This section presents expressions and equations that are useful in formulating chemically reacting flow problems. Species can exist in the gas phase, on surface sites, or in bulk mixtures. In some cases it is desirable to refer to information about species without regard to the phases, and in other cases it is desirable to determine information about species in one particular phase or group of phases. Therefore, before beginning to discuss our formalism in terms of mathematical expressions, we introduce a nomenclature that facilitates manipulating species information.

Information about a species (say a thermodynamic property) is presumed to be available in ordered arrays beginning with the first gas-phase species, continuing through the surface species, and ending with the last bulk species. In the expressions and equations below we presume that there are species, and we use the index to refer to a specific species. There are gas-phase species, which, by convention, are always the first entries in the species arrays. The index of the first gas-phase species is ( by our convention) and the last gas-phase species index is (). Thus, the gas-phase species indices are . In a similar way surface species indices are in the range and bulk species are in the range . The surface species may be arranged on any number of sites, and the bulk species may exist in any number of bulk mixtures. Furthermore, situations can occur in which there are no surface species and/or no bulk species.

As discussed in Development of a Surface Formalism, the species are grouped in “phases.” The first is the gas phase, whose index . The next phases (if they are present) are the surface sites, whose phase indices are bounded by . The final phases are the bulk mixtures, whose indices are bounded by . In each phase there are species, and those species have indices in the range .