Using standard potentials, what is the largest approximate E° value that can be achieved when two half-cell reactions are combined to form a battery?

• A. 3 V
• B. -6 V
• C. -3 V
• D. 6 V

Answer: (D)

The key idea is that the overall cell potential comes from the difference between the standard reduction potentials of the two half-reactions: E°cell = E°cathode − E°anode. To get the largest positive value, place the half-reaction with the most positive E° as the cathode and the one with the most negative E° as the anode. The strongest positive standard potential is for fluorine reducing to fluoride, and the strongest negative standard potential is for lithium metal forming Li+. When you take that large positive potential and subtract the large negative potential, you get a very large positive E°cell—on the order of several volts, near the upper limit of what standard potentials can give. That explains why the largest approximate value is around six volts, far larger than the other options.