# How does the rate determining step determine the overall speed of the reaction if it is the first step in the reaction?

I can imagine it determining the rate of the whole reaction if it is the last step since that means that we will have lots of the reactants needed for the rate determining step, synthesized in earlier steps. Therefore, the rate of this step represents the rate of conversion of these reactants to our final products. But if it is in the beginning , you will have to include the speeds of the other steps in the overall rate which means the overall rate is not dependent on the rate determining step alone. So why is it called a rate determining step if it is in the beginning ?

NOTE : If it is the last step, then we only consider the speeds of the earlier steps until we have excess of the reactants which feed into the rate determining step. As for the rest of the time, we will only have a store of reactants which are waiting for the rate determining step.

• If it's the first step that's rate determining, then as soon as that transformation is complete, everything else immediately happens by comparison. – Zhe Jul 10 '17 at 13:35
• The rate determining step is so slow that we just pretend all the other steps happen instantaneously. You're right that it still takes time but we ignore it. Generally a RDS is a few orders of magnitude faster than fast steps, so if the first step takes ten hours, you're not going to care if the second step takes a few seconds. – iammax Jul 10 '17 at 13:37
• If you aim to arrive as a group while riding a bicyle, the slowliest team member defines the rate of advancement of the ensemble -- regardless if s/he is in front, in the centre, or trailing. Similar, it does not matter if the lowest rate is observed for the first, or the last reaction, or somewhat in the middle. Related question (and answers) here : chemistry.stackexchange.com/questions/72162/… – Buttonwood Jul 10 '17 at 14:44
• @Buttonwood I encourage you to add that as an elaborate answer. – Pritt says Reinstate Monica Jul 10 '17 at 15:31