[leaf345]

Hello,
I was wondering if someone could help me out.
I'm currently taking an upper year chemistry course on reaction mechanisms. In our opening lecture/review, our professor mentioned that in a multi-step reaction, the rate limiting step would be the reaction step with the greatest energy difference between the transition state and the reactant. So in this reaction co-ordinate, Step A would be the rate limiting step.

I have no problem with this, as this is what I've learned throughout high school and university. However, he then mentioned that for the reverse reaction, Step A would also be the rate limiting step. He said that it was a common misconception for students to believe that Step B would be the rate limiting step, due to confusing wording in most textbooks. I later asked him to clarify why this is, but I didn't get a satisfactory answer. Can anyone else explain to me why this is true?

[Kyle1990]

If you input enough energy to surpass the activation energy (Ea) of step B, then you still have the obstacle of getting over the "hill" of step A. So you need enough energy to pass the Ea of step A in order to have a reverse reaction. Thus, step A is the rate-determining step.