Many important reactions happen at interfaces. Because reactions between aqueous species and mineral surfaces are so important in low temperature geochemistry, this chapter focuses on reactions at solid/solution interfaces.

Reaction rates at solid/solution interfaces are controlled by the area of the interface as well as by the chemical and physical conditions that occur there. Surface reactions are approximately confined to a two-dimensional region, so their rates are expressed in terms of how fast species are created per unit of surface area, and this means that the rates have units of flux (J, mol/m2sec). The flux notation (J) and terminology is used throughout this book. The environment at the solid/solution interface is a hybrid of the bulk solid and bulk solution, so models of the chemical and physical conditions controlling the reaction rates must account for this transitional character. Equilibrium thermodynamics provides a powerful starting point for constraining the surface conditions. At equilibrium the chemical potential of each component must be the same throughout the system, so the chemical potential of the components in the surface are equal to their chemical potentials in the solid and solution phases. At low temperatures, the slow rate of equilibration between the bulk solid and the surface may void this requirement for the solid but it should apply for the components in the bulk solution. Also, at equilibrium the principle of detailed balance requires that the rates of forward reactions in the interface must equal the rates of the reverse reactions. In addition, the forward and reverse reaction steps must be the same. Models of reaction rates at equilibrium are well constrained by these principles but as the system departs from equilibrium these requirements fall away and we must search for other principles to model interfacial reaction rates.