By default, the rate-of-progress of a reaction is evaluated by Equation 3–4 , which uses the concentration of each reactant or product species raised to the power of its stoichiometric coefficient. Thus, the rate-of-progress of a reaction that includes species A with a coefficient of 2 will be second-order with respect to the concentration of A. Equation 3–4 would always be valid when mass-action kinetics are obeyed, and the mechanism is written in terms of elementary reactions.

Often, these elementary assumptions do not apply to the global reactions of interest. For example, an experimental measurement may show that the rate of reaction is proportional to the concentration of a species raised to an arbitrary power (different from its stoichiometric coefficient). In such cases, Ansys Chemkin allows the user to declare that the rate-of-progress of a reaction is proportional to the concentration of any species (regardless of whether that species even appears as a reactant or a product in the reaction) raised to any specified power. To modify the reaction order for the reaction in the forward or reverse direction, the user must declare the FORD or RORD auxiliary keywords, respectively, in the Pre-processor input file. These keywords are discussed in Table 3.7: Alphabetical Listing of Gas-phase Reaction Auxiliary Keywords of the Chemkin Input Manual Input Manual. These options are available both for gas-phase and surface reactions.

When the order-dependence of reaction is changed via the FORD or RORD keywords, the rate-of-progress variable for the reaction is evaluated by

(3–13)

where is the reaction order specified through the FORD keyword and is the reaction order specified through the RORD keyword for species . The default for species participating in reaction is the normal mass-action kinetics values

(3–14)

and

(3–15)

if an order-change parameter is not given for species .

The user should exercise caution when specifying a change of reaction order, as such a change may produce unexpected and unphysical results in a kinetic simulation. For example, the user should consider the kinetics of the reverse reaction when changing reaction-orders for the forward reaction. Such a reaction may no longer satisfy microscopic reversibility. At equilibrium, elementary kinetics ensure that

(3–16)

A reaction for which one has specified a change in reaction order will not have the proper equilibrium behavior unless

(3–17)

The user specifying may also wish to adjust such that Equation 3–17 is satisfied; Gas-phase Kinetics does not do this automatically. Another alternative would be to simply specify that the reaction is irreversible. A user program can call subroutine CKIORD for gas-phase reactions and SKIORD for surface reactions, to determine if a reaction has user-specified orders and the values of those parameters.