In some cases there are experimental data that indicate that the Arrhenius expression for the
rate constant, Equation 3–5
, is modified by the coverage
(concentration) of some surface or bulk species. Ansys Chemkin Surface
Kinetics allows optional coverage parameters to be specified for species
and reaction 
, through use of the auxiliary keyword COV, described in
Table 4.6: Alphabetical Listing of Surface Reaction Auxiliary Keywords
of the
Chemkin Input Manual
Input Manual. In
this case, the rate constant for the forward reaction is modified as
 | (4–7) |
where the three coverage parameters are 
, 
, and 
for species 
and reaction 
. The product in Equation 4–7
runs over only those surface
species that are specified as contributing to the coverage modification. Note that the surface
site fractions appear in Equation 4–7
rather than molar concentrations
(mole/cm2) for surface species, while bulk
activities appear for bulk species. The term associated with 
now makes it possible for the rate-of-progress of a reaction to be
proportional to any arbitrary power of a surface species concentration. Also, using this
modified expression for 
, the net pre-exponential factor may be a
function of coverage
 | (4–8) |
and the activation energy is a function of the coverage
 | (4–9) |
For reactions with optional coverage dependence, the rate of progress is calculated employing Equation 3–4 , with the forward rate coefficient from Equation 4–7 . The reverse rate constant is calculated via Equation 3–6 .
If the form of Equation 4–7 is not flexible enough to describe a certain coverage behavior, one can repeat the same reaction several times with different values for the coverage parameters such that the sum of the rate constants approximates the desired form.