3. Reaction with multiple stoichiometry, isochore, isotherm, of order 1

So far, we have only considered the effect of internal diffusion on the apparent rate of a single reaction. But, in practice, catalysts are most often used to promote one reaction over others that lead to worthless products. In other words, the effect of diffusion on catalyst selectivity is at least as important as its effect on the apparent rate of a reaction.

We can imagine an unlimited number of systems involving series and parallel reactions of varying kinetic order, so no general results can be presented. We shall confine ourselves to briefly recalling results relating to three simple cases.