Gas Phase Equilibria
For equilibria in a gas phase, fugacity, f, is used in place of activity. However, fugacity has the dimension of pressure, so it must be divided by a standard pressure, usually 1 bar, in order to produce a dimensionless quantity, f / p. An equilibrium constant is expressed in terms of the dimensionless quantity. For example, for the equilibrium 2NO2 N2O4,
Fugacity is related to partial pressure, p, by a dimensionless fugacity coefficient ϕ: f = ϕ p. Thus, for the example,
Usually the standard pressure is omitted from such expressions. Expressions for equilibrium constants in the gas phase then resemble the expression for solution equilibria with fugacity coefficient in place of activity coefficient and partial pressure in place of concentration.
Read more about this topic: Keq, Types of Equilibrium Constants
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