Intermolecular forces
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Why does CCl4 have higher boiling point than SiCl4 , both are non-polar simple molecules and so should rely on van der Waals forces which increase with an increase in relative molecular mass. Does it have anything to do with the decomposition into SiCl2 and Cl2 like in PbCl4?
Jenkin writes
No, it is simply that the van der Waals forces are greater in SiCl4 than in CCl4 - but see follow up below.
This is because there are more electrons in the SiCl4 molecule than in the CCl4 molecule. The relative molecular masses can be useful if you are asked to compare the van der Waals forces in different molecules but they are not the reason for any difference. Van der Waals forces result are present because of the temporary dipoles caused by movement of electrons, so the more electrons there are the greater these temporary dipole–temporary dipole interactions will be.
Hi, sorry this doesn't help as the data states that CCl4 has a higher boiling-point than SiCl4. Your answer suggests that SiCl4 would have a higher boiling-point due to an increase in van der Waals forces.
Looking forward to your reply ...
Jenkin replies This a very odd one and my earlier response shows my own surprise. The only reference I have found says:
‘It is rather remarkable that SiCl4 is more volatile than CCl4, especially since this effect does not appear in any other series of halides. Hildebrand ascribes this phenomenon to the comparatively large increase in molecular volume in going from the carbon compound to the silicon compound.’
(The original paper reference is J.H.Hildebrand, J.Chem.Phys., 15, 727, (1947))
I haven’t looked at this paper, but would guess that the increase in molecular volume in some way reduces the effect of the intermolecular van der Waals forces because the electrons are more ‘spread out’ than one might expect. It is perhaps also worth remembering that in the liquid state the molecules are not in a neat. close-packed structure anyway, but a jumble in which they slide around one another.
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updated: 01 December 2006
