a quick scan does not indicate anything about exothermic chemical reactions converting mass into energy or endothermic chemical reactions converting energy into mass. I'll do a more thorough read later.
"1.5. The extreme size of this conversion figure was interesting in several respects. In the first place, it explained why the equiva-lence of mass and energy was never observed in ordinary chemical combustion. We now believe that the heat given off in such a combustion has mass associated with it, but this mass is so small that it cannot he detected bv the most sensitive balances avail-able. (It is of the order of a few billionths of a gram per mole.)"
Another common misconception.