Читать книгу Geochemistry - William M. White - Страница 72

The Gibbs free energy is perhaps misnamed. By analogy to the Helmholtz free energy, it should be called the free enthalpy (but enthalpy is an energy), because it is derived as follows:

(2.122)

and

(2.123)

or

which reduces to:

(2.124)

Notice the similarity to the Helmholtz free energy; in that case we subtracted the TS term from the internal energy; in this case we subtracted the TS term from the enthalpy. The Gibbs free energy is the energy available for nonPV work (such as chemical work). It has two other important properties: its independent variables are T and P, generally the ones in which we are most interested in geochemistry, and it contains the entropy term (as does the Helmholtz free energy), and hence can be used as an indication of the direction in which spontaneous reactions will occur.