MCAT General Chemistry Question 221: Answer and Explanation

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Question: 221

6. Based on the reaction mechanism shown below, which of the following statements is correct?

2 NO + O2 → 2 NO2

1) 2 NO → N2O2 (fast)

2) N2O2 + O2 → 2 NO2 (slow)

  • A. Step 1 is the rate-determining step and the rate of the overall reaction is k[N2O2].
  • B. Step 1 is the rate-determining step and the rate of the overall reaction is k[NO]2.
  • C. Step 2 is the rate-determining step and the rate of the overall reaction is k[NO2]2.
  • D. Step 2 is the rate-determining step and the rate of the overall reaction is k[N2O2][O2].

Correct Answer: D

Explanation:

D The rate-determining step (RDS) of a reaction mechanism is the slowest step of that mechanism, eliminating choices A and B. The rate law of an elementary step can be determined from the coefficients of the reactants in the elementary step. Because Step 2 is the RDS, the overall rate law will be equivalent to the rate law for the RDS. Therefore, rate = k[N2O2][O2].

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