As we know from molecular kinetic theory, atoms or molecules in liquids and gases are in a state of constant motion. From time to time, individual molecules of a liquid, moving fast enough, can “break away” from its surface. Thus, above any liquid, a certain number of molecules of a given substance will be in the form of vapor. The pressure of these molecules, if there are no foreign impurities, is called the vapor pressure of this substance. Sometimes you can feel this presence of steam above the liquid – remember the characteristic sensation of moisture on the seashore or ocean.
We also know that to convert a substance from a liquid to a gaseous state (cm. Phase transitions) you need to spend some energy. This energy is called heat of evaporation or heat of vaporization… The Clapeyron-Clausius equation precisely describes the relationship between the heat of vaporization H, steam pressure p and temperature T substances:
ln p = H/RT + constant
where ln p Is the natural logarithm taken from the value of the vapor pressure, and R Is Rydberg’s constant. Temperature T measured in kelvin.
The first to bring this dependence in 1834 was the engineer-constructor of steam engines Benoit Clapeyron. Naturally, due to his specialty, Clapeyron was primarily interested in the heat of vaporization, and he used his equation mainly for engineering and applied purposes. For science, the equation for the heat of phase transition was rediscovered almost two decades later by Rudolf Clausius, the author of the formulation of the second law of thermodynamics.
Most often, the Clapeyron-Clausius equation is used for simple calculation or measurement of the heat of vaporization of various substances. By measuring the vapor pressure at different temperatures and plotting it on a graph, along one axis of which the value of ln is plottedp, and on the other – the value 1 / T, scientists from the obtained linear relationship (slope of a straight line) determine the heat of vaporization of a substance.