THE LAWS OF THERMODYNAMICS DESCRIBE HOW THE ENERGY IN A SYSTEM CHANGES AND WHETHER THE SYSTEM CAN PERFORM USEFUL WORK ON IT'S SURROUNDINGS. THERMODYNAMICS IS USED TO ANALYZE THERMAL SYSTEMS SUCH AS HEATING AND AIR-CONDITIONING SYSTEMS, REFRIGERATORS, AND WATER HEATERS.

1. CLASS 11 CHEMISTRY
CHEMICAL THERMODYNAMICS
PART 3

2. 2
Q. State the various statements of second law of
thermodynamics.
A. The second law of thermodynamics can be stated in many
different forms:
1. Planck’s statement of the second law: It is impossible to
construct a machine operating in cycles that will convert heat into
work without producing any other changes in the surroundings.
2. Clausius or Kelvin statement of the second law: It is
impossible to construct a heat engine which will continuously
abstract heat from a single body and convert the whole of it to
work without leaving changes in the working system.
Contd.

3. 3
3. All spontaneous process are thermodynamically irreversible.
4. Without the help of an external agency, a spontaneous process
cannot be reversed., e.g. heat cannot by itself flow from a colder
body to hot body.
5. There exists a function ‘S’ Called entropy which is a state
function. The entropy of the universe remains constant in a
reversible process but it increases in an irreversible process, i.e.
entropy of the universe tends to increase.

4. 4
Q. What are the limitations of second law of
thermodynamics?
A. The limitations of the Second Law Thermodynamics are:
1. The second law of thermodynamics only deals with the
irreversibility of heat conversion to another form.
2. The second law of thermodynamics only deals with
closed systems.

5. 5
Q. What is the significance of the second law of
thermodynamics?
A. The second law of thermodynamics helps us to
determine the direction in which energy can be
transformed. It also heps us to to predict whether a
given process or chemical reaction can occur
spontaneously or not.

6. 6
Q. What is entropy?
A. Entropy is a Greek word with no equivalent in common
language. Entropy stands for ‘trope’ [a Greek word
meaning change], the suffix ‘en’ is used to identify it with
energy.
Entropy is defined as: ‘the property of a substance
which measures the disorder or randomness of a
system.’

7. 7
Q. Why is entropy a state property?
A. Quantities that are state properties do not depend on
the path by which the system arrived at its present state.
Therefore, the most useful way to quantify a state
property is to measure its change.
Entropy is a property of the state of a system. The
change in entropy in going from initial to final state is
independent of the path taken. The entropy change will
be the same for both reversible and irreversible
processes linking the two states.

8. 8
Q. What is the physical significance of entropy?
A. Entropy is the measure of disorderedness because
because spontaneous processes are accompanied by
increase in entropy as well as increase in the order of
the system. Thus, increase in entropy implies increase in
disorder. For a spontaneous process, entropy change is
positive and if it is zero, the system remains in a state of
equilibrium.

9. 9
Q. What are the properties of entropy?
A. The properties of entropy are as follows:
1. It is a thermodynamic function.
2. It is a state function. It depends on the state of the system
a not the path that is followed.
3. Entropy is an extensive property which means it scales
with the size or extent of a system.
4. Its unit in S.I units J K-1
mole-1
, its unit in C.G.S unit is cal K-
1
mole-1
.
5. It is represented by ‘S’.

10. 10
6. Value of ΔS is zero for a cyclic or reversible
process.
7. Total entropy of the universe remains unaltered in
a reversible process but increases in an irreversible
process [ΔS > 0].
8. Entropy of the system increases up to equilibrium
state and attains maximum value after the
establishment of equilibrium.
9. The value of entropy depends on the independent
variables used to define the state of the system.

11. 11
Q. What is ‘Entropy Change’?
A. Entropy is a state function like enthalpy and internal energy.
So, it depends upon the initial and final states of a system. Thus,
entropy change, ΔS can be mathematically represented as:
ΔS = S[final state] – S[initial state], when a system undergoes a change
from initial to final state.
For any chemical process, ΔS = S[products] – S[reactants]
For a reversible process, ΔS = qrev / T at equilibrium, where qrev is
the amount of heat supplied at temperature T in a reversible
process.

