Pretty lame for me to comment on a 6 year old discussion, but I found myself asking the same question Dr. Syntax was asking and then realized that no one really answered the question. Here is my take: In an exothermic chemical reaction mass can be "lost" in two different ways: One way is in the form of photons leaving the system as radiation created when electrons change energy levels. Secondly, exothermic reactions can also lose heat (and yes relativistic mass) through conduction, convection and advection which all result in a loss of mass in reactants as the atoms of the reactants lose kinetic energy and transfer that energy to their immediate environments as heat--this type of heat exchange excites the atoms of adjoining matter outside of the reaction increasing the kinetic energy of those atoms outside of the reaction. Thus, the lowered total kinetic energy of atoms of the reactants (becoming a product) results in lowered total mass of those atoms, and the higher kinetic energy of the atoms outside of the reaction results in higher mass of those atoms---as we know that a particle's mass (relativistic mass) changes with its velocity.
