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I’m trying to understand how breaking chemical bonds turn into kinetic energy. If AB is a chemical which breaks into A and B which zip away from each other, is it because when the bond breaks they suddenly repel each other? Another way I'm thinking of this is what happens if one single molecule of AB alone in a vacuum breaks apart.

I think the point I'm trying to make is the reaction is making the products move faster than when they started. It takes a force to make things move. So it has to be either gravity, electric, magnetic, or one of the nuclear. I'm pretty sure we can rule out nuclear and gravity.

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  • $\begingroup$ Maybe we can explain it by temperature. If PV = nRT, and the energy from the bond is turned into heat, then n, T and P must increase in the immediate vicinity of the molecule. ( n increases because AB has become A + B ) So to get back to equilibrium the molecules move apart. If I had better evidence I'd make an answer, but this is pure speculation. $\endgroup$ – user137 Aug 14 '14 at 0:10
  • $\begingroup$ Breaking increases the potential energy of the system, making lowers it. When making bonds the lost potential energy is converted to kinetic energy. A reaction involves making and breaking bonds. Perhaps you have come across the infamous ATP example and a flawed text has said that breaking bonds in ATP releases energy. It does not. The reaction involves breaking existing bonds and making new bonds for a net reduction in the potential energy of the system. $\endgroup$ – Brinn Belyea Aug 14 '14 at 3:06
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Breaking chemical bonds does not turn their potential energy into kinetic energy. All bond dissociation enthalpies are positive (for stable bonds that actually exist), which means that breaking them is an endothermic or heat absorbing process, and therefore they absorb kinetic energy (heat is a transfer of molecular kinetic energy).

What you might be thinking of is exothermic reactions, where the enthalpy of the reactants is lower than the products, and so the reaction releases heat, or increases the kinetic energy of the surroundings.

The short answer to your question is "because energy is conserved, it has to go somewhere, and heat and light are the only places it can go."

I'll try to think about a better way to explain it that isn't too far away from reality, but at the same time isn't too complicated. If I can, I'll edit this answer later to include it.

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You are thinking about it a little wrong. The fact that the bond exists means it is in a state of lower energy than when A and B are separated -- therefore you would have to input energy into the system to get the bond to break. But if you had two species A-B and C and you could form a more stable bond B-C which means it has a lower energy level, then the excess energy E(A-B) - E(B-C) for the reaction: A-B + C -> A + B-C could be released as kinetic energy of the A and B-C species moving away from each other.

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As the other answers have mentioned, a simple A-B bond dissociation reaction is significantly endothermic and does not happen spontaneously. A single molecule may break apart and its pieces may be scattered if energy is introduced in the molecule (for example, a collision with a very energetic target or the absorption of a photon may trigger strong vibrational excitations which end up breaking the molecule apart). It is, however, also possible for an unstable molecule to spontaneously rearrange into a more stable isomer, and part of the released energy may cause the rearranged molecule to shake apart.

In any event it's a case of causing a vibration to stretch atoms strongly enough that the bond between them snaps and the pieces fling away, not unlike a rubber string being pulled until it tears. Perhaps this video may help clarify what's going on, though it doesn't specifically show reactions occurring.


Edit: I somehow had completely misread the question as $\ce{A +B -> AB}$ instead of $\ce{AB -> A +B}$, so I apologize for that. My previous answer isn't actually an answer, but I figure I might as well leave the information below in case anyone is interested:

As far as I understand, most often the chemical potential energy released when two molecules react exothermically turns into vibrational potential energy in the bonds of the product molecule/entity for a small amount of time, and this vibrational energy is quickly dispersed into other molecules by collisions (timescale of a few picoseconds), turning mostly into rotational and translational kinetic energy dispersed over several molecules, as vibrational excitations are "frozen out" at temperatures close to ambient.

If the two molecules meet in an absolute vacuum and can react exothermically but have nowhere to dump the energy, then the molecules actually have a large chance of not reacting at all, even if it's extremely spontaneous! The only way they can meet and stay bound is if, during the short time they are in proximity, a low-probability event may occur where the reaction transition state emits a photon carrying away most of the releasable chemical potential energy, otherwise the molecules will just fly apart again. This is a very important consideration in astrochemistry. You can read more about this specific topic in "The Physics of the Interstellar Medium", 2nd edition, 1997 J.E. Dyson and D.A. Williams (IoP), chapter 3, section 3.4.

Actually, I should clarify that the second paragraph is only really valid for reactions of the type $\ce{A +B->C}$. A reaction of the type $\ce{A +B->C +D}$ can happen more easily as one of the species being formed can also act as a dump for excess energy.

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