The activation energy is the height of the energy barrier of the reaction. The transition state is the point of maximum energy on the diagram which represents a species possessing both reactant-like and product-like properties. Because it is so high in energy, the transition state is very reactive and can never be isolated due to its extremely short lifetime. The relative energy of the reactants and products, the ΔE on the diagram, determines whether the reaction is exothermic or endothermic. A reaction will be exothermic if the energy of the products is less than the energy of the reactants. A reaction is endothermic when the energy of the products is greater than the energy of the reactants. The is for an exothermic reaction. Below is a reaction coordinate diagram for an endothermic reaction.