Enthalpy Phase Change Predictions from Van Der Waals Equation

Abstract

It is pointed out that a P,v,T relation, or equation of state, predicting a phase change has calorimetric data embedded in it, such as the latent heat or enthalpy change at the phase transition. This is illustrated by solving van der Waals equation over a range of sub-critical temperatures, employing Maxwell's construction for the saturation pressure and deriving the data from Clausius—Clapeyron. The results allow a comparison with real data. For example, water substance data are shown to be only modestly represented by a van der Waals model. Nevertheless, the procedure is applicable to more elaborate P,v,T relations for real substances.