The relative magnitude of chemical and diffusion reaction rates

1 THE RELATIVE MAGNITUDE OF CHEMICAL AND DIFFUSION REACTION RATES 

It is convenient to label the relative slowness of encounter pair reaction as due to an activated process and to remark that the chemical reaction (proton, electron or energy transfer, bond fission or formation) can be activation-limited. This is an unsatisfactory nomenclature for several reasons. Diffusion of molecules in solution not only involves a random walk, but oscillations of the molecules in solvent cages. Between each solvent cage in which the molecule oscillates, a transformation from one state to another occurs by passage over an activation barrier. Indeed, diffusion is activated (see Sect. 6.9), with a typical activation energy 8—12 kJ mol-1. By contrast, the chemical reaction of a pair of radicals is often not activated (Pilling [35]), or rather the entropy of activation 

When the activation process is comparable with or slower than the rate of approach of reactants to form encounter pairs, it is no longer satisfactory to say that the reactants can not co-exist within a distance R of one another. Because the rate of reaction, /eact, of the activation process is finite, so too is the lifetime (and hence concentration) of encounter pairs non-zero. The inner boundary condition, which describes reaction of A and B together in the diffusion analysis, is unsatisfactory. Collins and Kimball [4] suggested an alternative boundary condition and the remainder of this section analyses their work following Noyes [5]. Firstly, the boundary condition is developed and then included in the diffusion equation analysis to obtain the density distribution. Finally, the rate coefficient is obtained. 

3 nm) and the radius of p-iodotoluene (0.4—0.5 nm). Consequently, the rate of reaction may be a little less than diffusion-limited. 

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