If a cell contains identical electrodes and only the ion concentrations differ between half-cells, what type of cell is it and how is its potential determined?

• A. A galvanic cell; Ecell = E° - (0.05916/n) log Q.
• B. A concentration cell; Ecell = (RT/nF) ln(a_high / a_low); At 25°C, Ecell = (0.05916/n) log(C_high/C_low).
• C. A fuel cell; Ecell = ΔG/nF.
• D. An electrolytic cell; Ecell > 0.

Answer: (B)

When both electrodes are the same and only the ion concentrations differ between the half-cells, the cell is a concentration cell. The standard potential for such a setup is zero because the two half-reactions are identical, so there’s no inherent driving force from standard potentials. The voltage comes entirely from the concentration difference, which is captured by the Nernst equation.

Ecell = E°cell − (RT/nF) ln Q. Since E°cell = 0 for identical electrodes, this simplifies to Ecell = −(RT/nF) ln Q. If you treat ions moving from the higher-concentration side to the lower-concentration side, Q = a_high / a_low, giving Ecell = (RT/nF) ln(a_high / a_low). At 25°C, this becomes Ecell = (0.05916/n) log(C_high / C_low).

So the correct description is that this is a concentration cell, with its potential determined entirely by the concentration ratio of the ions in the two half-cells.