Читать книгу Geochemistry - William M. White - Страница 71

We can rearrange eqn. 2.58 to read dU − TdS = − PdV. The −PdV term is the work term and the TdS term is the heat function. TdS is the energy unavailable for work. Therefore, dU − TdS is the amount of internal energy available for work, or the free energy. We define it as A, the Helmholtz free energy:

(2.119)

As usual, we are interested in the differential form (since we are more interested in changes than in absolutes):

(2.120)

or substituting eqn. 2.58 into 2.120:

(2.121)