Re: The heat given out in a chemical reaction is due to what?

The total mass of the reactants in an exothermic reaction (or any reaction, for that matter) is identical to the total mass of the products. Chemical reactions simply involve the redistribution of electrons from some atoms to other atoms. There are no nuclear reactions involved, and in the absence of nuclear reactions, no change in mass is possible.

If the energy released by an exothermic reaction isn't created by the conversion of mass into energy, where does it come from? The answer is readily apparent once you realize that the total energy after the reaction must be equal to the total energy before the reaction. The reaction proceeds from reactants to products because the products have lower total internal energy than the reactants. This energy is contained within the molecular structure of the species in question. It is a form of potential energy which derives from the chemical bonds within the molecules and from the particular geometry taken on by the molecules.

Since the products of the reaction have lower total energy than the reactants, and since energy cna not just disappear, the energy must go somewhere. The energy is released (this is the release of energy which makes the reaction exothermic), and it usually goes into raising the temperature of the reaction vessel.

I hope this helps answer your question.