Question

The rate determining step in a reaction mechanism:

A. is the fast step

B. is the slow step

C. is always the first step

D. is the transition state

Answer 1

B) is the slowest step

In any multi-step reaction, the slowest step has to be the rate determining step.

This is understood by considering the simple fact that in any process which consists of a number of steps, the total time taken to complete the process is mainly governed by the most time consuming step.

So, if we consider the process of making tea, the most time consuming step is boiling the tea. If the overall process takes 10 minutes to be completed and the tea has to be boiled for about 6 minutes, it is easily concluded that the major part in the process of making tea is involved in boiling the tea. In other words, boiling is the rate determining step in this case.

A similar analogy is drawn in a multi-step reaction where among the numerous steps, the most time consuming step is the rate determining step.

Example:

In an SN1 type of reaction, in the first step a carbocation is formed and in the second step the nucleophile attacks the carbocation from both the faces to obtain a racemic mixture of products. The formation of carbocation is the slowest among the two steps and hence is the rate determining step. This is the reason why the SN1 reactions are first order and depends on the concentration of substrate only and not on the nucleophile.