In most cases (that is, for unrestricted chemical equilibrium problems), an equilibrium calculation allows for all possible chemical reactions that may occur in a system. As discussed in Chapter 2 of Smith and Missen¹, as far as thermodynamics is concerned, the number of reactions used to represent the system is the most important aspect.

As an example, consider the system consisting of the species C₆H₅CH₃ (toluene), H₂, C₆H₆ (benzene), and CH₄ (methane). Since there are 4 species and 2 elements, this system is described stoichiometrically by at most 2 independent reactions. A possible set of such reactions is:

C₆H₅CH₃ + H₂ = C₆H₆ + CH₄
6CH₄ = C₆H₆ + 9H₂

To solve unrestricted equilibrium problems, EQS4WIN does not require a specification of the reactions, but only individual species free-energy data. (If desired, however, the problem may also be solved by entering an allowable set of reaction data.) However, in some situations, one wishes to calculate the equilibrium composition resulting from cases in which the stoichiometry is restricted in some way (that is, when a particular set of reactions is specified, the number of which is less than the maximum number possible). An example of such a case, discussed by Björnbom² and by Schneider and Reklaitis³, occurs for the above system when it is desired to calculate the equilibrium composition under the restriction that only the first of the above two reactions is allowed.

By utilizing the Reaction Input Data feature of EQS4WIN, restricted equilibrium problems may be easily solved. This is done by entering the actual restricted set of reactions which govern the equilibrium. The particular example described above is included in all versions of EQS4WIN.