Bimolecular Reaction Rates in Solids and Liquids

Abstract

The purposes of this article are: (1) to call attention to the fact that bimolecular reactions (A+B→AB) in solids and liquids do not, in general, follow simple second-order kinetics and, (2) to discuss some of the complications arising when deviations occur. In a previous paper a general expression for the rate of bimolecular reactions in liquids and solids was derived. The general reaction rate is second-order in the concentrations but the ``rate constant'' is time dependent. That expression is simplified and presented here in terms of three physically significant parameters, D, the sum of the diffusion coefficients of species A and B, r0, the A—B capture radius, and s, the ratio of the probability that a pair of particles A—B separated by a capture radius r0 will react, to the probability that they will diffuse apart before reaction can occur. The dependence of the ``rate constants'' on these parameters and on the time is illustrated graphically. The physical significance of the limiting cases is discussed. A simple, accurate, physically meaningful approximation to the rather abstract general rate expression is presented. An experimental method of distinguishing bimolecular reactions when they do not follow simple second-order kinetics is discussed. Reactions involving particles with long-range forces are considered briefly.