HeLlow
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thermodynamically unstable diamond
Hi everyone,
Please don't flame me if my question is too basic or what.. Sorry.. I am new here.
Why is it that diamonds are thermodynamically unstable in common room temperatures and pressures? Got any good explanation for that? Or have any good
websites that can provide me with a answer? (err I mean as in not whole web page with 10 million words and that is the good explaination for my
question ok? THanks so much.
Is chlorate unstable too? It will decompose into chlorides and perchlorates is it?
Thanks so much.
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Nevermore
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diamonds thermodinamically instable?
can you explain better?
diamonds are made of C, they are instable as long as C is instable.
Nevermore!
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Mr. Wizard
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Understanding why diamonds are not the most common type of carbon. Here's a try: They are not unstable, in fact they are very stabil, as
witnessed by their immense bonding energy (strength). Sometimes to make one form of a material you have to have very high energies to arrive at the
most stabil cofiguration. For example you could have a bucket of iron filings which would not be very strong, but after melting them in a furnace and
allowing them to cool, the resulting solid iron would be tough and stabil. Carbon subjected to high pressure and temperature will also rearrange
itself into a diamond. This is just an idea analogy to get the feel of it. Imagine rolling a rock down a hill. It gives off energy and becomes more
stabil. Before it can roll though, you must give it a push to get it out of it's already stabil position higher up on the hill. This energy you
put in to get the reaction going, and is sometimes large, and the rock doesn't roll very far when it does roll. Diamonds are very nearly at the
bottom of the hill as far as carbon arrangements go, and the rocks roll only with a lot of help. This is just a little 'mind models' to help
grasp the concept.
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vulture
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Graphite is more stable then diamond, so all diamond will eventually convert itself into carbon.
Diamonds which haven't been moved for long will stain a tiny little bit because of the graphite deposited on the surface.
To get a really good grasp of this, comprehension of enthalpy, free energy and entropy is necessary.
IIRC, deltaH for C(diamond) = +1,86kJ/mol
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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Geomancer
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Nevermore: Note, he was asking about the stability of diamond, as a form of carbon, not the stability of all carbon allotropes.
Mr. Wizard: When chemists say "stable", they usually mean kineticly stable, which is what you responded to. This is because the concept of
absolute thermodynamic stability is rather useless most of the time. Any reasonably complex compound is unstable with respect to some hideous mixture
. When chemists do talk about the thermodynamic stability of complex molecules, they usually restrict the discussion to a small set of possible
configurations (say, the conformations of cyclohexane).
[Edit: apparently you did mean thermodynamic stability. In that case you're just wrong, at standard pressure.]
As for graphite, I suspect it owes its stability to the delocalization of the pi electrons, like benzene only more so. This disadvantage of this
arrangement is low density, so sufficient pressure will make other forms (diamond) favored.
[Edited on 8-3-2004 by Geomancer]
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Proteios
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it aint so much that diamonds are unstable its just that graphite is a more stable allotrope. The details of this arise MIGHT be rationalisable by
quantum mechanics...... (else the argument will involve a lot of hand waving)
diamond is still HUGELY more stable than monotomic C.... unlike the noble gases which are much more stable as monotomic species.
The interconversion between allotropes of carbon essentially does not take place due to the high activation energies.
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Mr. Wizard
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You guys know a lot more about it than I do. So am I to understand that diamonds do (slowly) convert to graphite at STP?
I can understand why the outer edge of a diamond, where the 'loose ends' of the carbon bonds leave it open to change. Is the diamond the
more stabil form only at very high pressures?
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Proteios
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Diamonds are indefinately stable....probably for the timescale on the age of the universe under our humble conditions.
Surface chemistry is different. The outside of a diamond is not carbon. For that matter most diamonds are not entirely carbon either, they have
trace amounts of all sort of things in there!
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Marvin
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The surface of a diamond is usually a monatomic layer of hydrogen. This terminates the lattice. I have my doubts about vultures description of the
outer edges turning to graphite in any human timescale.
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vulture
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Interesting...
You could be right about the graphite not forming, but something in my head tells me I read that somewhere. Could be an urban legend ofcourse.
[Edited on 9-3-2004 by vulture]
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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BromicAcid
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Diamond is thermodynamically unstable with respect to graphite:
C(diamond) ---> C (Graphite) (delta)G = -3 KJ/Mol
However the reaction is kenetically very very slow, as in we would never notice it in our life time. There are lots chemicals that are
thermodynamically unstable such as H2O2 that we can go grab off the shelf that thermodynamics say should not be there. Also the activation energy
comes into play here, for example, any form of carbon would be better off as carbon dioxide if thermodynamics had anything to say about it, however
we've got graphite in our pencils that does not spontaneously burst into flames, and for phosphorus, we see more white and red phosphoruse then
black phosphorus even though it is the most thermodynamically stable because of the activation energy required to make it.
Diamond will go to graphite but regardless, the reaction is not one we should worry about in our lifetime, however it does happen. I'm sure
there is some book out there listing a rate for the reaction, I would be interested in seeing that.
[Edited on 3/10/2004 by BromicAcid]
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