PHYSICAL CHEMISTRY IN BRIEF

CHAP. 3: FUNDAMENTALS OF THERMODYNAMICS [CONTENTS] 66

dS >

̄dQ

T

, [irreversible process]. (3.7)

U Main unit:JK−^1
Entropy is defined by equation (3.6) up to the integration constant. The second law of
thermodynamics can be used only to calculate entropy changes during reversible processes and
not values in a given states.
It follows from the second law of thermodynamics that it is impossible to construct a
perpetual motion mechanism of the second type—a hypothetical cyclically working
machine that receives heat from its surroundings and converts it into work without any losses.

Example

Prove that entropy increases during irreversible processes in an adiabatically isolated system.

Proof

In an adiabatic process ̄dQ= 0. By substituting for ̄dQinto the inequality (3.7) we obtain

dS > 0. This shows that entropy increases.

Note:Entropy is the degree of disorder in the movement of molecules making up a thermo-

dynamic system. The higher the disorder, the higher is the system’s entropy. For example,

the entropy of a gas is higher than the entropy of a crystal at the same temperature and

pressure.

3.1.4 The third law of thermodynamics

At a temperature of 0 K, the entropy of a pure substance in its most stable crystalline form is

zero

lim

T→ 0

S= 0. (3.8)

This postulate supplements the second law of thermodynamics by defining the natural referen-

tial value of entropy. Equation (3.8) allows for the calculation of entropy in a given thermody-

namic state of a system [see3.2.8and3.5.5].