The net gas-phase chemical production rate of each species, , results from the competition between all the chemical reactions involving that species. For reactions stated as reversible in the Gas-phase Kinetics and Surface Kinetics input files, the reverse rate parameters can be determined from the forward rate coefficients and the thermodynamic properties of the species involved in the reaction, via the law of mass action. In these cases, Surface Kinetics requires activities to be defined for bulk-phase species, in order to determine the species thermochemistry. In these reactor models, the program sets the bulk activities, , equal to their mole fraction in the bulk phase:

(8–41)

More details regarding chemical reaction specifications, reaction-rate determinations, and thermochemical properties, are available in Gas-phase Chemical Rate Expressions and Surface Chemical Rate Expressions .