Diffusion and Activation Control in Heterogeneous Reactions.

Abstract

The study of heterogeneous reactions is concerned chiefly with the determination of the nature of the processes controlling the observed velocity of the reaction. It was Nernst (40) who first postulated that reactions of this type are controlled by diffusion, either the diffusion of the "corrosive" medium to the phase boundary or the diffusion of the reaction products away from the phase boundary. This postulation involves the tacit assumption that the reaction at or with the boundary occurs extremely rapidly, so rapidly, in fact, that its rate has little or no effect on the observed velocity of the reaction. Bruner (5) and Noyes and Whitney (41) likewise made similar suggestions. Upon publication of these views there arose two schools of opinion, the one (49) maintaining that the principle had only limited validity, and the other (see, e.g., 22) maintaining that the Nernst-Noyes-Bruner theory or a modification of it is applicable to most heterogeneous reactions. There have been data contributed for countless reactions in which diffusion is rate-determining; however, there are also data for other reactions which indicate that the observed velocity cannot be identified with the velocity of diffusion alone, but rather that the speed of the chemical process, per se, may exert a considerable influence.These facts suggest that the two viewpoints (i.e., diffusion control vs. activation control) represent limiting extremes of behavior. The following generalities may be set up:1. Diffusion to or from a phase boundary may be important in determining the rate of a heterogeneous reaction, and it is in many of them.2. The rate of the chemical reaction proper, that is, the rate of activation of the reactants, may be important in determining the rate of a heterogeneous reaction, and it is in a fairly large number of known reactions.3. The rate of diffusion to or from a phase boundary may be comparable in magnitude to the rate of reaction at or with the phase boundary. As will be seen later, some few reactions seem to fall into this category.4. A particular reaction may be predominantly diffusion-controlled under a