General Reaction Kinetics Diffusion Resistance as the Rate-Determining Step

2 General Reaction Kinetics Diffusion Resistance as the Rate-Determining Step 

Deriving an explicit expression for a/ao in a general case of reaction kinetics is very difficult and often impossible. The reason for this is that the surface concentrations must be solved in Equation 8.51. When the rate-determining step is diffusion-resistant both in the product layer and in the gas film, a generally applicable derivation is possible. 

If the diffusion steps are slow compared with the chemical reaction, reactant A is instantaneously consumed after it has diffused to the interface. At the same time, the surface concentration c is very small in comparison to the bulk-phase concentration c. The difference in Equation 8.53 can be approximated with sufficient accuracy accordingly, 

It is easy to show that the result obtained actually is Equation 8.69, which was obtained for first-order reactions. In cases in which diffusion resistances in the gas film and/or in the product layer are rate-determining, we can use the general expressions for a/ao as given in Table 8.2 (the relationship between the integrated time dependencies of the bulk-phase concentrations c ). 

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