MCAT General Chemistry Question 74: Answer and Explanation

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Question: 74

14. The following system obeys second-order kinetics.

2 NO2 → NO3 + NO(slow)
NO3 + CO → NO2 + CO2(fast)

What is the rate law for this reaction?

  • A. rate = k[NO2][CO]
  • B. rate = k[NO2]2[CO]
  • C. rate = k[NO2][NO3]
  • D. rate = k[NO2]2

Correct Answer: D

Explanation:

To answer this question, recall that the slow step of a reaction is the rate-determining step. The rate is always related to the concentrations of the reactants in the rate-determining step (not the overall reaction), so NO2 is the only compound that should be included in the correct answer. The concentration of NO2 is squared in the rate law because the stoichiometric coefficient of NO2 in the rate-determining step is 2.

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