Physical Transformations of Pure Substances Vaporization and Melting

Physical Transformations of Pure Substances Vaporization and Melting 

The general starting point to calculate the equilibrium state is the Clausius inequality  

For a constant pressure, the work done by the system on the surrounding (e.g., by a piston) is p( Kap — Vuquid) =pAV, so we obtain a positive term for an expansion of the system  

The entropy of a liquid is approximately independent of pressure and can be expressed by the standard value S° at po = 1.013 bar. For the vapor phase (ideal gas) we have to consider the change of entropy if the pressure changes from po to p by  

Rearranging yields the vapor pressure equation in its simplest form  

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