Composition Dependence of Nonequilibrium Effects in Gas-Phase Reactions

Abstract

The process of a chemical reaction perturbs the translational and internal distributions of reactants and products. The composition and affinity dependences of the effects of the nonequilibrium distributions are derived for variously defined rate coefficients of reactions occurring in a dilute, homogeneous, and isothermal gas phase by a first‐order perturbation solution of appropriate Boltzmann equations for all reactants and products. In general, the ratio of forward‐to‐reverse rate coefficient differs from the equilibrium constant to the extent of the influence of the nonequilibrium distributions; one exception is the ratio of the apparent forward‐to‐reverse rate coefficient in the respective limits of infinite affinity for an isomerization or excitation reaction proceeding in large excess of inert gas. Detailed calculations are presented for this and some other simple cases including a bimolecular reaction of hard‐sphere molecules and a reaction with components in only two internal states. In the limit of zero product concentrations the nonequilibrium contributions to the rate coefficient differ from those obtained by an analysis entirely neglecting the presence of products in violation of the law of microscopic reversibility. The difference vanishes if one reactant is present in large excess. The analysis is applied to dissociation‐recombination reactions of diatomic molecules and is compared with other approaches to the problem of nonequilibrium kinetics, that is, the use of the master equation and the use of the conventional steady‐state approximation.