The Table of Standard Reduction Potentials gives a reduction potential of 0.771 V for the reduction of Fe3+(aq) to Fe2+(aq) under standard acidic conditions. What is the reduction potential under standard basic conditions?
There appears to be no explicit pH dependence in the reduction under standard acidic conditions:
Fe3+(aq) + e– → Fe2+(aq)
However, this does not take into account all of the reactivity under basic conditions. Under standard basic conditions (1.00 M OH–, pH = 14.00) both iron cations will precipitate as hydroxide salts:
Fe(OH)3(s) + e– → Fe(OH)2(s) + OH–(aq)
Thus, the contribution of the precipitation chemistry must also be accounted for in determining the reduction potential.
Fe3+(aq) + e– → Fe2+(aq) E°red = 0.771 V ΔG° = –nFE° = –(1)(96485)(0.771) = –74400 J
Fe(OH)3(s) → ← Fe3+(aq) + 3 OH–(aq) Ksp = 4×10–38 ΔG° = –RTlnKeq = –(8.314)(298)ln(4×10–38) = +213000 J
Fe2+(aq) + 2 OH–(aq) → ← Fe(OH)2(s) Keq = 1/Ksp = 1/8.0×10–16 = 1.3×1015 ΔG° = –RTlnKeq = –(8.314)(298)ln(1.3×1015) = –86200 J
Add the three reactions together to give the net reaction:
Fe(OH)3(s) + e– → Fe(OH)2(s) + OH–(aq)
ΔG° = –74400 + 213000 –86200 = 52000 J
E° = –ΔG°/nF = –(52000)/(1)(96485) = –0.54 V