For the reaction: NO2 (g) + F2 (g) ⇌ 2 NO2F (g), a proposed mechanism has the slow step NO2 + F2 ⇌ NO2F + F and a fast step NO2 + F ⇌ NO2F. What is the rate law for this mechanism?

• A. A rate = k [NO2]^2 [F2]
• B. rate = k [NO2]
• C. rate = k [NO2][F2]
• D. rate = k [F2]

Answer: (C)

The rate law is determined by the slow (rate-determining) step. That step is bimolecular, involving one NO2 and one F2 molecule reacting to form NO2F and F. For an elementary step, the rate law follows the reactants in that step: rate = k [NO2][F2]. The fast step that follows does not set the overall rate, since it occurs after the slow step and involves intermediates (NO2F and F) rather than the starting reactants. Therefore the rate depends on both NO2 and F2—first order in each—giving rate = k [NO2][F2]. The other options imply dependence on only one species or on a different combination, which doesn’t match the slow, bimolecular step.