Comments on the ``Hildebrand Rule''

Abstract

According to the ``Hildebrand rule,'' normal liquids should have the same entropy of vaporization at temperatures at which their vapors have the same molal volume. Pitzer has shown that liquids which obey the theory of corresponding states should have equal entropies of vaporization at temperatures at which their ratio of vapor volume to liquid volume is the same. He pointed out, however, that this rests upon several assumptions, including, ``potential energy depending only on intermolecular distances and universal shape of potential energy curve.'' In this paper, two pairs of liquids with widely different liquid molal volumes but otherwise closely similar, are compared with the result that the former rule is more closely obeyed, supporting the suggestion that it is an oversimplification to regard the molecular fields of polyatomic molecules as radial from the center of the molecule. The entropy of vaporization of a van der Waals liquid, R[1n(V—b)−1n(v—b)], where V refers to vapor and v to liquid, indicates that equal V and equal v—b should give equal entropy of vaporization. In comparing ethane and diisopropyl, for example, the separation of the methyl groups is more significant than the separation of the molecular centers.