Rate of Reaction for Shrinking Spherical Particles

When no ash forms, as in the burning of pure carbon in air, the reacting particle shrinks during reaction, finally disappearing. This process is illustrated in Fig. 25.8. For a reaction of this kind we visualize the following three steps occurring in succession. 

Step 1. Diffusion of reactant A from the main body of gas through the gas film to the surface of the solid. 

Step 2. Reaction on the surface between reactant A and solid. 

As with particles of constant size, let us see what rate expressions result when one or the other of the resistances controls. 

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