12. 12
Q. What do you mean by total entropy change?
A. Total entropy change is equal to the sum of the entropy change of
the system and surroundings. Total entropy change, Δstotal = Δssystem +
Δssurroundings
If the system loses an amount of heat q at a temperature T1, which is
received by surroundings at a temperature T2, then total entropy Δstotal
can be calculated:
Δssystem = -q/T1
Δssurroundings = q/T2
Δstotal = -q/T1 + q/T2
Contd.

13. 13
1.If Δstotal is positive, the process is spontaneous.
2.If Δstotal is negative, the process is non-spontaneous.
3.If Δstotal is zero, the process is at equilibrium.
Q. What is entropy of phase transition?
A. The change of one state[solid, liquid, gas] to another is
called phase transition. It occurs at definite temperatures
such as melting point [solid to liquid], boiling point [liquid to
vapour], etc. Phase transitions are accompanied by
absorption or evolution of heat. The entropy change for
these transitions may be calculated as: ΔS = q/T, where q is
the heat evolved or absorbed during transition and T is the
temperature.

14. 14
Q. What is entropy of fusion?
A. When a solid melts, the change in entropy is given by:
ΔSfusion = Sliquid – Ssolid = ΔHfusion/T
Where, ΔHfusion = The enthalpy of fusion
Tfusion = Absolute temperature at which solid melts
Sliquid = Entropy of liquid
Ssolid = Entropy of solid
Since, ΔHfusion is always positive, therefore, ΔSfusion is
positive. Hence, Sliquid > Ssolid.

15. 15
Q. What is entropy of vaporisation?
A. One mole of a liquid can be changed into vapours at its boiling
point [Tb] by supplying latent heat of vaporization. The change in
entropy of vaporization, ΔSv is given by:
ΔSv = ΔHv / Tv, where, ΔSv = Entropy of vaporisation per mole, Tv =
boiling point in K.
For water, ΔHv = 40.73 kJ mol-1
, Tv = 373k
Therefore, ΔSv = 40730/373 = 109JK-1
mol-1

16. 16
Q. What is entropy of sublimation?
A. It is the entropy change when one mole of solid
changes into vapour at a particular temperature. For
example, for the reaction, I2[s] → I2[g], the entropy change
is given as: ΔSsub = ΔHsub / T
ΔS[sub] = SI2[g] – SI2[s],
ΔHsub = Enthalpy of sublimation at the temperature T in
Kelvin,
ΔHsub = ΔHfus + ΔHvap

17. 17
Q. What is entropy change for change of one
crystalline form to another?
A. The change in entropy when 1 mole of a solid
changes from one crystalline form to another is given by
ΔS = ΔHt / T, where ΔHt is the molar heat of transition
of the substance and T is the transition temperature.
Molar heat of transition is the amount of heat absorbed
or evolved by one mole of a substance when it
undergoes change of state from one crystalline form to
another at the transition temperature.

18. 18
Q. What is entropy of universe?
A. An isolated system includes the system under
investigation and surroundings. The entropy change for
an isolated system includes the entropy change in the
system as well as surroundings and the total entropy
change is called the entropy of the universe. ΔSuniverse =
ΔSsystem + ΔSsurroundings
ΔSuniverse is zero for reversible process.
ΔSuniverse is grater than zero for irreversible process.

19. 19
Q. What is entropy change in adiabatic process?
A. No heat enters or leaves in an adiabatic process.
Therefore, qrev=0 and hence entropy change ΔS= qrev/T is
also zero.
Thus, there is no entropy change in an adiabatic process.
A process in which there is no change in entropy is said to
be isoentropic.

20. 20
Q. What happens to the entropy when an egg is boiled
hard?
A. On hard boiling of an egg, entropy increases due to
denaturation of egg protein which results in change of
structure of protein from helical form [more ordered] to
random coiled form [less ordered].
Q. What happens to the entropy when the rubber band
is stretched?
A. On stretching a rubber band, the long flexible
macromolecules get uncoiled. These uncoiled molecules
are arranged in a more specific manner resulting in
discrease in disorder or entropy.

