The Clausius–Clapeyron Equation

The Clausius–Clapeyron equation describes how a substance’s vapor pressure changes with temperature. It shows that vapor pressure increases exponentially with temperature and depends on the enthalpy of vaporization, the gas constant, and a constant specific to the substance.

This equation can be rearranged into its logarithmic form to get a linear equation.

According to this, when the natural logarithm of vapor pressure is plotted against the reciprocal temperature, the slope of the line gives the negative of the enthalpy of vaporization over the gas constant.

The Clausius-Clapeyron equation can also be expressed in a two-point format by considering the vapor pressure P₁ at temperature T₁ and vapor pressure P₂ at temperature T₂.

Since the constant A remains the same for a given substance at both temperatures T₁ and T₂, the two logarithmic expressions can be equated. This provides the two-point form of the Clausius-Clapeyron equation, which enables us to find the change in vapor pressure between two temperatures.

This way, by knowing the enthalpy of vaporization of a liquid and its vapor pressure at a particular temperature, the two-point form of the Clausius-Clapeyron equation can be used to find out the liquid's vapor pressure at a different temperature.