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For the reaction CH4 + Br2 -> CH3Br + HBr, the experimentally determined rate law is: d[CH3Br]/dt = k1[CH4][Br2] / (1 + k2[HBr]/[Br2]). Which of the following statements about this reaction is/are correct?

1. The reaction proceeds in a single elementary step
2. The reaction is second order overall during the initial stages when [HBr] is approximately zero
3. The reaction is second order overall during the final stages when [Br2] approaches zero
4. The molecularity of the overall reaction is two

Correct answer: The reaction is second order overall during the initial stages when [HBr] is approximately zero

Solution

The complex rate expression (with HBr in the denominator) immediately rules out a single-step reaction and means molecularity cannot be defined for the overall process. In early stages ([HBr]~0), the denominator ~ 1 and rate = k1[CH4][Br2] — second order overall. In later stages ([Br2] very small), the rate = k1[CH4][Br2]² / (k2[HBr]) — order changes but is not simply second order. Molecularity applies only to elementary steps, not complex reactions.

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