Clausius-Clapeyron Equation

Recall that the vapor pressure of a liquid increases with a rise in temperature. However, this dependence is not linear.

To illustrate, the vapor pressure of water at 50 °C is 0.122 atm, while at 100 °C, it is 1 atm. Vapor pressure curves sharply upward with increasing temperature, resulting in an exponential curve.

In comparison, when the natural log of vapor pressure is plotted against reciprocal temperature, a straight line is obtained, and its equation is called the Clausius–Clapeyron equation.

Here, R is the ideal gas constant; c, a constant characteristic of the liquid, is the y-intercept; and the slope of the line is equal to the negative of the molar heat of vaporization over the gas constant.

The equation allows the calculation of the molar heat of vaporization from the experimental measurements of equilibrium vapor pressures and temperatures.

For example, suppose the natural log of ethanol vapor pressure plotted as a function of reciprocal temperature gives a straight line with the slope of negative 4638 kelvins.

The equation for the slope of the line, along with the value of R, gives the molar heat of vaporization of ethanol as 38 560 joules per mole.

If the molar heat o