Does pH affect Gibbs free energy?

Apr 21, 2018

Sometimes...

It does if the reaction involves an acid... The Gibbs' free energy is a function of the reaction quotient:

$\Delta G = \Delta {G}^{\circ} + R T \ln Q$

and if $Q$ is a function of $\left[{\text{H"_3"O}}^{+}\right]$, then $\Delta G = \Delta G \left({10}^{- \text{pH}}\right)$. Some examples are...

${\text{HC"_2"H"_3"O"_2(aq) + "H"_2"O"(l) rightleftharpoons "C"_2"H"_3"O"_2^(-)(aq) + "H"_3"O}}^{+} \left(a q\right)$, ${K}_{a} = 1.8 \times {10}^{- 5}$

${\text{NH"_3(aq) + "H"_2"O"(l) rightleftharpoons "NH"_4^(+)(aq) + "OH}}^{-} \left(a q\right)$, ${K}_{b} = 1.8 \times {10}^{- 5}$