
11
The Clapeyron Equation
In this chapter we shall derive one of the most important equations in thermody-
namics, the so called Clapeyron Equation, named after Paul Emile Clapeyron (1799-
1864), one of the founders of thermodynamics. The Clapeyron equation is magical
in its ability to explain a myriad of facts concerning phase equilibria. Nevertheless,
its derivation, at least in its most commonly used form, is rather straightforward. We
shall present a few alternative ways of obtaining it. We shall also give a generaliza-
tion of it due to the Austrian and Dutch physicist Paul Ehrenfest (1880-1933), which
enables it to be applied to the so called