The change in Gibbs Free Energy for a reaction (#DeltaG_(rxn))# depends on the concentration of reactants and products, so an increase in pH increases #DeltaG_(rxn)# if #H_3O^+# is a reactant, and decreases #DeltaG_(rxn)# if #H_3O^+# is a product.
The general relationship between Gibbs Free Energy change for a reaction and the concentrations of reactants and products is
#DeltaG_(rxn)=DeltaG^0+RTlnQ#
where #DeltaG^0# is the free energy change for the reaction under standard conditions (unit concentrations of reactants and products), and #Q# is the reaction quotient (actual product concentrations divided by actual reactant concentrations).
Example: #H_3O^+(aq) + NH_3(aq) harr H_2O + NH_4^+(aq)#
#Q=([NH_4^+])/([H_3O^+][NH_3])#
Note that #H_2O# does not contribute to #Q# because it is a solvent.
An increase in pH decreases the hydrogen ion concentration, #[H_3O^+]#, thereby increasing #Q# (because it appears in the denominator of #Q#), and increasing #DeltaG_(rxn)#.
An increase in #DeltaG_(rxn)# decreases the thermodynamic driving force for the reaction, and if #DeltaG_(rxn)# becomes positive the reaction is no longer spontaneous (i.e., the reverse reaction becomes spontaneous).
