In the proposed mechanism for NO2 + F2 reaction, which step is rate-determining?

• A. NO2 + F2 -> NO2F + F (slow)
• B. NO2 + F -> NO2F (fast)
• C. NO2 + F2 -> NO2F + F (fast)
• D. NO2F formation step is rate determining

Answer: (A)

The rate-determining step is the slowest step in a mechanism, and it sets the overall rate of the reaction. If the step where NO2 reacts with F2 to form NO2F and F is labeled as slow, that step controls how fast the reaction proceeds. All the subsequent steps described as fast occur quickly after this slow step, so they don’t limit the rate. Therefore the step NO2 + F2 -> NO2F + F, identified as slow, is the rate-determining step, and the rate law would be based on this bimolecular interaction. The other steps, being fast, do not set the overall rate, and saying the “NO2F formation step is rate determining” is vague without specifying which particular step in the sequence is the slow one.