“chem·i·stry (kem′ i strë), n., pl.-tries. The science that deals with or investigates the composition, properties, and transformations of substances and various elementary forms of matter.” The dictionary definition emphasizes chemical transformation as a central theme of chemistry.

By the end of the nineteenth century, the young science of physical chemistry had characterized the dependence of the rate of the chemical transformation on the concentrations of the reactants. This provided the concept of a chemical reaction rate constant k and by 1889 Arrhenius showed that the temperature dependence of the rate constant often took on the simple form k = A exp(−Ea∕RT), where A is referred to as the pre-exponential factor and Ea as the activation energy. Arrhenius introduced the interpretation of Ea as the energetic barrier to the chemical rearrangement. Only later did we understand that reactions also have steric requirements and that the Arrhenius A factor is the carrier of this information.

It was next realized that the net transformation often proceeds by a series of elementary steps. A key progress was the identification of the reaction mechanism, which is a collection of elementary processes (also called elementary steps or elementary reactions) that leads to the observed stoichiometry and explains how the overall reaction proceeds. A mechanism is a proposal from which you can work out a rate law that agrees with how the observed rate of the reaction depends on the concentrations.