An energy profile for a two-step reaction shows the second peak rising higher above its own starting valley than the first peak rises above the reactants. Which step controls the overall reaction rate?

Core Concept

Each step's activation energy is measured from its own starting level up to its peak. The taller relative climb for Step 2 means the larger Ea, so fewer molecules complete that step per unit time. The slower step is the rate-determining one, regardless of where it sits in the sequence. Overall progress therefore waits on the second barrier.