The units of rate constants in chemical kinetics

Abstract

This short article discusses the units of rate constants as used in chemical kinetics and, in particular, the aspect of non-integral powers of base units, which some might find unusual for units in the SI system. In many ways the fact that the units of the rate constants as usually defined convey information about the order of the reaction or reactions involved is very useful, but in other ways having the same (or at least very similar) quantity that has different units under different conditions is not so desirable. Furthermore, just as with chemical equilibrium constants, taking functions of the rate constant (such as the logarithm when representing the Arrhenius equation in the form ln \(k\,{\text {vs.}}\,1/T\)) needs special attention. Here we examine a possible alternative definition of rate constants in terms of an explicit ratio to the concentration standard state and although we acknowledge that this approach unlikely to be adopted by the community, it serves as a basis to discuss the meaning of rate constants.