For many catalytic reactions the values of <5 are small and the condition for the optimal catalyst may be expressed in a simple form:

(4.9)

Equation (4.9) indicates that for a reaction (4.1) proceeding in two separate steps (4.1a) and (4.1b), when 2a < 2b, it is the interaction of the reactant with oxygen at the surface of-the catalyst which is rate determining, the replenishment of the surface with oxygen being very rapid. On increasing the heat of interaction ~ of the reactant with surface oxygen from a small value this step is accelerated and so is the overall reaction (4.1) until ~ becomes equal to 2b. From this moment step b begins to be rate-determining and on further increasing ~ the overall rate now decreases because of the decreasing heat 2b of the replenishment of catalyst surface with oxygen. A volcano=-shaped curve is thus obtained, which is the manifestation of a very general relationship, constituting the basis of lnany theoretical considerations.