For an exothermic reaction drawn on an energy diagram, the activation energy of the reverse reaction is measured

Core Concept

The reverse reaction starts from the products, so its barrier runs from the product level up to the same transition-state peak. For an exothermic forward reaction the products sit lower, which makes the reverse barrier visibly taller than the forward one. The difference between the two barriers equals the magnitude of ΔH. Reading diagrams correctly means anchoring each activation energy to the side the reaction begins from.