According to the law of conservation of energy, energy cannot be destroyed or created, it can only change its form, i.e. during the chemical reaction it can be absorbed from the surroundings and stored as potential energy of the products of that reaction (this is endothermic reaction) or it can be released as heat, while the potential energy of the products is less than potential energy of the reactants (this is exothermic reaction). Since in this reaction, energy is released as heat, this mean that some of the potential energy from reactants was transformed into heat, while the rest presents the potential energy of the products.
