Ina chemical reaction, no mass can be destroyed or created and no energy can be destroyed or created.
Thus, the energy released from a chemical reaction, be it heat or light, have to come from the reactants of this reaction.
Kinetic energy can be release, but not on chemical reactions, kinetic energy change is not part of chemical reactions.
The energies involved in chemical reactions are bond energies.
The energy can't come from the products, because for this energy to be released the bonds have to be broken, and the products are the result, so they will no be broken.
Thus, the energy can only com from the stored bond energy in the reactants.