Trouton's rule

It is expressed as a function of the gas constant R: A similar way of stating this (Trouton's ratio) is that the latent heat is connected to boiling point roughly as Trouton’s rule can be explained by using Boltzmann's definition of entropy to the relative change in free volume (that is, space available for movement) between the liquid and vapour phases.

[2][3] It is valid for many liquids; for instance, the entropy of vaporization of toluene is 87.30 J/(K·mol), that of benzene is 89.45 J/(K·mol), and that of chloroform is 87.92 J/(K·mol).

Because of its convenience, the rule is used to estimate the enthalpy of vaporization of liquids whose boiling points are known.

For example, the entropies of vaporization of water, ethanol, formic acid and hydrogen fluoride are far from the predicted values.

The entropy of vaporization of XeF6 at its boiling point has the extraordinarily high value of 136.9 J/(K·mol) or 16.5 R.[4] The characteristic of those liquids to which Trouton’s rule cannot be applied is their special interaction between molecules, such as hydrogen bonding.

A log–log plot of the enthalpies of melting and boiling versus the melting and boiling temperatures for the pure elements. The linear relationship between the enthalpy of vaporization and the boiling point is Trouton's rule. A similar relationship is shown for the enthalpy of melting.
Enthalpies of melting and boiling for pure elements versus temperatures of transition, demonstrating Trouton's rule