[Mike Dacre]

Hello all,

Apologies if this is the wrong place to post, or if this question has been answered already, I know it is a very simple question, but I couldn't find it easily by searching.

I am trying to figure out what actually happens physically when two entities for a bond and release 'thermal' energy. As an example, let's look at 2Cl reacting to form Cl₂.

I understand that in this case, each Cl atom has some amount of potential energy, and that when the two bond, their potential energy decreases, some energy gets invested into the bond energy, and some 'thermal' energy is released.

What I want to know is how is this energy actually being released? Does the Cl₂ molecule vibrate faster than the two Cl atoms and thus have more kinetic energy, resulting in higher energy collisions with neighboring molecules and thus an increase in the average kinetic energy (temperature) of the whole system? Alternatively, are photons of electromagnetic energy being released by the bond formation, similar to what happens when an electron moves from a higher to a lower orbital?

Is there a good source for me to learn more about the physics of chemical reactions?

Thanks!

[Corribus]

This is a complex topic, and entire textbooks have been written on energy relaxation pathways in molecular systems. If you're really a glutton for punishment, I suggest Nick Turro's "Modern Molecular Photochemistry". It's a fantastic textbook, though perhaps a bit advanced for your needs, that will sadly never receive an update because Turro recently passed away.

Anyway, keeping things brief: in isolated atoms the only way for excited electrons to relax is by photon emission. Molecules, on the other hand, can transfer excited electronic energy between isoenergetic vibronic and rotational states, a process known as internal conversion. Even here, however, if the molecule is isolated, the only way to release energy is by electromagnetic radiation - and while excited electronic states have reasonably fast rates of spontaneous emission, excited rotational/vibrational states have extremely long lifetimes, so the probability of spontaneous emission in a vibrationally excited molecule is quite low. Of course, molecules are never truly isolated, so there are competing relaxation pathways that take the form of partial transfer of excess kinetic energy through inelastic collisions with nearby molecules. This process is technically pressure dependent (basically, the likelihood of a collision taking place, plus the likelihood of energy being transferred) but generally extremely fast in condensed phases. The result is dispersal of excess energy in the form of heat (energy distributed among the various vibrational and rotational modes of a body of molecules). In most cases, internal conversion and heat dispersal is faster than radiative decay, which is why only in a select few molecules is fluorescence observed to an appreciable degree. As you might imagine, there are a lot of rules that impact the various rates of energy dispersal, but... brief, brief.

When two atoms come together to form a molecule, the molecule is initially in an excited state. In the absence of other nearby molecules, what would typically happen is that the two atoms would bounce off each other and go on their merry way, because they need to lose energy to become in a bound state. In a classical model, they simply have too much kinetic energy to stick together. In a real system, other nearby atoms/molecules can collide with the excited complex and steal just enough of its energy so that it becomes trapped in a potential well, such that the two component atoms no longer have enough kinetic energy to fly apart.  At this point they're bound together in molecule form and will shed the rest of their energy via the normal ways that vibrationally excited molecules do so - primarily through further collisions with the surrounding molecular bath, dispersing that extra energy into the surrounding environment in the form of  heat. Unless of course a collision transfers enough kinetic energy to break the complex apart again.

So, the quick answer is that while electromagnetic radiation can certainly be used to drive bond breaking, bond formation most often results in release of heat because photon emission usually just isn't competitive with energy transfer via collisions. The exception is the case in which the product molecule is electronically excited, in which case all bets are off, because now the rate of photon emission is competitive with nonradiative decay. (This is essentially what happens in the luminol reaction.)

(If that was too complicated an answer, I can try to simplify it. I don't know what your education level is. You have mixed some concepts in here such as electronic relaxation and vibrational relaxation, which also doesn't make your question straightforward to answer. The most common case is that bond formation results in vibrationally excited molecules in which all electrons are in their lowest energy states, so you don't have to worry about electrons moving from one orbital/state to another. It takes quite a bit of energy to move electrons around, more than is usually available from latent heat available during a chemical reaction. You'll have to know a bit of statistical mechanics to really understand why that's the case, though.)

EDIT:

Scratch that. Apparently Turro's book does have a modern edition, but with a different title. How did I not know this? I just ordered a copy!

http://www.amazon.com/Modern-Molecular-Photochemistry-Organic-Molecules/dp/1891389254

[Mike Dacre]

Hi Corribus,

That was a fantastic answer, thank you very much. Just the right amount of detail, and now I am hungry for more and thinking of getting a copy of Turro's book.

Thanks again!