Chemical kinetics and diffusion control

In a heterogeneous process, a sequence of steps involving diffusion and chemical reaction is involved. For example, a reactant must diffuse to the surface of a solid catalyst before adsorption and chemical reaction can occur. Either diffusion or the chemical reaction may be rate-controlling and, over a sufficiently large temperature range, the Arrhenius plot of In k vs. 1/T is no longer straight, but curved. 

Consider the reaction of gaseous A at a solid surface B. The rate at which A is transported towards the surface is given by Pick s Law 

It is usual to assume that the concentration gradient in the gas film close to the surface is linear and 

If the reaction of A at the surface obeys first-order kinetics, then 

At a steady state, the rate of removal of A by reaction is equal to the rate at which A is transported to the surface by diffusion. 

---

For mixed chemical kinetics and diffusion control, Rabinovich suggested the relation (51)... 

Let us consider here the effect of segmental mobility on curing rate. The Eiring theory of reaction rates, which considers formation and decomposition of an activated complex, can deal with the simultaneously occuring chemical kinetics and diffusion control characterized, respectively, by some rate constants k and kd. Under certain conditions (prevailing chemical kinetics or diffusion control), the apparent reaction rate constant, kapp, expressed via the Rabinowitch equation... 

---