21. 21
Q. Why diamond has lower entropy than graphite?
A. In diamond, all the C-atoms are linked to form a
network structure. This results in less disorder or
entropy. On the other hand, graphite has more disorder
or entropy due to the presence of free electrons and
slipping of layers over each other.
Q. Why does entropy of a solid increase on fusion?
A. In a solid, constituent particles have fixed positions.
On melting, they fall apart and are free to move. This
results in increase in randomness or entropy.

22. 22
Q. State the second law of thermodynamics in terms
of entropy.
A. Naturally occurring processes are accompanied by
increase in entropy. Hence, entropy of the universe is
continuously increasing.
Q. What is meant by thermal death of the universe?
A. Since all natural processes are accompanied by
increase in entropy, the universe is slowly drifting
towards the state of maximum entropy, all production of
useful work will cease and so life would come to an end.

23. 23
Q. What are the application of second law of
thermodynamics?
A. The application of second law of thermodynamics are:
1. The second law of thermodynamics states that heat
always move from a hot body to a cold body. All heat
engine cycles, including Otto, Diesel etc., as well as all
working fluids employed in the engines, are covered in
this rule.
2. Refrigerators
3. Heat Pumps

24. 24
Q. What are spontaneous processes?
A. A process which under some conditions may take place
by itself or by initiation, independent of the rate is called
spontaneous process.
A spontaneous process is simply a process which is
feasible.
The rate of the process may vary from extremely slow to
extremely fast.

25. 25
Q. Give examples of spontaneous process.
A. Examples of spontaneous processes which take
place by themselves:
1. Dissolution of common salt in water.
2. Evaporation of water in an open vessel.
3. Flow of heat from hot end to cold end or from a hot
body to cold body.
4. Flow of water down a hill.
Contd.

26. 26
Examples of spontaneous processes which take
place on initiation:
1. Lightning of candle involving burning of wax.
2. Heating of calcium carbonate to give calcium oxide
and carbon dioxide.
CaCO3[s] → CaO[s] + CO2[g]
3. Combination of hydrogen and oxygen to form water
when initiated by passing an electric spark.
H2[g] + ½ O2 [g] → H2O[g]

27. 27
Q. What are non-spontaneous processes? Give
examples.
A. A process which can neither take place by itself nor by
initiation is called a non-spontaneous process. For example:
1. Flow of water up a hill.
2. Flow of heat from cold body to a hot body.
3. Diffusion of gas from low pressure to high pressure.
4. Dissolution of sand in water.

28. 28
Q. What is meant by free energy?
A. Free energy refers to the amount of internal energy of
a thermodynamic system that is available to perform
work.
The free energy of a system is a measure of its capacity
to do useful work. It is a part of the energy of a system
which is free for conversion to useful work and is
therefore called free energy.

29. 29
Q. What is Gibb’s free energy?
A. Gibb’s free energy is that thermodynamic quantity of a
system, the decrease in whose value during a process is equal
to the maximum possible useful work that can be obtained from
the system.
Gibb’s free energy is the energy available in a substance to do
work. However, this work does not involve mechanical work,
meaning the substance does not expand or contract to push on
something. It refers to the ‘chemical work’ involved in chemical
reactions. One could think of chemical work as the energy
involved in transforming one chemical to another. Gibb’s free
energy is a chemical potential energy in a substance.

30. 30
Q. What is the significance of Gibb’s free energy?
A. Gibb’s free energy can be used to determine if a reaction
will be spontaneous, non-spontaneous, or at equilibrium.
When Gibb’s free energy is negative, the reaction is
spontaneous. When Gibb’s free energy is positive, the
reaction is not spontaneous. When Gibb’s free energy is zero,
the reaction is in equilibrium.
On earth, objects always want to fall and reduce their potential
energy. In the world of chemical transformations, chemicals
always want to minimize their Gibb’s free energy. What this
means is that chemicals chemicals will tend to transform to
other states or chemicals that have less Gibb’s free energy.

