The entropy of vaporization is the increase in entropy upon vaporization of a liquid. This is always positive, since the degree of disorder increases in the transition from a liquid in a relatively small volume to a vapor or gas occupying a much larger space. At standard pressure Po = 1 bar, the value is denoted as ΔSovap and normally expressed in J mol−1 K−1.
In a phase transition such as vaporization, both phases coexist in equilibrium, so the difference in Gibbs free energy is equal to zero.
where
According to Trouton's rule, the entropy of vaporization (at standard pressure) of most liquids is about 85 to 88 J mol−1 K−1.