For an exothermic reaction, the potential energy diagram is most likely to be a curve with a negative slope indicating that the activation energy has a negative value. An exothermic reaction is a reaction in which energy is given off, or, in other words, a reaction that has a ΔH < 0. On the other hand, an endothermic reaction the energy diagram will be a curve with a positive slope indicating that there is a positive value in the activation energy. In this type of reaction energy is absorbed in the reaction from the environment.