Thermodynamics, Statistical Physics, and Quantum Mechanics

THERMODYNAMICS AND STATISTICAL PHYSICS 163

b) Consider the particular case of equilibrium between liquid and vapor.

The volume of the liquid is usually much smaller than that for the vapor

so we can disregard in (S.4.57.4) and write

Phase Transition (MIT)

where L is the latent heat per mole, is Avogadro’s number, and Ris

the gas constant.

For a system at equilibrium with an external reservoir, the Gibbs free energy

is a minimum. Any deviation from equilibrium will raise G:

where is the pressure of the reservoir (see Landau and Lifshitz, Statistical
Physics, Sect. 21). Expanding in we have

Using the ideal gas law for vapor, we get

or

We can see that Rewriting (S.4.57.6) in usualunits gives

4.58

Since we may rewrite (S.4.58.2) as