHYSICAL CHEMISTRY
Time: 2 Hours
Maximum Marks: 50
Note: Attempt all the Parts. Answer five questions
from each of the Parts A, B, C and D. Use of Log tables and Non-programmable
calculators is allowed.
Part-A
1 Each
Note: Attempt any five questions.
1. What are azeotropes?
Ans:- Azeotropes are mixtures of two or more liquids
that boil at a constant temperature and have the same composition in both the
liquid and vapor phases. These are also called continuously boiling liquids.
Azeotropes are
mixtures of two or more components in liquid state whose proportions cannot be
changed by simple distillation. This occurs because when the azeotrope is
boiled, the proportions of the components in the vapor are the same as in the
unboiled mixture.
2. What is the number of net atoms in a face centered
cubic unit cell?
Ans:- A face-centered cubic (FCC) unit cell contains a
total of four atoms:
(i) Atoms at
the corners: There are atoms at the corners of every unit cell.
(ii) Atoms at
the center of each face: Every unit cell has atoms at the center of each
face.
A unit cell is a
repeating unit formed by vectors spanning the points of a lattice. Atoms at the
corners, edges, and faces of a unit cell are shared by multiple unit cells.
In comparison, a
simple cubic unit cell has one atom per unit cell, and a body-centered cubic
unit cell has two atoms per unit cell.
The atoms
present in a face centered cubic unit cell are:-
Face centered cubic unit
cell:-
(i) When the
constituent particle is present at the center of each face, it is known as face
centered unit cell.
(ii) The
coordination number of face-centred cubic (FCC) is 12.
(iii) One eighth
of an atom at each of the eight corners i.e. 8x1/8=1 atoms from the corners.
(iv) Half of one
atom on each of the six faces i.e. 6x1/2=3 atoms from the face.
Hence, a face
centered cubic unit cell contains a total of 4 atoms.
3. What is the physical significance of decrease in Gibbs
free energy?
Ans:- The physical significance of the reduction in
Gibbs free energy is that it indicates whether a process is spontaneous or not.
The changes in Gibbs free energy are negative, meaning that the Gibbs free
energy decreases. In spontaneous transformation the Gibbs energy always
decreases and never increases.
Other physical
significance of the reduction in Gibbs free energy include:-
(i) Maximum
non-PV work: Minimizing the Gibb's work at constant temperature and
pressure in a process gives an estimate of the maximum non-PV work (useful
work) that can be obtained from the system in a reversible manner.
(ii) The
reaction can proceed: If the free energy of the system decreases, the
reaction can proceed and work can be done.
(iii) Chemical
equilibrium: Once the free energy reaches its lowest possible value, all
net changes stop, which represents a state of chemical equilibrium.
(iv)
Exothermic reaction: Decrease in free energy of the reacting system
indicates an exothermic reaction.
4. What will be the effect of adding 0.1 M KCl on the C.
S. T. of phenol-water system?
Ans:- By adding 0.1 M KCl to phenol-water mixture, it
will dissolve only in water. The critical solution temperature (CST) of the
phenol-water system is the maximum miscibility temperature. The CST of pure
phenol-water system is 67.5 °C.
Adding KCl to
the phenol-water system shows less increase in CST as it reduces the
miscibility to some extent. Adding salt can reduce the miscibility of phenol
and water, which causes phase separation. The water molecules will bind with
the salt ions and hydrate them.
The critical
temperature is affected by the presence of impurities inside the system. If the
impurity present or added is soluble in both liquids, the CST is reduced.
It was found
that the addition of ionic compound KCl to the phenol-water system results in
less increase in CST as they reduce the miscibility to some extent.
5. Calculate the concentration of H+ ions of a
solution whose pH is 4.
Ans:- This means, for example, that a solution with a
pH of 4 has a hydrogen-ion concentration of 10-4mol/l, which means that there
are 0.0001 mol of hydrogen ions in 1 liter of solution.
6. State Stark-Einstein's law of photochemical
equivalence.
Ans:- The Stark–Einstein law of photochemical
equivalence states that for each photon of light absorbed by a chemical system,
only one molecule is activated for the subsequent reaction. This
"photoequivalence law" was derived by Albert Einstein during the
development of the quantum (photon) theory of light.
The law of
photochemical equivalence states that:-
(i) In a
photochemical process each molecule activated by light absorbs a quantity of radiation
which causes activation
(ii) One mole
of reactant will absorb equivalent quanta of light.
(iii) Each
photon absorbed will cause an (elementary) chemical or physical reaction
Photochemical
reactions are effected by the absorption of radiation in the visible and
ultraviolet regions of wavelengths. Examples of photochemical reactions
include:
(i)
Photosynthesis
(ii)
Production of Vitamin D by humans due to exposure to sunlight
(iii)
Bioluminescence
7. Give an example of a solid in a gas type of colloid.
Ans:- The solid present in the gas type colloid is
called aerosol. An aerosol is a colloidal system of solid or liquid particles
in a gas. For example, smoke or fog.
