Pressure, Temperature and Phase Changes

The advantage of carrying out thermal processes under vacuum pressures can be seen if the liquid-gas boundary is considered. 

The Clapeyron equation states the relationship between a phase change for a pure material and changes in pressure and temperature. For vaporisation  

Using the Clapeyron equation, calculate the enthalpy of vaporisation (evaporation) of water from the following data  

As vvap is much larger than vh in Equation (6.1) (vvap - v7) can be replaced by vvapm, where the subscript m refers to a unit amount (mol-1, kmol-1), or, better, vm(vap). If it is assumed that the vapour behaves ideally (vm = RT/p), then the Clausius-Clapeyron equation is obtained  

If it is further assumed that A//vapm does not depend on temperature then Equation (6.2) integrates to  

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