In an electrolytic cell, what is the sign of Ecell under the applied conditions to drive the nonspontaneous reaction?

• A. Ecell must be positive.
• B. Ecell must be negative under the applied conditions to drive the nonspontaneous reaction.
• C. Ecell must be zero.
• D. Ecell sign is irrelevant.

Answer: (C)

The main idea is that spontaneity is indicated by a positive cell potential. An electrolytic cell is built to force a nonspontaneous reaction, so the cell potential for the reaction as written is negative under the applied conditions. The external power source supplies energy to push the electrons opposite to the natural tendency, so the overall Ecell becomes negative. A zero Ecell would mean no driving force, and a positive Ecell would mean the reaction would proceed spontaneously, which isn’t the case when you’re deliberately driving it.