The Rationale of Trouton's Rule and of the Trouton-Hildebrand-Everett Rule

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 volume 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 cm3 mol-1 for all substances. The introduction 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 correlation. It thus offers no improvement to Trouton's rule nor to the THE rule. © 1996, Verlag der Zeitschrift für Naturforschung. All rights reserved.