Statement of the principle of conservation of energy: “Energy cannot be created or destroyed in a chemical reaction or physical process. It can only turn from one form to another."
In chemical reactions energy is released, if they are exothermic, and absorbed in the case of being endothermic. The reactants of an endothermic reaction, plus a certain amount of heat (energy) give the products. It can be shown that the energy of the products is equal to the sum of the energy that the reactants had plus the heat input.
Experience indicates that in any physical or chemical process energy is conserved, although it can pass from one form to another. Chemical energy can be converted into heat, light, electrical energy, etc.