Often in real-world applications the elementary kinetics are not known. In some cases, an experimental measurement finds that the rate of reaction is proportional to the concentration of a species raised to a some arbitrary power (different from its stoichiometric coefficient). The user may 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 Gas-phase Kinetics input file. (These keywords are discussed in Reaction Order Parameters .)

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

(4–9)

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:

(4–10)

and

(4–11)

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. In such cases, it may be advisable to specify that the reaction is irreversible.