Computational chemistry answers some of these questions. When you look at just a molecule by itself “what happens if you apply heat” is somewhat simple. Covale

Computational chemistry answers some of these questions. When you look at just a molecule by itself “what happens if you apply heat” is somewhat simple. Covalent bonds just break because the molecule is vibrating too much - think of a covalent bond as a flexible strut, if you put too much pressure on it, it snaps. This can result in the temporary formation of unstable molecules that then recombine. You could predict which particular bonds in a molecule are unstable based on the total structure, angles, electronegativity, polarity, etc. But of course those small unstable molecules can further breakdown, react with each other, and react with the parent molecule to form new stuff. So basically the parent molecule is part of some huge “power set” of potential molecules all interacting with each other.