Relation of Equilibrium Constants to Composition

The standard state for a gas is the ideal-gas state of the pure gas at tire standard-state pressure P° of 1 bar. Since tire fugacity of an ideal gas is equal to its pressure, /,-° = P° for each species/. Thus for gas-pliase reactions= fi/P°, etnd Eg. (13.10) becomes  

The equilibrium constant K is a function of temperature only. However, Eq. (13.25) relates K to fugacities of the reacting species as tliey exist in the real equilibrium mixture. These fugacities reflect the nonidealities of the equilibrium mixture and are functions of temperature, pressure, and composition. Tliis means that for a fixed temperature the composition at equilibrium must change with pressure in such a way that W ifilP ) remains constant. 

Substitution of this equation into Eq. (13.25) provides an equilibrium expression displaying the pressure and the composition  

If the assumption that the equilibriimi mixture is an ideal solution is justified, then each becomes (pj, thefugacity coefficient of pure species i at T and P [Eq. (11.81)]. In this case, Eq. (13.26) becomes  

Each 0 for a pure species can be evaluated from a generalized correlation once the equilibrium T and P are specified. 

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