Entropy of vaporization

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Not to be confused with Enthalpy 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.

 \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.

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