Chemical EnergySome substances have chemical energy stored in them. This energy is due to the chemical binding of the atoms in such substances. A stable chemical compound has less energy than that of its individual parts because the arrangement and motion of electrons and nuclei in a compound is different from that in its individual parts. This difference in energy is called chemical energy. This energy is released or absorbed in a chemical reaction.
If the total energy of the reactants is more than that of the products, the difference in energy is released in that reaction. Such reactions are called exothermic reactions. But, if the total energy of the reactants is less than that of the products, the difference in energy is absorbed in that reaction. Such reactions are called endothermic reactions. Burning of coal or a match-stick is an exothermic reaction. Hydrolysis is an endothermic reaction.