1. Third Law of Thermodynamics
No finite sequence of cyclic processes can succeed in
cooling a body to absolute zero.
Nernst Postulate (Weak Form)
The entropy change associated with any condensed
system undergoing a reversible isothermal process
approaches zero as temperature approaches 0 K,
where condensed system refers to liquids and
solids.
Walther Nernst
(1864 – 1941)

2. Nernst Postulate (Strong Form)
The entropy of all perfectly crystalline substances is
zero at absolute zero.
Max Planck
( )

3. Third Law Entropies

5. Standard State Entropy for a Gas

7. Boltzmann’s Interpretation

11. Entropy of Substances Tabulated Results Function of Molecular Size
Species So(298) J/K mol Species So(298) J/K mol
Li 29.1 HF 173.7
Na 51.4 HCl 186.8
K 64.7 HBr 198.6
Rb 69.5 HI 206.3
Cs 85.2
Function of Molecular Complexity
NO 211 CH4 186
NO2 246 C2H6 230
N2O4 304 C3H8 270
C4H10 310

12. Residual Entropy
Ice I Structure
Ice II Structure