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.
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:[1] where
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:[2] [3] According to Trouton's rule, the entropy of vaporization (at standard pressure) of most liquids has similar values.
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