A Surface Kinetics mechanism may include I surface reactions that involve up to K chemical species, where K is the total number of species (gas, surface, bulk) in the system. The reactions can be represented in a general form that is equivalent to that stated in Equation 3–1 for gas-phase reactions in Basic Rate Expressions . As with gas-phase reactions, reactions may be reversible, where both forward and reverse rates are included, or irreversible where only forward rates are considered.

For surface reactions, the net production rate is in units of (mole/cm² /sec) for each of the species, regardless of phase. The net production rate for each species is the sum of the rates-of-production for all reactions involving the k th species, as shown in Equation 4–5...pointed to "Equation 4-2" in "CHEMKIN_Theory.xml".

(4–5)

As with gas-phase reactions, the stoichiometric coefficients may be non-integers for global rate expressions, and the net stoichiometric coefficient is given by Equation 3–3 . Net stoichiometric coefficients for surface reactions are determined by Surface Kinetics library routine SKNU. Again, the rate-of-progress variable qi for the i th reaction is given by the difference of the forward rates and the reverse rates. By default, the law of mass action, as stated in Equation 3–4 is used to determine the forward and reverse rates based on the stoichiometric coefficients and the species concentrations. This assumption can be over-ridden as described in later sections of this chapter. For surface reactions, the Surface Kinetics library routine SKROP is used to determine net rates of progress.

As discussed in Surface State Variables , the form of the concentrations depends upon whether species k is in the gas-phase, on the surface, or in the bulk. Furthermore, the units of the rate constants will depend on the identity of the reactants and products in a particular reaction.

The forward rate constants for the reactions are (by default) assumed to have the Arrhenius temperature dependence, as given in Equation 3–5 . The three Arrhenius parameters (returned by Surface Kinetics library routines SKABE and SKRAEX) are required input to the Surface Kinetics Pre-processor for each reaction. However, there are a number of ways in which the rate expression for a reaction can be altered from this basic expression. These alternative formulations are described in the remaining sections of this chapter.

For reversible reactions, the reverse rate constants are related to the forward rate constants through the equilibrium constants, as stated in Equation 3–6 . The user can over-ride this use of the equilibrium constant by explicitly declaring Arrhenius coefficients for the reverse reaction in the Pre-processor input via the auxiliary keyword REV. This option is explained in the Table 4.6: Alphabetical Listing of Surface Reaction Auxiliary Keywords of the Chemkin Input Manual Input Manual. An executable can call the Surface Kinetics library routine SKIREV to find out if reverse coefficients were explicitly input for a given reaction and the coefficient values.