Is this reaction endothermic or exothermic? Explain your answer in two ways: first, using the energy values, and second, by referring to the shape of the graph. The exothermic reaction this reaction is the activation energy for the reaction is the energy at the peak of the diagram. The far left part of the curve is horizontal and has an energy value of 50 kJ. The curve then rises to a peak of 250 kJ before falling to a horizontal region at 200 kJ.