Entropy of vaporization

In thermodynamics, 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 Template:Tmath = 1 bar, the value is denoted as Template:Tmath and normally expressed in joules per mole-kelvin, J/(mol·K).

For a phase transition such as vaporization or fusion (melting), both phases may coexist in equilibrium at constant temperature and pressure, in which case the difference in Gibbs free energy is equal to zero:

$\mathrm{\Delta }{G}_{\text{vap}}=\mathrm{\Delta }{H}_{\text{vap}}-T_{\text{vap}}\times \mathrm{\Delta }{S}_{\text{vap}}=0,$

where $\mathrm{\Delta }{H}_{\text{vap}}$ is the heat or enthalpy of vaporization. Since this is a thermodynamic equation, the symbol Template:Tmath 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:

$\mathrm{\Delta }{S}_{\text{vap}}=\frac{\mathrm{\Delta }{H}_{\text{vap}}}{T_{\text{vap}}}.$

According to Trouton's rule, the entropy of vaporization (at standard pressure) of most liquids has similar values. The typical value is variously given as 85 J/(mol·K), 88 J/(mol·K) and 90 J/(mol·K). Hydrogen-bonded liquids have somewhat higher values of Template:Tmath

See also

• Entropy of fusion

References

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