Chemical affinity

Chemical affinity is the thermodynamic driving tendency of a specified chemical reaction, defined at fixed temperature and pressure aswhere $G$ is Gibbs energy, $\xi$ the extent of reaction, ${\nu }_i$ signed stoichiometric coefficients and ${\mu }_i$ chemical potentials.^[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.