Internal Volume and the Entropy of Vaporization of Liquids

Abstract

An effective internal volume, Ve, of a liquid is defined by the equation ΔS = R ln Vv/Ve, where ΔS = entropy of vaporization, and Vv = molal volume of the vapor in equilibrium. A condition for equilibrium is found which gives ΔS in terms of Ve/V and dVe/dV, where V is the actual molal volume of the liquid. The experimental data are considered in the light of this relationship, and the reason for the approximate validity of Trouton's or Hildebrand's rule is discussed. The nature of the relation between the internal and actual volume and its connection with the specific heat of the liquid is considered. Finally suggestions are made in way of explanation of the fact that entropies of vaporization of associated liquids are practically invariably high.

Modified Trouton's Rule for Predicting the Entropy of Boiling

“…Even at constant vapor volumes it can be observed (Rice, 1937;Halford, 1940;Hermsen and Prausnitz, 1961;Hildebrand and Scott, 1950) that the entropy of boiling is not entirely constant. Conformational and rotational restrictions in the liquid that are not present in the vapor will also contribute to the overall entropy.…”