Rapid preequilibria as a cause for change of activation energy with temperature

A reaction scheme is considered where the substance taking part in the rate-determining step is present in two forms which are in mobile equilibrium (isomers; or differently solvated or protonated species). Equations are developed which show that if only one of the two forms is kinetically active, while both are present at comparable concentrations, the apparent energy of activation decreases with temperature. On the other hand, if the two forms are present at different concentrations, but nevertheless contribute about equally to the observed reaction rate, having different, independent, rate constants, the apparent energy of activation is shown to increase with increasing temperature. The meaning of the results, and their possible applicability, are discussed.