What happens to the cell potential Ecell as the reaction quotient Q increases for a galvanic cell?

• A. E increases.
• B. E stays the same.
• C. E becomes negative.
• D. E decreases.

Answer: (D)

The key idea is how the reaction quotient Q affects the cell potential through the Nernst equation. For a galvanic cell, Ecell = E°cell − (RT/nF) ln Q (or at 25°C, Ecell = E°cell − (0.0592/n) log Q). As Q increases, the logarithmic term grows, so the subtraction makes Ecell smaller. In other words, accumulating products (or reducing reactants) drives the reaction away from spontaneity in the written direction, reducing the measured cell potential. If Q gets large enough, the potential can even become negative, but the immediate effect of increasing Q is a decrease in Ecell.