The heat is on

On somebody asked

“Where does the heat from a burning candle originate?”

The answer talks about “the energy stored in the bonds” being released. So far so good. But what does that mean, really?

I remember being slightly dissatisfied with that answer when I took chemistry. It was a while before the answer occurred to me; when it did, I wondered why my chemistry teachers hadn’t explained it to me. (Or maybe they had, and I was asleep…)

Remember potential energy from physics? The classic example involves a weight held above the ground; it has potential energy because of its distance from the earth, the energy that was required to lift it there. When released, it falls to the ground, and part of that potential energy is converted to kinetic energy.

A more general way of thinking about that example is that two objects (in this case the earth and the weight) are attracted to each other (in this case by gravity). The farther apart they are, the more potential energy they have as a system. Bring them closer together, and some of that energy is released, converted into another form of energy.

OK now, think about molecules, say hydrogen and oxygen — two atoms in each molecule.  Look a little closer and you see protons and electrons attracting each other. What happens when the hydrogen burns, producing water, is that the atoms are rearranged in a way that reduces the average distance between proton and electron. That difference in distance corresponds to an energy difference — the energy it would take to pull the electrons out to that greater distance.

Two hydrogen atoms sharing their electrons share them equally — the average distance between proton and electron is the same. Same for the two oxygen atoms in an oxygen molecule. But when oxygen atoms and hydrogen atoms share electrons, as they do in water, the eight protons in the oxygen nucleus are able to pull the hydrogen atoms’ electrons in closer. That difference in distance between protons and electrons is a difference in potential energy, which is released as light and heat.

OK, that’s a simplification. For starters, the nuclei of atoms repel each other, and so do the electrons. But it’s at least part of the story, and it seems like a useful intuition about chemistry, about why some reactions release heat and others require that energy be added. At least my son thought so!


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