According to their definitions expressed in the above Wikipedia articles, these two concepts seem to be totally identical. Is that true? If not, what is the difference?
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1 Answer
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The concepts are extremely closely related. The difference is that the mass action ratio is a formally a ratio of concentrations, or, for gases, of partial pressures. The mass action ratio, and the term "mass action" in general, refers to situations where the chemical potential of the reactants and products is directly proportional to the concentration, or pressure, or mass, of the species involved.
The reaction quotient is is a tiny bit more general; the implicit assumption of mass action is not necessarily made. The reaction quotient is the ratio of chemical potentials of reactants and products. It works even in systems that don't follow the law of mass action, i.e. where the activity coefficients vary during the equilibration.
For all intents and purposes, the two concepts are the same. If you want to split hairs, you'd probably do it the way I did here.
