Intermolecular forces
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Why is the boiling point of a mixture of propanone and trichloromethane higher than expected?
Jenkin writes:
When we say ‘higher than expected’, in this context it means ‘higher than if the mixture showed ideal behaviour’.
If it behaved ideally, the intermolecular forces between propanone and trichloromethane would be similar to those between molecules of the pure, separate liquids. But here there are hydrogen bonds between the hydrogen atom of trichloromethane and the oxygen atom of propanone. These ‘higher than expected’ intermolecular forces mean that the mixture will have a ‘higher than expected’ boiling point.
Notre that hydrogen bonding involving C-H bonds is very, very unusual. However in trichloromethane, the three electronegative chlorine atoms attached to the carbon atom have the effect of polarising the C-H bond sufficiently to allow a degree of hydrogen bonding with teh oxygen atom in propanone.
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updated: 18 January 2006
