What is the difference between a rate-determining step and a slow step?

The rate-determining step and the slow step in a reaction are essentially the same thing.

In a chemical reaction, the rate-determining step, also known as the slow step, is the slowest step in a reaction mechanism. It is the step that determines the overall rate of the reaction. This is because a reaction cannot proceed faster than its slowest component step. The rate-determining step is therefore crucial in understanding the kinetics of a reaction, as it provides the basis for the rate equation and the activation energy of the reaction.

The term 'rate-determining step' is used to emphasise the role of this step in controlling the rate of the reaction. It is the step with the highest energy barrier to overcome, and thus requires the most time to complete. The rate of the reaction is directly proportional to the rate of the rate-determining step. If this step is fast, the reaction is fast. If this step is slow, the reaction is slow.

On the other hand, the term 'slow step' is used to describe the speed of this particular step relative to the other steps in the reaction mechanism. In a multi-step reaction, some steps may occur almost instantaneously, while others may take a significant amount of time. The slow step is the one that takes the longest time to complete.

In summary, the rate-determining step and the slow step refer to the same concept in the study of reaction kinetics. They both describe the step in a reaction mechanism that takes the longest time to complete and thus determines the overall rate of the reaction. The difference in terminology is simply a matter of emphasis: 'rate-determining step' highlights the role of this step in controlling the reaction rate, while 'slow step' describes its relative speed compared to the other steps.