Influence of Temperature and Pressure on Transformations

Abstract

The chemical potential can be regarded as constant only in a first approximation. Frequently, temperature and pressure have a decisive influence on the chemical potential and therefore on the course of chemical processes. Water freezes in the cold and evaporates in the heat. Ice melts under the blades of ice skates and butane gas (the fuel of a cigarette lighter) becomes liquid when compressed. Therefore, a more detailed approach has to consider the temperature and pressure dependence of μ. Often linear approaches to these dependencies suffice. If the corresponding temperature and pressure coefficients are given, it is easily possible to predict the behavior of the substances when they are heated, compressed, etc. The melting, sublimation points, etc., can be calculated, but also the minimum temperature needed for a particular reaction. Only the pressure coefficient of gases shows a strong pressure dependence itself; therefore, the linear approach is only valid in a small pressure range. For wider application, a logarithmic approach has to be used.