# When they combine

1. Mar 26, 2005

### sid_galt

Let's say we have a Na+ and a Cl- ion moving towards each other. When they combine, energy will be released.
My question is - in what form will the energy be released and in what will the kinetic energy of the Na+ and Cl- nuclei be converted too?

My guess is that the energy will be released in the form of radiation and the kinetic energy of the nuclei will be converted to the oscillatory energy of the molecule of NaCl. Am I correct?

2. Mar 27, 2005

### chem_tr

They will not combine-they will be attracted to each one, but repelled after some degree. Let's say Na+ ion's outer shell of electrons repel the ones belonging to Cl-. This is the most simple approach, but enough for a start.

However, positron and electron does not behave like this, their combination will give radiation, you are right in this way.

3. Mar 27, 2005

### GCT

The proper way to denote this situation is by referring each element in their gaseous state $Na(g) and Cl(g)$ there are no charges associated with these elements in gaseous state. At the temperature associate with such gaseous states, these elements, I imagine, will not combine to reform $NaCl$. You'll only have the latter compound under lower temperatures, it's all temperature related.

Think about it this way. You've got some solid $NaCl (s)$, you heat it up so that they become the corresponding gases, well it makes no sense to say that they simply recombine to reform the solid, one would be neglecting the actual event of gaseous formation in the first place.

I'm no expert and I'm pretty sure that there are more fascinating details. Nevertheless, my guess is that with the internal energy possesed by the gaseous molecules, that the most that can happen is that the loss of KE will be in the form of radiation as chemtr said, that is possibly the loss of electrons, resulting in [itex]Na^+(g) and Cl^-(g)[itex]. When these two collide....also radiation, loss gain of electrons. However, solid NaCl will not form.

Last edited: Mar 27, 2005
4. Mar 27, 2005

### DH

hix,
When Cl- and Na+ combine, they dont release Energy!
- When NaCl disolves in water, it releases Energy!

When Na + Cl2 -> NaCl it release energy!

5. Mar 27, 2005

### sid_galt

I wasn't talking about positron and electron. My basic question is, when a chemical reaction occurs say Na + Cl, then in which form is the energy released such that all the momentum laws are conserved.

Is the energy released in the form of oscillatory motion of the molecule or vibrational or translational. Which one?

But I am not talking about temperatures associated with gaseous states. I am talking about low temperatures where only one Na atom and one Cl atom are present and the rest is vacuum.

6. Mar 28, 2005

### Staff: Mentor

I think non-elastic collision gives very good conceptual approximation of what will happen. Part of the kinetic energy will be preserved as the pair Na(+)Cl(-) will be moving, part of the energy will be preserved in the form of oscillatory energy as you expect.

Now, the situation is much more complicated as the pair Na(+)Cl(-) is very difficult to describe - it is not yet a crystal, it is not a covalent compound and it is not stable in a solution. Not to mention hydration and solubility effects :)

While we can talk about translational energy and oscillatory energy they are both forms of kinetic energy and the amount of energy preserved at the moment of combination in this or another form will be not important in most cases - due to frequency of collisions between particles in a solution energy will be dissipated to the equlibrium between translational and oscillatory almost instantly.

7. Mar 28, 2005

### GCT

note that temperature is essentially a function of the KE of the molecules themselves. Gaseous sodium and chlorine atoms will always possess high KE (in general). Now I understand that each of the corresponding ions can exist, and they will probably be attracted to each other, however, I don't believe that these inter attractions will be strong enough for NaCl to form; what typically leads to the stability of such elements is the lattice energy (you're talking about large range intermolecular bonding) KE of these elements in a solid is much lower. I wish I can tell you more about the details, however I need to do more research on this subject.

If such NaCl gases form, it will probably not be very stable; how would you form such gases? You would heat the NaCl solid to a very high temperature, it makes no sense at all to say that they will spontaneously recombine to NaCl solid, you would denying the actual event which led to the formation of the gases in the first place.

8. Mar 28, 2005

### sid_galt

Thanks for the help.

I agree. However I am not actually talking about real life. I am just talking about my thought experiment. Anyway, thanks for the information.

9. Mar 28, 2005

### pack_rat2

There's no such thing as a molecule of NaCl. Salt exists as a crystal with Na+ and Cl- arranged in a lattice. All of the (+) ions are attracted to the (-) ions in their general area, and vice versa. When it dissolves in H2O, the crystals break up and the ions go their various ways. The same thing happens if it vaporizes.

10. Mar 29, 2005

### Staff: Mentor

While in general you are right it is not as easy. Friend of mine did some calculations as part of his MSc thesis back in eighties and apparently before there are at least 100 ions of Na+ and Cl- what you get is something that is not crystal yet but it is not just a bunch of a molecules at the same time.

As the original question was about single ions meeting it is far too early to speak about crystal lattice. Also, there is not such a thing as 100% ionic bond so NaCl particle in gaseus form is not something entirely not possible.

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11. Mar 29, 2005

### GCT

Well, ionic bonds technically have certain energies associated with them, and the dipole is pretty much 100 percent since you taking into account the complete ionization energy(s) of one and the electron affinity (-second ionization energy(s)) of another.

The OP should investigate the physical chemistry, dynamics, what what exactly occurs during the reformation of NaCl, from decreasing the temperature (I'll have to do some research myself). Metals are quite interesting. One phenomena I remember, was that if you heat some metals, and then try to cool them...they will subsequently cool, spontaneously reheat, then cool again. Not quite sure the reason for this at this time.