Physical Chemistry Third Edition

3

The Second and Third Laws

of Thermodynamics: Entropy

PRINCIPAL FACTS AND IDEAS

1. There are two principal physical statements of the second law of
   thermodynamics: (1) If a system undergoes a cyclic process it cannot turn
   heat put into the system completely into work done on the surroundings.
   (2) Heat cannot flow spontaneously from a cooler to a hotter object if
   nothing else happens.


2. The second law implies that no heat engine can have an efficiency as
   great as unity.


3. The mathematical statement of the second law establishes a new state
   function, the entropy, denoted bySand defined through its differential by

dS

dqrev

T

4. The mathematical statement of the second law provides a means of
   calculating the entropy change of any process that begins and ends at
   equilibrium states.


5. The second law implies that no macroscopic process can decrease the
   entropy of the universe.


6. Entropy is connected with lack of information through the Boltzmann
   definition of the statistical entropy:

SstkBln(Ω)

wherekBis Boltzmann’s constant andΩis the number of mechanical

states that corresponds to the thermodynamic state of the system,

corresponding to a fixed energy.

7. The third law of thermodynamics allows the entropy of any pure perfect
   crystalline substance consistently to be set equal to zero at absolute zero
   of temperature.


8. The second and third laws of thermodynamics imply that zero
   temperature on the Kelvin scale is unattainable.

105