Here are some
examples of solid types of colloids in gas:-
Smoke,
automobile exhaust, dust storm, fog, mist, forest exhaust, geyser steam.
Other
examples of colloids include:-
Whipped cream,
shaving cream, beaten egg whites, colored glass, gems, alloys, paint, ink, egg
white, clay, curd, pudding, cheese.
Part-B
2 Each
Note: Attempt any five questions.
8. Deduce the dimensions of gas constant R.
Ans:- Therefore, the gas constant is dimensionally
represented as [M1 L2 T-2 K-1].
9. Calculate the standard entropy change of the
combustion of methane at 298.15 K.
You can use
of the following data:
Ans:- According to the third law of thermodynamics,
the entropy of a closed system will never decrease with time. Rather, entropy
starts increasing. This applies to the entire universe as well.
The combustion
of ethane in oxygen produces carbon dioxide and water according to the balanced
chemical reaction:
The change in
entropy for a reaction is given by the standard entropy of formation of each
species according to the following equation:
Where m and n
are the coefficients of the balanced reaction equation.
From the
literature, the entropy of formation of each of these molecules at 298.15 K is:
From these we
find the standard state entropy change as:
10. What is an extensive property? Give two examples.
Ans:- Extensive properties are thermodynamic
properties of a system that depend on the mass of the system. For example, if
the value of a property of a system is equal to the sum of the values of the
system's parts, it is called a comprehensive property.
An extensive
property is a property that depends on the amount of substance in a sample.
Mass and volume are examples of extensive properties.
Here are some
examples of comprehensive assets:-
Mass, volume,
internal energy, enthalpy, entropy, total charge.
An intensive
property is a thermodynamic property that does not depend on the mass of a
system. For example, viscosity is an intrinsic property because it does not
change when the amount of substance changes.
Other
examples of intensive properties include:-
Temperature,
molar heat capacity, density, solubility, color, brightness, freezing point,
ductility.
11. The molar conductance at infinite dilution for sodium
acetate, hydrochloric acid and sodium chloride are 0.009101, 0.04261 and
0.01264 Sm2 mol-1 Calculate the molar respectively.
conductance at infinite dilution for acetic acid.
12. State any two differences between a chemical reaction
and a photochemical reaction.
Ans:- Here are some differences between chemical
reactions and photochemical reactions:-
(i) Chemical
reactions: The formation and breaking of chemical bonds between atoms
causes change in the position of electrons. Only atoms present in the reactants
can be converted into products.
(ii)
Photochemical reactions: These are chemical reactions which occur in the
presence of light. Molecules gain energy by absorbing light, which causes the
molecules to move into a more excited state. Photochemical reactions depend on
the number of protons that are known to activate these reactions.
Here are some
examples of chemical reactions and photochemical reactions:-
(i) Chemical
reaction: Ultraviolet light reacts with oxygen molecules in the
stratosphere to produce atomic oxygen.
(ii)
Photochemical reaction: Silver chloride is a silver salt which reacts with
light and decomposes into metallic silver and chlorine gas.
Photochemical
reaction is a form of chemical reaction in which the reactants receive energy
in the form of photons while thermal reaction is a form of chemical reaction in
which the reactants receive energy in the form of heat. This is the key
difference between photochemical and thermal reaction.
13. In the formation of ammonia:
N2
(g) + 3H2 (g) → 2NH3 (g)
the rate of
consumption of hydrogen at a particular instant is 4.78 × 10-4 Ms-1.
What is the rate of formation of ammonia at that instant?
14. Why does a mixture of two immiscible liquids boil at
a temperature which is lower than the boiling point of any of the pure liquids?
Ans:- A stirred mixture of immiscible liquids will
boil at a temperature lower than the boiling point of any pure liquid. Their
combined vapor pressure is forced to reach the external pressure before the
vapor pressure of any individual component gets there.
The vapor
pressure of a mixture of two immiscible liquids is the sum of their vapor
pressures in their pure state. It is independent of their relative quantities.
Therefore, the boiling point of the mixture is lower than the boiling point of
any liquid, and remains constant throughout.
Insoluble liquids
are nothing but those liquids which will not mix to give one phase. Water and
oil are examples of immiscible liquids. One liquid floats on top of another
liquid when mixed together.
We know that the
vapor pressure of a mixture of two immiscible liquids is equal to the sum of
the vapor pressures of the liquids in their pure state.
Vapor pressure is
independent of their relative amounts present in the mixture.
So, the boiling
point of a mixture of liquids is lower than the boiling point of a single
liquid.
Therefore a
mixture of two immiscible liquids at a constant pressure of 1.0 atm boils at a
temperature lower than the normal boiling point of either liquid.
As we know that a
solute (non-volatile substance) lowers the vapor pressure of the liquid, and
hence the boiling point of the solution becomes higher.
So here, there is
no solute in the mixture of two insoluble liquids. Therefore the mixture will
boil at a temperature lower than the boiling point of any liquid.
Part-C
3 Each
Note: Attempt any five questions.
15. Which one will have higher boiling point- ethanol or
dimethyl ether? Give reasons for your answer.
[COMING SOON]
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