Rate equations are quantitative models of the time course of chemical reactions. Although rate equations are based on macroscopic observations, they reflect processes that occur at the molecular scale. This chapter reviews some of the important models that link these two scales. These models are especially useful because they constrain the mathematical form of rate equations and they provide a conceptual basis for thinking about the reactions. Because water is so important in geochemical systems, this chapter focuses on models for reaction rates in the aqueous phase.

Transition-state theory

Chemical reactions break and reform bonds within and between molecules so that the bonding arrangement of the reactants gives way to the bonding arrangement of the products. For a reaction to occur: (1) the molecules must “collide” with each other to form a cluster; (2) the atoms in that cluster must be configured in the approximate geometry of the products; and (3) the cluster must contain sufficient energy to allow the rearrangement of the electrons from the breaking bonds to the developing bonds.