This question came to mind when studying voltaic cells. In voltaic cells, for some ions (Cu2+ for example), both oxidation and reduction are possible. Given both possiblities, how do we determine which reaction will take place? I first thought that comparing the signage in voltage of both reactant combinations would give the answer. But it then occurred to me that, even in both cases, signage could be positive. In other words. V1>0 V2>0 but V1>V2 how does one determine which reaction takes place then? the same can be said about free energy G1< 0 G2< 0 but G1<G2.