Reaction kinetics enter into a geochemical model, as we noted in the previous chapter, whenever a reaction proceeds quickly enough to affect the distribution of mass, but not so quickly that it reaches the point of thermodynamic equilibrium. In Part I of this book, we considered two broad classes of reactions that in geochemistry commonly deviate from equilibrium.

The first class, discussed in detail in Chapter 6, was reaction between a fluid and the minerals it contacts. The kinetics of the reactions by which minerals dissolve and precipitate was the subject of the preceding chapter (Chapter 16). The second class of reactions commonly observed to be in disequilibrium in natural waters, as discussed in Chapter 7, is redox reactions. The subject of this chapter is modeling the rates at which redox reactions proceed within the aqueous solution, or when catalyzed on a mineral surface or by the action of an enzyme. In the following chapter (Chapter 18), we consider the related question of how rapidly redox reactions proceed when catalyzed in the geosphere by the action of microbial life.

Kinetic redox reactions are simulated within the context of a redox disequilibriummodel, a geochemical model constructed to account for disequilibriumamong species of differing redox state, as described in Chapter 7. In such models, one or more redox coupling reactions are disabled. It is worth noting that the development here, although cast in terms of redox reactions, can be applied equally well to describe the kinetics of other reaction types, such as the formation and decomposition of complex species.