One can think of a chemical reaction as proceeding by the pulling apart of all of the atoms in the reactant molecules, and then the reassembling of these isolated atoms in new ways to make product molecules. The difference between bond energies of products and reactants, DE, is essentially the energy of the reaction:

DE = E(products) - E(reactants)

If the energy of the products is less than that of the initial molecules, the molecular energy decreases. The change in energy of the molecules and atoms during the reaction is negative, and heat is given off:

DE = E(products) - E(reactants) = <0

Conversely, if the product molecules have more energy than the starting materials, heat is absorbed, the total molecular energy increases, and DE is positive.