31. 31
Q. How free energy is related to enthalpy and entropy?
A. The equation for Gibb’s free energy shows the free
energy of a system [G] at any moment. It is defined as the
enthalpy [H] of the system minus the product of the
temperature [T] times the entropy of the system [S]. Gibb’s
free energy is usually expressed in kilojoules per mole
[kJmol-1
].
G = H - TS
Enthalpy and entropy are both thermodynamic properties
of the system.
Contd.

32. 32
Enthalpy is defined as the sum of the internal energy
[E] of the system plus the product of the pressure [P]
and volume [V], H = E + PV
Entropy is the molecular disorder or randomness of
the system. It measures the thermal energy per unit
of temperature that is unavailable for doing useful
work.

33. 33
Q. Discuss the effect of temperature on free energy for
exothermic reaction.
A. The conditions will depend upon the thermodynamic
equation ΔG = ΔH – TΔS. Here, G is the free energy, H is the
enthalpy, T is the temperature and S is the entropy. If a
reaction is exothermic [H is negative] and the entropy S is
positive [more disorder], the free energy change is always
negative. On increasing the temperature, whether Gibb’s free
energy will increase or decrease, will depend upon the entropy
of the given reaction. If the value of ΔS is positive, then the
value of -TΔS will become more negative. On increasing the
temperature, the value of free energy becomes very small.

34. 34
Q. What is Gibbs-Helmholtz equation?
A. The Gibbs-Helmholtz equation is a thermodynamic equation used to
calculate changes in the Gibb’s free energy of a system as a function of
temperature. It relates the free energy change to the enthalpy and
entropy changes of the process as: ΔG = ΔH -TΔS, the equation
was named after Herman von Helmholtz and Josiah Williard Gibbs.
Q. Name two factors which favour a spontaneous reaction.
A. Enthalpy is the total heat content of the system. Entropy is the
measurement of randomness of the system. Change in enthalpy and
change in entropy should be positive for a reaction to be spontaneous.

35. 35
Q. How can the spontaneity of a process can be
predicted on the basis of Gibb’s Helmholtz equation?
A. According to Gibb’s Helmholtz equatIon,
ΔG = ΔH-TΔS, this equation includes both the factors,
i.e., the energy factor, ΔH and the entropy factor,TΔS
which decide the spontaneity of a process.
Thus ΔG is the resultant of the energy factor, i.e.,
tendency for minimum energy and the entropy factor, i.e.,
the tendency for maximum randomness.
Contd.

36. 36
Depending upon the signs of ΔH and TΔS and their
relative magnitudes, the following different possibilities
arise:
1. When ΔH is negative but TΔS is positive i.e.,
energy factor as well as randomness factor favour
the process:
A. The process will be highly spontaneous and ΔG will
be highly negative at all temperatures.
Contd.

37. 37
2. When both ΔH and TΔS are negative i.e. when
energy factor favours the process but
randomness factor oppose it. Then,
A. If ΔH > TΔS, the ΔG is negative and the process is
spontaneous.
B. If ΔH < TΔS, the ΔG is positive and the process is
non-spontaneous.
C. If ΔH = TΔS, the ΔG is zero and the process is in
equilibrium.
Contd.

38. 38
3. When both ΔH and TΔS are positive i.e when
energy factor opposes the process but
randomness factor favours it. Then,
A. If ΔH > TΔS, the ΔG is positive and the process is
non-spontaneous.
B. If ΔH < TΔS, the ΔG is negative and the process is
spontaneous.
C. If ΔH = TΔS, the ΔG is zero and the process is in
equlibrium.
Contd.

39. 39
4. When ΔH is positive and TΔS is negative i.e.
when energy factor as well as randomness factor
opposes the process, ΔG will be highly positive and
the process will be non-spontaneous.
To sum up, if [ΔG]T,P < 0, the process is spontaneous.
if [ΔG]T,P = 0, the process is in equilibrium state.
if [ΔG]T,P > 0, the process is non-spontaneous.
Hence, only that process can occur spontaneously
which results in decrease in free energy, G.

40. 40
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