MCAT General Chemistry Question 245: Answer and Explanation

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Question: 245

7. A galvanic cell is constructed from two half-cells of platinum and iron. The half-reactions for these two elements are provided as follows:

Pt2+(aq) + 2e- → Pt (s)E° = +1.20 V
Fe3+(aq) + 3e- → Fe (s)E° = –0.036 V

Which of the following statements is true about the galvanic cell?

  • A. E° = 1.164 V, and Pt2+ is the reducing agent
  • B. E° = 1.164 V, and Fe3+ is the reducing agent.
  • C. E° = 1.236 V, and Pt2+ is the oxidizing agent.
  • D. E° = 1.236 V, and Fe3+ is the oxidizing agent.

Correct Answer: C

Explanation:

C The half-reaction with the less positive reduction potential (in this case, Fe3+(aq) + 3e- → Fe(s)) should be reversed in order to combine the half-reactions to obtain an E° > 0 and create a galvanic cell. When the Fe half-reaction is reversed, the sign of the potential must be reversed. Combining the two half-reactions then gives: 3 Pt2+(aq) + 2 Fe(s) → 2 Fe3+(aq) + 3 Pt(s) with an E° = 1.236 V (eliminate choices A and B). Since Fe3+ is the product of the reaction, it cannot be the oxidizing agent (eliminate choice D). However, Pt2+ is reduced (it gains electrons from the Fe that is oxidized), so it is therefore the oxidizing agent.

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