Two substances can react by electron transfer (redox) if:
- an oxidant of one couple and a reductant of another couple are present AND
- E°(couple containing the reactant oxidant) is more positive than E°(couple containing the reactant reductant).
In deciding whether both an oxidant and a reductant are present,
remember that in quoted E° values, the oxidant is always given the left.
In comparing the potentials of couples to identify the more negative one and the more positive one,
remember that for electrode potentials having the same sign, the magnitude of the numbers must be compared.
Consider two couples with positive potentials:
E° (Ag+, Ag) = +0.80 V
E°(Cu2+, Cu) = +0.34 V
Ag+ is the oxidant in the more positive couple.
Cu is the reductant in the more negative couple.
The reaction of Ag+ with Cu is spontaneous.
Consider two couples with negative potentials:
E°(Pb2+, Pb) = – 0.15 V
E°(Zn2+, Zn) = – 0.76 V
Zn is the reductant in the more negative couple.
Pb2+ is the oxidant in the more positive couple.
The reaction of Pb2+ with Zn is spontaneous.