we have seen that for a reaction such as aA bB cC dD,.....................(1)
which is at any stage of completion, the free energy change is given by
G = G o RT ln [C] c [D] d / [A] a [B] b.................................(2)
where the bracketed quantities [A], [B], [C] and [D] refer to the actual activities (or concentrations in ideal solutions, pressures or fugacities (divided by p o ) for gases) of these species in the reaction mixture. The quotient on the right hand side is denoted by Q and if the reaction has reached equilibrium, it becomes equal to K, the equilibrium constant. Using the relation between the change in free energy and emf,