MCAT General Chemistry Question 240: Answer and Explanation

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Question: 240

2. Given the following reactions:

Pb2+ + 2e- → Pb(s) Eº = –0.13 V
Fe(s) → Fe2+ + 2e- Eº = 0.45 V

Which one of the following is true?

  • A. Pb(s) is a better reductant than Fe(s)
  • B. Fe(s) is a worse reductant than Pb(s)
  • C. Fe2+ is a better oxidant than Pb2+
  • D. Pb2+ is a better oxidant than Fe2+

Correct Answer: D

Explanation:

D Oxidant and reductant are synonymous with oxidizing agent and reducing agent, respectively. Choices A and B are saying the same thing, so both can be eliminated. To compare the relative strengths of the ions as oxidizing agents, reverse the half reaction for Fe so that it reads as a reduction:

Pb2+ + 2e- → Pb(s)E° = –0.13 V
Fe2+ + 2e- → Fe(s)E° = –0.45 V

Note that the sign of E° is reversed in this process. Pb2+ has a more positive reduction potential than Fe2+, making it the better oxidant.

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