Читать книгу Geochemistry - William M. White - Страница 74

The Gibbs free energy is perhaps the single most important thermodynamic variable in geochemistry because it provides this criterion for recognizing equilibrium. This criterion is:

Products and reactants are in equilibrium when their Gibbs free energies are equal.

Another important quality of the Gibbs free energy is closely related:

At fixed temperature and pressure, a chemical reaction will proceed in the direction of lower Gibbs free energy (i.e., ΔG _r <0).

The reverse is also true: a reaction will not proceed if it produces an increase in the Gibbs free energy.

On an intuitive level, we can understand the Gibbs free energy as follows. We know that transformations tend to go in the direction of the lowest energy state (e.g., a ball rolls down hill). We have also learned that transformations go in the direction of increased entropy (if you drop a glass it breaks into pieces; if you drop the pieces they don't re-assemble into a glass). We must consider both the tendency for energy to decrease and the tendency for entropy to increase in order to predict the direction of a chemical reaction. This is what the Gibbs free energy does. Example 2.7 illustrates how Gibbs free energy of reaction is used to predict equilibrium.