Chemical Reactions involving Pure Condensed Phases

When considering chemical reactions of involving pure condensed phases, we can calculate equilibrium concentrations by considering the chemical potentials in the vapor that is equilibrium with the system. This leads to an important approximation that one can set the activities of the pure condensed reactants and products to one--in other words, they don't appear in the equilibrium constant. This allows us to make quantitative predictions on the reduction and oxidation of materials in terms of the partial pressure of oxygen.

The data for free energies of reaction for oxidation of metals is conveniently represented in an Ellingham diagram.