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.

A new calculation of the work of formation of bubbles and drops

“…The isotherms are continuous and do not represent the jump in specific volumes between liquid and gas that we would expect. This horizontal jump is added by means of a construction credited to Maxwell (Lewins 2003). This construction is to locate the level of the two-phase pressure that will join the small specific volume of liquid, left, to the large specific volume of gas, right, on an isotherm necessarily below the critical point.…”

“…Thus, the area of the loop below the saturation line is equal to the area above, figure 5. The numerical implementation of this construction is given in Lewins (2003).…”