Reactions between Gases and Pure Condensed Phases

The class of reactions in which an element reacts with a diatomic gas to form a compound is both of practical importance and amenable to simple thermodynamic analysis.

Reactions in this category include oxidation, nitriding, and hydriding of metals and halogenation of the electronic material silicon, usually by Cl2 or F2. The system is schematized in Fig. 9.5.

X2 gas in

X2 gas in metal 1 ^oxide, nitride, etc

Fig. 9.5 Metal M and compound MX2 in environment of X2 gas

The simplicity of the thermodynamics stems from the immiscibility of M and MX2; both are in their pure states, so the chemical potentials are equal to their molar free energies. Consider oxidation of a metal to form a dioxide according to reaction (9.3).., The chemical potential of oxygen gas is dependent on its partial pressure, and is described by Eq (9.17). With these simplifications, the equilibrium criterion, Eq (9.16), becomes:

Rearranging this equation into a more convenient form gives:

Equation (9.33a) can be viewed as a degenerate form of Eq (9.21), but without the partial pressures of M and MO2, which have been replaced by unity because these species are pure condensed substances.

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