Fugacity Criteria for Phase Equilibria

We began the chapter by showing that Gibbs energy was equivalent in phases at equilibrium. Here we show that equilibrium may also be described by equivalence of fugacities. Since

we may subtract G^ig from both sides and divide by RT, giving

Substituting Eqn. 9.22,

which becomes

Therefore, calculation of fugacity and equating in each phase becomes the preferred method of calculating phase equilibria. In the next few sections, we discuss the methods for calculation of fugacity of gases, liquids, and solids.