Two substances can react by electron transfer (redox) if BOTH conditions 1 and 2 are met.
- an oxidant of one couple and a reductant of another couple are present
When deciding whether both an oxidant and a reductant are present,remember that in quoted E°(oxidant,reductant) values, the oxidant of the couple to which the value applies is always given the left.
- E°(couple containing the reactant oxidant) is more positive than E°(couple containing the reactant reductant).
In comparing the potentials of couples, to identify the more negative one and the more positive one,
If the potentials have opposite signs, the one with the positive sign is more positive.
If both potentials are positive, the one with the larger number is more positive.
If both potentials are negative, the one with the smaller number is more positive.
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.
To calculate the cell potential subtract the more negative potential from the more positive one.
E°(cell) = 0.80 V - 0.34 V = +0.46 V
Consider two couples with negative potentials:
E°(Pb2+, Pb) = – 0.15 V
E°(Zn2+, Zn) = – 0.76 V
Pb2+ is the oxidant in the more positive couple.
Zn is the reductant in the more negative couple.
To calculate the cell potential subtract the more negative potential from the more positive one.
E°(cell) = – 0.15 V – (–0.76 V) = –0.15 V + 0.76 V = +0.61 V