NEET levelSingle correct optionDifficulty 1 of 3ec071
Given E∘(FeX3+/FeX2+)=0.77V and E∘(FeX2+/Fe)=−0.44V. The standard potential of FeX3++3eX−Fe is
A−0.04V
B+0.33V
C+0.17V
D−1.21V
Answer: A
Potentials of half reactions cannot be added directly; Gibbs energies can. ΔG∘(FeX3+Fe)=ΔG∘(FeX3+FeX2+)+ΔG∘(FeX2+Fe), so −3FE∘=−F(0.77)−2F(−0.44). Hence E∘=31(0.77)+2(−0.44)=3−0.11=−0.04V. Option B subtracts and option C averages the potentials, both wrong.