MCAT General Chemistry Question 67: Answer and Explanation

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Question: 67

7. A certain chemical reaction has the following rate law:

rate = k[NO₂][Br₂]

Which of the following statements necessarily describe(s) the kinetics of this reaction?

I. The reaction is second-order.

II. The amount of NO₂ consumed is equal to the amount of Br₂ consumed.

III. The rate will not be affected by the addition of a compound other than NO₂ and Br₂.

A. I only
B. I and II only
C. II and III only
D. I, II, and III

Correct Answer: A

Explanation:

If the sum of the exponents (orders) of the concentrations of each species in the rate law is equal to 2, then the reaction is second-order. The exponents in the rate law are unrelated to stoichiometric coefficients, so NO₂ and Br₂ could have any stoichiometric coefficients in the original reaction and still be a second-order reaction, invalidating statement II. Statement III is incorrect because the rate can be affected by a wide variety of compounds. A catalyst, for example, could increase the rate.

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