Multistep Reaction Energy Profile

A reaction energy profile plots potential energy against reaction progress. One smooth hump means one step: reactants climb over a single activation energy barrier to products. Two humps mean two steps, because the system has to climb, come down into a dip, then climb again. That dip is the intermediate, a species that actually exists for a short time, fully formed, before it reacts further. It is not a transition state. Transition states sit at the peaks and never exist as stable species; intermediates sit in the valleys and do.

Each hump has its own activation energy, measured from the energy of whatever the reaction starts that step with, up to that hump's peak. The step with the taller peak, measured this way, has the larger activation energy and is slower. Since the slowest step controls the overall rate, that taller peak marks the rate-determining step. There is no rule that later steps are automatically slower or faster. A reaction could easily have a fast first step over a short hill into a deep intermediate valley, followed by a slow second step over a tall hill. You have to look at the actual barrier heights, not guess from order.

The overall enthalpy change for the whole reaction is just the energy of final products minus initial reactants, read straight off the far ends of the diagram, and it equals the sum of the enthalpy changes of the individual steps, by Hess's law. This profile is where the rate law and the rate-determining step idea from earlier in kinetics become visual: the mechanism's slow step is literally the tallest mountain on the path.