On the surface sites or phases, we can describe the chemical state in terms of an array of surface species site fractions, . Surface site fractions are also often referred to as "site coverages." Like gas-species fractions, surface site fractions become dependent variables in the chemically reacting-flow problem when multiple surface site species are included in the chemistry set.

The array of surface site fractions is of length . It is composed of sections containing the site fractions of each of the species on a given site (phase) . The site fractions on each site are normalized, such that

(2–2)

The sum in Equation 2–2 runs from the first species in phase to the last species in phase . The surface molar concentration of a species is then

(2–3)

where is the density of sites of phase (in moles/cm²) and is the number of sites that each species occupies. Note that Equation 2–3 assumes that the surface site density is constant as a function of time. It is possible to override this requirement in the surface chemistry reaction set, in which case use of this equation should ensure that the correct value of is used. Non-conservation of sites is discussed further in Surface Site Non-conservation .

For the sake of parallelism, we adopt the nomenclature for bulk species:

(2–4)

Surface Kinetics takes the approach that the activity, , of bulk species is used in all chemical rate expressions. In the limiting case of an ideal solution, the activity of a species is equal to its mole fraction. However, Surface Kinetics does not explicitly evaluate the relationship between bulk mole fraction and the bulk activities. In most existing Ansys Chemkin executables, ideal solutions are assumed when bulk mixtures are present, although it is recognized that this approximation is often not a good one. Typically bulk activities become dependent variables in the reacting-flow problem only if there are more than one bulk species in any one bulk phase.