Given Fe3++3e−→Fe,E∘Fe3+/Fe =−0.036V...(i) Fe2++2e−→Fe,E∘Fe2+/Fe =−0.439V...(ii) we have to calculate Fe3++e−→Fe2+,∆G=? To obtain this equation subtract equ (ii) from (i) we get Fe3++e−→Fe2+...(iii) As we know that ∆G=−nFE Thus for reaction (iii) ∆G=∆G1−∆G −nFE∘=−nFE1−(−nFE2) −nFE∘=nFE2−nFE1 −1FE∘=2×0.439F−3×0.036F −1FE∘=0.770F ∴E∘=−0.770V O−−>F−>Na+>Mg++>Al3+