I know that fire is a chemical reaction, but how does having enough energy actually start a chemical change? What is happening on a quantum mechanical level? In layman's terms please.
On quantum level, an electron state changes from the one that used to be lowest in energy to a newly available state that's even lower in energy. That breaks the old chemical bond and forms a new one. The new state becomes available because a certain atom got very close to the one that electron belonged to. To get atoms close enough, certain amount of energy, called activation energy, is required. Typically, activation energy is supplied by heat.
Last year someone here asked a similar question. See https://www.physicsforums.com/showthread.php?t=487638
If you go to the youtube movie you will hear a five-minute talk by Physicist Richard Feynman all about fire!
If you want a technical description with lots of details, see:
CHEMISTRY AND PHYSICS OF FIRE
www.maiif.net/pdf/fire_chapter1.pdf
I did not find any descriptions of quantum mechanical processes in fire. If there are any someone else needs to show all of us!