On an energy diagram for a reaction, the activation energy of the forward reaction corresponds to which distance?

Core Concept

Activation energy is the minimum energy colliding molecules need to reach the transition state, so it is measured upward from the reactant level to the top of the barrier. The peak marks the highest-energy arrangement along the reaction coordinate. The vertical drop from reactants to products instead gives the overall ΔH. Measuring from the product level to the peak would give the reverse reaction's activation energy.