Editing Chemical equilibrium (section)

===Treatment of activity===

The expression for the equilibrium constant can be rewritten as the product of a concentration quotient, ''K''<sub>c</sub> and an [[activity coefficient]] quotient, ''Γ''.
:<math>K=\frac{[\mathrm{S}] ^\sigma [\mathrm{T}]^\tau ... } {[\mathrm{A}]^\alpha [\mathrm{B}]^\beta ...}
\times \frac{{\gamma_\mathrm{S}} ^\sigma {\gamma_\mathrm{T}}^\tau ... } {{\gamma_\mathrm{A}}^\alpha {\gamma_\mathrm{B}}^\beta ...} = K_\mathrm{c} \Gamma</math>

[A] is the concentration of reagent A, etc. It is possible in principle to obtain values of the activity coefficients, γ. For solutions, equations such as the [[Debye–Hückel equation]] or extensions such as [[Davies equation]]<ref>{{cite book|first=C. W. |last=Davies |title=Ion Association |publisher=Butterworths |date=1962}}</ref> [[Specific ion interaction theory]] or [[Pitzer equations]]<ref name="davies">{{cite web |first1=I. |last1=Grenthe |first2=H. |last2=Wanner |url=http://www.nea.fr/html/dbtdb/guidelines/tdb2.pdf |title=Guidelines for the extrapolation to zero ionic strength |access-date=2007-05-16 |archive-date=2008-12-17 |archive-url=https://web.archive.org/web/20081217001051/http://www.nea.fr/html/dbtdb/guidelines/tdb2.pdf |url-status=dead }}</ref> may be used. However this is not always possible. It is common practice to assume that ''Γ'' is a constant, and to use the concentration quotient in place of the thermodynamic equilibrium constant. It is also general practice to use the term ''equilibrium constant'' instead of the more accurate ''concentration quotient''. This practice will be followed here.

For reactions in the gas phase [[partial pressure]] is used in place of concentration and [[fugacity coefficient]] in place of activity coefficient. In the real world, for example, when making [[Haber process|ammonia]] in industry, fugacity coefficients must be taken into account. Fugacity, ''f'', is the product of partial pressure and fugacity coefficient. The chemical potential of a species in the [[real gas]] phase is given by
:<math>\mu = \mu^{\ominus} + RT \ln \left( \frac{f}{\mathrm{bar}} \right) = \mu^{\ominus} + RT \ln \left( \frac{p}{\mathrm{bar}} \right) + RT \ln \gamma </math>
so the general expression defining an equilibrium constant is valid for both solution and gas phases.{{Citation needed|date=September 2021}}