# Entropy of vaporization

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.

${\displaystyle \Delta G_{\text{vap}}=\Delta H_{\text{vap}}-T_{\text{vap}}\times \Delta S_{\text{vap}}=0,}$

where ${\displaystyle \Delta H_{\text{vap}}}$ is the heat or enthalpy of vaporization. Since this is a thermodynamic equation, the symbol T refers to the absolute thermodynamic temperature, measured in kelvins (K). The entropy of vaporization is then equal to the heat of vaporization divided by the boiling point.

${\displaystyle \Delta S_{\text{vap}}={\frac {\Delta H_{\text{vap}}}{T_{\text{vap}}}}.}$

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.