Abstract

In seeking a rational basis for the general rules regarding the enthalpy of vaporisation of a liquid, it is shown that whereas Trouton's Rule is consistent with the assumption that the effective molar volume, v, for translational motion in the liquid is a constant fraction (2 x 10" 5) of the molar vol-ume of vapour at 1 atm, the rather better approximation of Trouton, Hildebrand and Everett (THE), which corrects for the effects of varying molecular density in the vapour, re-quires that v be 1.5 cm 3 mol" 1 for all substances. The intro-duction of the apparently more reasonable assumption that v is a constant fraction of the molar volume of the liquid, is shown to lead to an expression exhibiting much poorer cor-relation. It thus offers no improvement to Trouton's rule nor to the THE rule.