Clausius Clapeyron Equation

CLAUSIUS CLAPEYRON EQUATION

Clapeyron equation involves the relationship between saturation pressure and saturation temperature. Both enthalpy of evaporation and specific volume of the two phases are also involved. This equation provides a basis for the calculation of properties in a two phase region. It gives the slope of a curve separating the two phases in p-T diagram.

When the phase is changing from saturated liquid to saturated vapour, the temperature remains constant. So, ds equation reduces to

Integrating the above equation between saturated liquid (f) and saturated vapour (g),

This equation is known as Clausius-Clapeyron or Clapeyron equation.

The above equation is valid for the change from a solid to liquid and solid to vapour. At very low pressures, vg = vfg is assumed. So, it approaches pv = RT. Therefore, the above equation (5.103) becomes

We know that the equation of state for ideal gas

Substituting vg in equation (5.104), it becomes

Integrating the above equation,

The equation (5.105) is also known as Clausius-Clapeyron equation.