The reaction profile diagram for an exothermic reaction shows the enthalpy change and activation energy.If the activation energy is large, the reaction will take place slowly at room temperature. If the activation energy is small, the reaction will take place rapidly at room temperature.The energy at the top of the curve is the energy of the transition state or activated complex. Bonds are in the process of being broken and made. The transition state has partial bonds, an extremely short lifetime which lasts femtoseconds, and cannot be isolated.