What is a Diffusion-Controlled Reaction

In your own work, you must use your judgement about which model is most appropriate. This judgement can be tested with the following spectrum of examples  

We discuss models of heterogeneous reactions in this chapter and models of homogeneous reactions in the next chapter. This artificial division finesses the embarrassing question of which model we should use. This is usually not a major problem. We shall find that the choice between heterogeneous and homogeneous models is often obvious, one that we shall make almost automatically. 

Throughout science and engineering we find references to diffusion-controlled mass transfer and diffusion-controlled chemical reaction. Those using these terms often have very specific cases in mind or are not aware of how broadly and loosely these terms are used. In this short section, we want to describe the cases to which these terms most commonly refer. Each of these cases will be analyzed in detail later. 

A diffusion-controlled process always involves various sequential steps. For example, the dehydrogenation of ethane on a single platinum crystal involves the diffusion of the ethane to the solid followed by the reaction on the solid surface. The reaction between protons and hydroxyl ions in water first has these species diffusing together, and then reacting. 

Reactions like these are said to be diffusion-controlled when the diffusion steps take much longer than the reaction steps. Four cases in which this is true are shown in Fig. 16.2-1. For the heterogeneous reaction in Fig. 16.2-1(a) reagent diffuses to the surface the 

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