MCAT General Chemistry Question 152: Answer and Explanation

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Question: 152

2. How many electrons are involved in the following half-reaction after it is balanced?

Cr₂O₇^2- + H⁺ + e^- → Cr²⁺ + H₂O

A. 2
B. 8
C. 12
D. 16

Correct Answer: B

Explanation:

First, balance the atoms in the equation:Cr₂O₇^2- + 14 H⁺ → 2 Cr²⁺ + 7 H₂O

Now, adjust the number of electrons to balance the charge. Currently, the left side has a charge of +12 (-2 from dichromate and +14 from protons). The right side has a charge of +4 (+2 from each chromium cation). To decrease the charge on the left side from +12 to +4, we should add 8 electrons:Cr₂O₇^2- + 14 H⁺ + 8 e^- → 2 Cr²⁺ + 7 H₂O