4.9.4 Phase Transitions

There is an enthalpy change at a phase transition. Such a transition is at constant pressure and heat is involved, though the temperature remains constant.^4.16 Thus $\Delta H$ changes. There is often a volume change involved as well. That means that the phase change involves work against any external pressure. This must be taken into account when converting $\Delta H$ changes to $\Delta U$ changes and vice versa.

Example: Sulfur exists in two solid forms, monoclinic and rhombic. The respective densities are 2.104 g/ml and 2.070 g/ml. How much pV work is involved in the transition from monoclinic to rhombic sulfur at 1.00 atm?

What we need are the volumes per mole. This is given by $\frac{M}{\rho}$, where M is the molecular weight in grams per mole and $\rho$ is the density in g/ml. With M = 32.066 this gives V_mono = 15.24 ml and V_rhom = 15.50 ml. Then $\Delta V$ = V_rhom - V_mono = 0.25 ml or 0.00025 liters. The work involved is then 0.00025 liter-atm or 0.025 joules.