Reaction processes can be driven by the transfer of mass into or out of a multicomponent chemical system held at equilibrium. Complex reactions, for example, can occur when a solid phase dissolves into or precipitates from a fluid held in equilibrium. This chapter discusses how such processes might be simulated and interpreted using computer modeling techniques.

Many minerals in the geochemical environment do not react rapidly enough with coexisting fluids to maintain thermodynamic equilibrium. Kinetic laws for this reason are required to predict the rates at which such minerals dissolve and precipitate. This chapter shows how rate laws of this class can be incorporated in multicomponent chemical reaction models, illustrates how such models behave, and provides advice on how the models might be best applied to advantage by scientists and engineers.

The distribution of chemical mass in a multicomponent system held at equilibrium changes when the system’s temperature varies, or when chemical potentials in a phase in contact with the system shift. This chapter discusses how to construct numerical simulations of reaction processes driven by polythermal conditions, or by changing chemical boundary conditions.

Natural waters near Earth’s surface commonly exist far from redox equilibrium and hence hold a thermodynamic drive for the oxidation of some aqueous species at the expense of others, which are reduced. The rates at which such oxidation and reduction reactions occur in the natural environment are described by kinetic laws, which may account for heterogeneous catalysis or promotion by enzymes. This chapter shows how to incorporate redox kinetics into multicomponent chemical reaction models and gives a fully worked example of how such models can be applied.

Valid solutions to the multicomponent equilibrium problem are commonly but not invariably unique roots of the governing equations. Modelers for this reason need to consider the possibility of the existence of more than one mathematically correct root to the equations describing chemical equilibrium in multicomponent systems. This chapter demonstrates the origin of nonunique roots to the equilibrium problem, provides several worked examples of how such roots may arise, and gives advice about coping with the possibility of nonuniqueness.

Chemical buffers, reactions that resist change in a system’s chemical state, exert strong controls on the chemistry of the natural environment. Important buffers in nature include heterogeneous buffers arising from reactions among aqueous species, as well as heterogeneous buffers caused by reactions of a fluid with an external solid or gas phase.

Any consideration of reaction processes in multicomponent chemical systems begins with a conceptual model of the setting of interest. This chapter describes how to develop a conceptual basis for constructing a geochemical reaction model and discusses the uncertainties inherent in evaluating such a model.