A potential energy diagram shows the changes of potential energy of a system as it undergoes a chemical reaction.
The activation energy (Ea) represents the the minimum energy required to initiate a reaction. The reaction rate highly depends on the activation energy. The lower the activation energy, the higher the reaction rate.
The enthalpy change of the reaction, ΔH, is estimated as the difference in "heat energy" between the reactants and products (at constant pressure).
The molecule at the top of the curve or energy barrier is called the activated complex or the transition state.
Figure 1. The change in potential energy as a function of reaction progress for the reaction 2H2O2 --> 2H2O + O2. This is an exothermic reaction.