# In terms of reaction mechanisms, what exactly is the rate-determining step?

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The easiest way to visualise this is to imagine a line of four dinner ladies serving you different parts of a school meal e.g. the first peas, the second mash potatoes, the third chicken and the fourth gravy. To move to the next station you must have the preceding station’s food on your plate (e.g. you must have mash to be served chicken).You're walking down the line of dinner ladies, you have your peas and mash, but then the lady serving chicken is reallyyyyy slow-she is slowing the entire process down! However fast you collect your peas and mash, it is impossible to reduce the time it takes to reach the gravy dinner lady because it’s the chicken collection that is the slowest stage, relative to the other workers. Similarly, in a multi-stage reaction, the stages usually follow on one from the other, the finishing materials of one stage acting as the starting materials of the next. Therefore, the RATE OF THE SLOWEST STEP WILL GOVERN THE RATE OF THE ENTIRE REACTION. This is the rate determining step.

N.B. any step that occurs AFTER the rate-determining step will not affect rate. Also, by studying the order of a reaction from the rate equation, you are able to learn more about the rate-determining step (what is involved, what isn’t).

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