What is E° for the cell Fe | Fe2+ (1.0 M) || Zn2+ (1.0 M) | Zn, given E°(Zn) = -0.76 V and E°(Fe) = -0.44 V, when Zn is the cathode?

• A. +0.32
• B. -1.20
• C. -0.32
• D. +1.20

Answer: (C)

The key idea is that the standard cell potential is found from the reduction potentials and which side is the cathode. E°cell equals E°cathode minus E°anode. Here Zn is specified as the cathode, so the reduction at the cathode is Zn2+ + 2e- → Zn with E° = -0.76 V. The anode is Fe → Fe2+ + 2e-, whose reduction potential is -0.44 V for the Fe2+/Fe couple. Using the formula gives E°cell = (-0.76) - (-0.44) = -0.32 V.

The negative result means the described arrangement would not be spontaneous as a galvanic cell; it would require an external power source to drive the reaction. If instead Fe were the cathode and Zn the anode, the cell potential would be +0.32 V, which is spontaneous in that orientation.