The Third Law
The third law is based on the Nernst theorem that states that as absolute zero of the
temperature is approached, the entropy change DS for a chemical or physical
transformation approaches 0,
lim
T!0
DS ¼ 0
This makes that the property S is used to express the disorder or randomness of a
system.
The Fourth Law
The fourth law is based on the Onsager reciprocal relations which express the
equality of certain ratios between flows and forces in thermodynamic systems out
of equilibrium, but where a notion of local equilibrium exists. In absence of
magnetic fields, Onsager established
L j;k ¼ L k;j
that, to a certain degree, makes equation L j,k = L k,j a thermodynamic equivalent of
Newton's third law.
A Fundamental Equation in Thermodynamics
A most used equation in thermodynamics is:
DG ¼ DH À TDS
That correlates the Free Gibbs Energy G (or useful work of a system) to
Enthalpy (E) and Entropy (S). The value of the properties at the standard state (T =
273 K, P = 0.101 MPa) is expressed as G°, H°, S°.
Appendix B
235