# ReactionEquilibrium:Background:Reaction Equilibrium Reaction Equilibrium

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For the elementary reaction

where *X* and *Y* may be *R* or *B*, chemical equilibrium requires

If we separate the chemical potential into various contributions, as described in the Standard Properties page, this is

We can collect terms as follows

We define the equilibrium constant *K*(*T*) by the right-hand side of this equation; on this Standard properties page the standard-state chemical potentials are given in terms of the potential parameters, so *K*(*T*) is known. Then

The fugacity coefficients are not known in general. For sufficiently low density they approach unity (1.0). It may also happen that the ratio of the is approximately 1.0 even when they are not individually. For our calculations we will make this approximation.

We can write an equilibrium equation for each of the elementary reactions listed on the System page. This gives us three equations to solve for the six unknowns in this system: . Two of the other three equations express the fact that the total number of *R* atoms and *B* atoms in the system cannot change

and the other is simply the normalization of the mole fractions