= Chemical affinity
<Chemical affinity> is the thermodynamic driving tendency of a specified <chemical reaction>, defined at fixed <temperature> and <pressure> as
$$
\mathcal A=-\left(\frac{\partial G}{\partial\xi}\right)_{T,P}
=-\sum_i\nu_i\mu_i,
$$
where $G$ is <Gibbs energy>, $\xi$ the <extent of reaction>, $\nu_i$ signed <stoichiometric coefficients> and $\mu_i$ chemical potentials.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book)>[Affinity of reaction, entry A00178]{ref}
For the chosen forward direction, $dG=-\mathcal A\,d\xi$: positive <chemical affinity> favours a small positive advance. At an unconstrained equilibrium, $\mathcal A=0$. For a reaction quotient $Q$ and equilibrium constant $K$, $\mathcal A=RT\log(K/Q)$ under the usual activity conventions. This is a thermodynamic condition, not a rate prediction; a favourable reaction can remain slow because of an <Gibbs energy of activation>.
Back to article page
