Bimolecular Reaction Rates in Solids and Liquids

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, r<sub>0</sub>, the A—B capture radius, and s, the ratio of the probability that a pair of particles A—B separated by a capture radius r<sub>0</sub> 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.