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, the value is denoted as and normally expressed in joules per mole-kelvin, J/.
For a phase transition such as vaporization or fusion, both phases may coexist in equilibrium at constant temperature and pressure, in which case the difference in Gibbs [free energy] is equal to zero:
where is the heat or enthalpy of vaporization. Since this is a thermodynamic equation, the symbol refers to the absolute thermodynamic temperature, measured in kelvins. The entropy of vaporization is then equal to the heat of vaporization divided by the boiling point:
According to Trouton's rule, the entropy of vaporization of most liquids has similar values. The typical value is variously given as 85 J/, 88 J/ and 90 J/. Hydrogen-bonded liquids have somewhat higher values of