Entropy of vaporization

Template:Distinguish Template:Unreferenced stub 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.

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

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

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