Trouton's rule

Trouton’s rule states that the entropy of vaporization is almost the same value, about 87 J K^-1 mol^-1, for various kinds of liquids. The entropy of vaporization is defined as the ratio between the enthalpy of vaporization and the boiling temperature.

Trouton’s rule is valid for many liquids; for instance, the entropy of vaporization of toluene is 87.30 J K^-1 mol^-1, that of benzene is 89.45 J K^-1 mol^-1, and that of chloroform is 87.92 J K^-1 mol^-1. Because of its convenience, the rule is used to estimate the enthalpy of vaporization of liquids whose boiling points are known.

The rule, however, has some exceptions. For example, the entropies of vaporization of water, ethanol, and formic acid are far from the predicted values. The characteristic of those liquids to which Trouton’s rule cannot be applied is their special interaction between molecules such as hydrogen bonding. The entropy of vaporization of water and ethanol shows positive deviance from the rule; this is because the hydrogen bonding in the liquid phase lessens the entropy of the phase. In contrast, the entropy of vaporization of formic acid has negative deviance. This fact indicates the existence of an orderly structure in the gas phase; it is known that formic acid forms a dimer structure even in the gas phase.