Theory of Chemical Reactions in Mixtures with Inert Gases

Chemical gas reactions are often studied in the presence of inert gases, particular examples being shock‐tube dissociations and unimolecular reactions in the low‐pressure limit. A linear‐mixture rule is usually invoked to separate contributions to the rate constant from the two types of possible collision processes. This separability assumption is investigated in this paper. A sufficient condition is that reactants below the critical energy are essentially in thermal equilibrium. If this equilibrium is seriously disturbed by the reaction, the linear mixture rule will break down. The observation of nonlinearity is suggested as evidence for nonequilibrium contributions to the rate constant. Unimolecular reactions and reactions with tunneling contain complications which must be carefully separated from nonequilibrium effects. Systems in which the reverse reaction cannot be ignored lead to predictable simple modifications in this mixture rule, it still being additive but nonlinear. If nonequilibrium effects are important, gross changes in the rate constants are expected depending on which collision partner dominates energy transfer in the region below the critical energy. In particular, if molecules determine energy transfer near $E_0$ at low temperatures but atoms control this region at high temperatures, both cases may give linear plots, but the effective efficiencies in these two regions will differ.