| MadSci Network: Chemistry |
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There's an
answer on our website that deals with the geometry of water (which also
affects its molecular polarity).
But you don't need to do a calculation to estimate the effect of weaker polarity (and weaker hydrogen bonding) on water. All you have to do is compare water, ammonia and methane, which have the same molecular mass to within about 10% (18, 17 and 16 respectively). Methane is completely non-polar. Not only does it have a low boiling point (-161°), but its melting point (-182.5° at one atmosphere pressure) is not very far below its boiling point; this means that intermolecular forces are weak enough that the liquid phase is not terribly stable relative to the gas. Ammonia is a good bit more polar than methane, but not so polar as water. Its boiling point is -77°, which shows that its intermolecular attractions are much greater than those in methane; they are also much greater than those in its family homologs, phosphine and arsine, which are much less polar than ammonia. Water is the most polar of the three and also has the highest boiling point. This is usually attributed to its unique ability to hydrogen-bond. This actually is probably true: compare the boiling point of hydrogen fluoride, 19°, even though its molecular mass is 20 and we expect it to have a higher dipole moment than water. (Incidentally, this is not true: hydrogen fluoride has a dielectric of 84, while water's is 88.) Summing up: molecular polarity is a strong predictor of boiling point, as is shown by the series methane - ammonia - water, which have almost the same molecular mass but have boiling points that change by about 100° with each step. The ability to form hydrogen bonds is strongly related to the molecular polarity, so that we can attribute at least some of the trend to hydrogen bonding. Dan Berger
Dielectric constants for a very large number of substances can be found here. |
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