MCAT General Chemistry Question 177: Answer and Explanation

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Question: 177

12. For a cell with the following half-reactions:

Anode: SO₂ + 2 H₂O → SO₄^2- + 4 H⁺ + 2 e^-

Cathode: Pd²⁺ + 2 e^- → Pd

How would decreasing the pH of the solution inside the cell affect the electromotive force (emf)?

A. The emf would decrease.
B. The emf would remain the same.
C. The emf would increase.
D. The emf would become zero.

Correct Answer: A

Explanation:

A change in pH has a direct correlation to the hydrogen ion (H⁺) concentration. Decreasing the pH increases the H⁺ concentration, which means the concentration of products has increased in the oxidation of sulfur dioxide. This means it would be harder to liberate electrons, thus decreasing the emf. One could also view this decrease in oxidation potential as an increase in reduction potential. If E^°_red,anode increases, then E^°_cell must decrease according to E^°_cell = E^°_red,cathode - E^°_red,anode.

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