A reaction, such as this one, starts with a reactant. It needs to have activation energy in order to change to the product, as shown by the hump in the graph. At the top of the hump is the transition state. This is the point where, after enough energy, the reactant will turn into the product. This reaction is exothermic. Since the product has less energy than the reactant, it had to have released energy somehow, and that energy was released in the form of heat, which makes it exothermic. (Exothermic reactions release heat, vs. endothermic reactions which absorb heat).
Bonus: a catalyst can be used to lower the activation energy by giving the reaction a “kick start,” requiring less energy in the reaction for the reactant to turn into the product.
-Lemme know if I can do anything else!
