In a multi-step mechanism, the rate law is determined by which step?

In a multi-step mechanism, the rate law is determined by which step?

• A. The fastest step
• B. The slowest step
• C. The first step
• D. The last step

Answer: (B)

The rate is determined by the slowest step. In a sequence of elementary steps, the step that proceeds most slowly acts as the bottleneck, setting the pace for the entire reaction. The rest of the steps may be fast enough to quickly establish the necessary intermediates and then quickly proceed, but they cannot push the overall rate beyond what the slowest step allows. As a result, the rate law reflects the reactants (and sometimes intermediates related via equilibria) involved in that slowest, rate-determining step. The fastest step cannot control the overall rate, and the first or last step only set the pace if they happen to be the slowest step, which is not generally the case.

The rate is determined by the slowest step. In a sequence of elementary steps, the step that proceeds most slowly acts as the bottleneck, setting the pace for the entire reaction. The rest of the steps may be fast enough to quickly establish the necessary intermediates and then quickly proceed, but they cannot push the overall rate beyond what the slowest step allows. As a result, the rate law reflects the reactants (and sometimes intermediates related via equilibria) involved in that slowest, rate-determining step. The fastest step cannot control the overall rate, and the first or last step only set the pace if they happen to be the slowest step, which is not generally the case.