[Sungrazer (OP)]

I'm looking at a chemical equation describing how a molecule of methane fuel is burned with two molecules of molecular oxygen to produce one molecule of carbon dioxide, two molecules of water, and heat.  Where does the heat come from?  What mass of the reactants is "lost" and converted to energy?

[daveshorts]

The electrons in the water and carbon-dioxide etc are at a lower potential energy than the electrons in the methane & oxygen. So potential energy has been converted into heat in the same way as if you let two magnets snap together, energy is released which will eventually  turn into heat.

If you have two magnets apart they will be slighty more massive, than if they are snapped together, so the methane etc should be slightly heavier than the water etc.