StellarIn a multistep reaction energy profile, there are multiple humps, each representing a different elementary step. The number of humps matches the number of steps in the mechanism. Each hump has its own activation energy, measured from the point where that step begins up to its peak. This is how you can visually identify how many steps a reaction has and which step is hardest to complete.
The rate-limiting step is the step with the largest activation energy hump because it is the slowest. Even if the other steps are fast, the slowest one controls the overall speed of the mechanism. On a reverse reaction, the activation energy is measured from the products’ side of the relevant step up to the peak. So multistep profiles give you both kinetic and mechanistic information at the same time.
Example 1: If an energy diagram has three humps, then the mechanism has three elementary steps.
Example 2: If the second hump is the tallest, then the second step is the rate-limiting step.