Intermolecular forces
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Why is CH3F insoluble in water? if something like propanone is soluble whilst having only a C=O bond, why can hydrogen bonds form between water and the O from C=O but not between water and the F of C-F, when the C-F bond would be more polar than the C=O bond?
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Ulex says
Fluoromethane is a gas at ordinary temperatures. Like most gases, its solubility is strongly influenced by both temperature and pressure. At low temperatures and high pressures the solubility of fluoromethane would increase dramatically from the 2.295 g dm-3 or so which is sometimes quoted for standard conditions. There is a limit to this, however, a limit set by the water which becomes solid below 273 K, fluoromethane remaining a gas at this temperature (bpt 195 K).
The comparison with propanone and water is not valid because, at all reasonable temperatures, both are liquids.
If you think of all the familiar examples, most gases have limited solubility in water at ordinary temperatures. The ones which do dissolve freely (ammonia, hydrogen halides, sulphur dioxide etc) do so because they undergo some actual reaction with the water.
The solubility of gases in liquids (in the absence of chemical reaction) is described by Henry’s Law (which you might like to research). Thinking of this in entropy terms, a substance in the gaseous state has a much higher entropy than the same substance dissolved in a liquid; the tendency is, therefore, for a gas to remain undissolved. The entropy changes resulting from molecular interactions are not great enough to upset this effect.
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updated: 26 May 2009
