1) Entropy is a measure of the disorder or randomness in a molecular system. It increases when a system changes from a solid to liquid or liquid to gas as the molecules gain more freedom of movement. 2) The mathematical expression for entropy is the ratio of the heat change (q) to the temperature (T) of a reversible cyclic process. 3) According to the second law of thermodynamics, entropy is related to unavailable energy - as heat is applied to a system, some energy is used for work but the remaining heat becomes unavailable, increasing the entropy. Entropy is also associated with increased randomness and more probable molecular states.

1. ENTROPY
AND
FOUR
PROCESS

2. ENTROPY
•
•
Measure of degree of Disorder or
randomness in a molecular system is
called ENTROPY
EXAMPLES:
When solid change to liquid (entropy
increases).
When liquid change to solid (entropy
decreases).
When gas change to liquid (entropy
decreases).

3. Mathematical Expression Of
Entropy
Mathematical expression is
Ratio of the heat change (q) to the
Temperature of the reversible cyclic
Process units is Cal/Deg or Jk-1 mol-1 or
cal k-

4. Significance:
 ENTROPY
& UNAVAILABLE
ENERGY
 ENTROPY
&RANDOMNESS
 ENTROPY&PROBABILITY

5. Entropy & unavailable Energy:

When the heat is applied to system
some part of system is only used to do
work. It is called as available energy
and remaining part of heat is
unavailable energy. According to the
2nd law
Entropy = Unavailable energy
Temperature

6. Entropy & Randomness:

It is increase in entropy there is change
from order state to disorder state
Entropy & Probability:
•
A irreversible spontaneous process
tends to proceed from less process to
more probable state and so in
spontaneous process entropy increase

7. Entropy changes in isothermal
expansion of ideal gas
According to 1st law of thermodynamic
dE=q-PdV
or dE=q-W
(i)
In reversible isothermal process change
in
Internal energy(dE=0)
Equation (i) becomes
qrev –W=0
qrev =W
(ii)

8. The work done is expansion of moles of
Gas from volume V to V at constant
Temperature „T‟is
W=nRT In V /V
(iii)
Sub(iii) in (ii)
qrev=nRT In V /V
We know that
So, qrev =
*T
T =nRT In V /V
= 1/T nRT In V /V
1
2
2
1
2
2
1
1
2
1

9. =nRTInV2/V1
=2.303nRlog V2/V1
P V =RT (or)
P2V2=RT (or)
1
1
(iv)
V =RT/P
V2=RT/P2
1
1
=2.303nRlog P1/P2

10. Entropy change in reversible
process (Non-spontaneous) :
Consider isothermal expansion of
ideal gas
 If system absorbs „q‟ amount of heat
from surroundings at Temperature “T”
then Entropy increase of system

Entropy decrease of surrounding

11. Net change in entropy
=q/T +(-q/T)
Therefore in reversible isothermal
process
No change in Entropy

12. Entropy change in irreversible
(spontaneous) Process:
Consider system at higher
temperature „T1‟ ,surrounding at lower
temperature T2
 „q‟ amount of heat passes irreversibly
from system to surrounding
 Decrease in entropy of system


Increase in entropy of surrounding

13. Net change
As

14. Entropy change in Physical
transformations :
Entropy change takes place when
system under goes physical
transformation like vapourise fusion
etc.
 Let
be quantity of heat absorbed in
calories at constant temperature and
pressure.
 Entropy change

(i)

15. But enthalpy change
(ii)
L
Latent heat in calories
M
Molecular weight in Grams
T
Temperature in Kelvin
Sub (ii) in (i)

Latent heat of Vapourisation of H2O:
540cal /gm at 100°C
Latent heat of fusion of ice: 80cal/gm at
0°C