h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Just idly wondering...
Are there chemical reactions which require a low (or indeed, very low) temperature? Obv. there's all sorts of ones that only occur at or above certain temperatures, or which are more vigorous at higher temp, just wondering if there are ones which work in reverse.

Would seem, to my layman's mind, that they wouldn't, since temp is providing energy for things to happen, so if you take energy out of the system then it is going to be more difficult for things to happen. But then physical changes do occur at low and very low temps so presumably those changes in physical properties of the matter could change the way it interacts and thus produce reactions which would not occur at higher temps?

Absolutely.

There are many chemicals that we work with that are *extremely* reactive.

Use them at room temperature and you will get violent side reactions that do something very much other than what you want.

In order to do them under controlled circumstances you need low temps.



If I want to use liquid ammonia (in the absence of water) in a reaction then I have little option but to do it at low temp since it boils at -33 degrees C



In organic chemistry where carbon based molecules are made then shape and connectivity of the molecule can be important. Sometimes the one you want it formed more quickly than the one you don't. However it may be formed in a reversible reaction that will slowly revert back and then produce an alternative - but more energetically stable molecule that you don't want.
In such circumstances you want to do it cold - that way you maximise the amount of the faster produced desired-product and minimise the reverse reaction (and the alternative forward, slow reaction) so produce the more stable product.

I could go on...

But I guess you have your answer.

Cheers Orcus.
Although you mention the lack of side reactions that you don't want - I presume that means that the reaction you do want occurs both at low and high temp (relatively speaking) so you are using temp as a way of filtering out undesirable reactions?

But are there reactions that only occur at lower temp? That, side reactions and reverse reactions aside, simply don't occur at higher temps?

By way of further explanation of my thought processes - I was thinking on from things like bose-einstein condensates where the physical properties are markedly different? Presumably such a form of matter would interact rather differently? Or would it not interact at all since it is so cold? The other was super-conductors at low temperature where the physical properties have changed to allow resistance free conductance. Does this also change how such a material state of matter would react with other matter?

To give you an idea of what I mean by cold by the way - I routinely use temperatures of -78 °C in the lab.

This is widely used in chemistry since it just happens to the temperature you get when you saturate the solvent acetone with solid CO2 (dry-ice) so it is very convenient to make.
The lowest temperatures typically used in what I do are about -110 °C.

That's if your not counting freezing things in liquid nitrogen to do stuff like freeze-dry (not a reaction really so I don't think that counts).

Also, we routinely use superconducting magnets (in NMR machines - same thing as MRI) - these are not superconducting at room temp - they need to be cooled with liquid helium - that's *really* cold. Although not a chemical reaction as such - this property does rather illustrate that things can have different physical (and hence chemical) properties at low temperatures.

on the superconducting thing.

Since only really liquid helium out of normal matter forms a Bose-Einstein condensate and helium is completely inert then there are no chemical reactions to deal with with that.


Mmmm, the sorts of reactions I'm talking about will *in principle* still occur at high temps but in practise won't as it's difficult to filter out a reaction when the rest is going *KABOOM*
But yes you are right on that front.

heh

Cheers for the detailed reply!

"it's difficult to filter out a reaction when the rest is going *KABOOM*"

You also have the case in chemical engineering, when producing a whole range of chemicals including ammonia and nitrogen peroxide, that lower temperatures will increase the yield - in other words the percentage of the reactants that are actually converted to the products. Of course, lowering the temperature, as you rightly point out, will slow down the reaction kinetics, and a large part is finding the right compromise between a fast rate and a suitably large yield.

If you're talking super-cool temperatures (ie. close to 0K), then the whole concept of 'chemical reaction' goes up the spout because electrons (and, indeed, matter in general) behave in rather bizarre ways. Reactions are more likely to be driven by other factors - nuclear stability and nuclear forces - but I would need to look up some of the details.

B

As regards superconductors - the first superconducting alloys were operating at temperatures low enough that any chemical reaction happened so slowly as not to worry anyone. Nowadays, the T(c) - the maximum temperature at which a specific material superconducts - is being raised by new oxides and precision-moulds, and there is indeed a subsect of superconduction theory that deals with making brackets and suchlike that needs to come into close contact but won't actually react with the superconducting material.

B

"Are there chemical reactions which require a low (or indeed, very low) temperature? Obv. there's all sorts of ones that only occur at or above certain temperatures, or which are more vigorous at higher temp, just wondering if there are ones which work in reverse."

Yes, look up association reactions. For example, nitrogen dioxide gas at low temperatures (and/or high pressure) will dimerise to dinitrogen tetroxide NO2 + NO2 --> N2O4. Nitrogen tetroxide is significantly more reactive than NO2. Heat it up and the nitrogen tetroxide will fall apart back to NO2.

The following link may be pertinent:

http://en.wikipedia.org/wiki/Le_Chatelier%27s_principle#Effect_of_change_in_temperature