kyanite
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The theoretical determination of aggregation states
What determines wheter if a chemical will be either solid, liquid or gas? Am i missing some thing, or is there no real way to find out? thanks.
[Edited on 27-7-2004 by chemoleo]
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DDTea
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This relates to intermolecular forces, which are largely determined by the molecules themselves. Speaking very generally, there are dipole-dipole
interactions, London Forces, and a special type of dipole-dipole interaction called Hydrogen Bonding.
Dipole-Dipole forces are the attraction between atoms in molecules with respective positive and negative charges, as caused by difference in
electronegativy. Take the example of H2O. Oxygen is much more electronegative than H, and thus draws electrons closer to itself which gives it a
slight negative charge and the H a slight positive charge. Thus when two water molecules come together, the Hydrogens from one molecule are attracted
to the Oxygen of another. Now naturally, this can apply to a number of other molecules--but you must be careful to distinguish between this and ionic
bonds (e.g. in NaCl).
Extending the idea of dipole interactions are Hydrogen bonds (actually, what occurs in water is an H-bond, but I just used it to make an example).
Hydrogen bonds are a special dipole interaction because Hydrogen's atomic radius is so small, therefore it forms much closer interactions than
other atoms creating an unusually strong bond. This occurs especially if the other atom has a small atomic radius also, e.g.: Fluorine, Oxygen,
Nitrogen.
London dispersion forces are based off the idea that since electrons move around so much, that at some point they are dispersed unevenly around the
atom causing one end to have a negative charge and another end to have a positive charge. This interaction is much more pronounced in larger
molecules and atoms (this is why Iodine is a solid while Fluorine is a highly volatile gas).
As you can imagine, as intermolecular forces are stronger, the molecule moves more from a gas (which has little intermolecular interaction) to a
solid (with much intermolecular interaction.)
Another thing to remember... As Carbon chains get longer and more massive, they move from the gas phase to the liquid phase. Think of Methane versus
Hexane or Octane, and even more the case with Tar and such. This is because lengthy Carbon chains can become tangled in each other.
I hope this explains everything adequately. 
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kyanite
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whoah, looks like i have alot more chemistry to learn... 
so i guess it that means that it'd be really hard and even then it wouldnt be easy to tell wether it is or isnt...
Thanks for the information!
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Marvin
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I dont disagree with whats been said apart from possibly the tangled explanation for the high boiling point of tars.
Its helpful to remeber that a compound isnt solid liquid or gas - in so much as any one of these at a particular temperature.
The biggest factor typically is the size of the molecule, more specifically its molecular mass. Heavier molecules need more thermal energy to change
states solid>liquid>gas. The text book explanation escapes me currently, but its sufficiant to remeber that at the same temperature Xenon atoms
move around very slowly, and helium atoms move around very quickly - entirly due to the mass difference. Some books use the words 'escape
velocity' particually for liquid>gas transisitons to try to impress how speed rather than energy is more important.
Then add to that trends for types of compound. At room temp, ionic compounds (salts) will be solid at room temperature becuase the whole crystal is
basically one big molecule. Alcohols will almost always be liquids, alkanes will obey the simple mass rule well, small molecules with double bonds
will tend to be gasses or low boiling point liquids.
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BromicAcid
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| Quote: |
Then add to that trends for types of compound. At room temp, ionic compounds (salts) will be solid at room temperature becuase the whole crystal is
basically one big molecule.
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Remember that a very unsymetrical pairings in ionic compounds can cause the lattice energy to be so incredibly low that the ionic compound may be a
liquid at STP. Although this is more of an exception, ionic solids are by far the most common.
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Magpie
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This discussion has me wondering why mercury is a liquid at room temperature, at atomic weight 200.6. My textbook says that it freezes at -38.9 C,
but does not say why. Does anybody have an explanation for this perversity?
The single most important condition for a successful synthesis is good mixing - Nicodem
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BromicAcid
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The General Chemistry Online FAQ has an answer to the question "Why is mercury a liquid at STP?"
The climax of the reasoning:
| Quote: | Mercury hangs on to its valence 6s electrons very tightly. Mercury-mercury bonding is very weak because its valence electrons are not shared readily.
(In fact mercury is the only metal that doesn't form diatomic molecules in the gas phase).
Heat easily overcomes the weak binding between mercury atoms, and mercury boils and melts at lower temperatures than any other metal. The thin valence
electron sea makes mercury's ability to conduct electricity and heat much poorer than expected for a metal at that position in the periodic
table. |
Curtesy of ask.com
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