Introduction: Simple Analogies for Finding the Rate-Umiting StepThe idea of a rate-limiting step for a chemical reaction is one of the most fundamental concepts for understanding reaction rates. It would be difficult to discuss a reaction mechanism without knowledge of the rate-limiting step, and not surprisingly, almost all mechanistic studies have been directed at questions which center around the rate-limiting step. This concept is important enough to be introduced to undergraduates in introductory chemistry courses and is frequently presented in a statement similar to the following: "An important principle that governs all multistep reactions is that the overall rate of a multistep reaction cannot exceed the rate of its slowest step. Thus the slowest step of a multistep reaction is often called the rate-limiting or rate-determining step." This, and similar, statements can serve as a definition of rate-limiting step and have often been illustrated by analogies such as sand or water flowing through a sequence of funnels of varying stem size. These analogies have been popular for developing the concept of the rate-limiting step, since the funnel of smallest stem size will correspond to the ratelimiting funnel just as the reaction with the smallest rate constant will always correspond to the slowest step (i.e., the rate-limiting step) in a sequence of irreversible chemical reactions. These analogies have been important in illustrating that a rate-limiting step can exist and have gained widespread acceptance in teaching kinetics to undergraduates. However, many of the reactions introduced in chemistry courses are reversible, and as a result, use of the flow analogies frequently leads to an incorrect prediction of the rate-determining step.