24. Thermodynamic Functions

In addition to the terms discussed so far a number of other quantities and functions are used in thermodynamics without which textbooks that follow the conventional concept cannot manage. Because knowing these additional terms is essential for understanding traditional textbooks and the corresponding data collections, we will deal with the most important of them in this chapter and establish the relations to the concept chosen in this book. The major subsidiary terms are the four energetic quantities inner energy U, enthalpy H, Helmholtz energy A, and Gibbs energy G. The same quantity can serve different purposes depending on the variables chosen. The function U(S, V, …) characterizes the system under consideration. It is almost never explicitly stated (the abstractness of the variable S can be considered the underlying cause for this), but its differential plays a central role for all derivations. The functions U(T, V, …) and H(T, p, …) serve to describe the heat exchanged between system and surroundings under different experimental conditions (the first at constant volume, the second at constant pressure). The functions A(T, V, …) and G(T, p, …) play a similar role. Both are used to calculate the energy released during the considered process. This enables us to predict whether or not the process may run spontaneously. In the last section, we will discuss quantities such as activity, fugacity, etc. These quantities are used for describing deviations from what is considered ideal behavior of dissolved substances and gases.