The modeling of reactions in COMSOL uses the mass action law by default. Consider a general reaction belonging to a set of j reactions and involving i species:

For such a reaction set, the reaction rates rj (SI unit: mol/(m3·s)) can be described by the mass action law:

Here, and denote the forward and reverse rate constants, respectively. The concentration of species i is denoted as ci (SI unit: mol/m3). The stoichiometric coefficients are denoted νij, and are defined as being negative for reactants and positive for products. In practice, a reaction seldom involves more than two species colliding in a reacting step, which means that a kinetic expression is usually of order 2 or less (with respect to the involved concentrations).

Here, A denotes the frequency factor, n the temperature exponent, E the activation energy (SI unit: J/mol) and Rg the gas constant, 8.314 J/(mol·K). The pre-exponential factor, including the frequency factor A and the temperature factor Tn, is given the units (m3/mol)α − 1/s, where α is the order of the reaction (with respect to volumetric concentrations).

The default settings for the reaction given by Equation 2-1 and assuming equilibrium, yields the equilibrium expression in Equation 2-4: