In the energy profile of a two-step reaction, what lies in the valley between the two peaks?

Core Concept

Each peak is the transition state of one elementary step, and the minimum between them holds a real but unstable species: the intermediate. It forms in the first step and is consumed in the second, so it cancels from the overall equation. Its depth relative to the neighboring peaks influences how readily it continues onward. Intermediates occupy energy minima, the exact opposite of the maxima where transition states sit.