When more than one chemical reaction achieves equilibrium, a mass action law must be written for each. The analytical tool of choice is the reaction progress variable method.
Example: Dissociation of water vapor.
At low temperatures, water vapor (steam) does not dissociate, and chemical reactions can be neglected. At high temperature (> ~ 1500 K), dissociation into H2 and O2 becomes significant; at very high temperatures, as might occur in the exhaust of a rocket, the hydroxyl radical, OH, appears. The equilibrium reactions governing the concentrations of the four species are:
Assuming the total pressure to be 1 atm, the mass-action expressions for the above equilibria are:
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