A ⇌ B + C
the true equilibrium constant is given by
K = aBaC/aA
where aA, aB, and aC are the activities of the components, which function as concentrations (or pressures) corrected for nonideal behaviour. Activity coefficients (symbol γ) are defined for gases by γ = a/p (where p is pressure) and for solutions by γ = aX (where X is the mole fraction). Thus, the equilibrium constant of a gas reaction has the form
Kp = γBpBγCpC/γApA
The equilibrium constant of a reaction in solution is
Kc = γBXBγCXC/γAXA
The activity coefficients thus act as correction factors for the pressures or concentrations. See fugacity.
Subjects: Chemistry — Physics.