Intermolecular forces
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Please could you explain why iodine is not soluble in water but is in cyclohexane and why trichloromethane is soluble in propanone, in terms of intermolecular forces/attractions?
Igloo says
The mixing of two covalent substances involves not only the creation of new intermolecular bonds between the molecules of each but also the destruction of the intermolecular forces within each of the original substances.
In the case of the first example, fairly strong Van der Waals forces hold molecules of pure iodine together and strong hydrogen bonds exist between water molecules. Iodine would be unable to form anything other than weak Van der Waals forces with water molecules, so there is no energy available to break the relatively forces of attraction in each of the original substances. This is why no mixing occurs.
Mixing is possible with cyclohexane because the Van der Waals forces between its molecules and iodine molecules are strong enough to allow for breaking the weak Van der Waals forces in both cyclohexane and in water as the two sets of molecules mix.
In your second example, dipole-dipole forces are the strongest type of intermolecular attraction between propanone molecules and this is true also of the molecules in pure trichloromethane. When trichloromethane and propanone are mixed the new intermolecular forces formed are also dipole-dipole attractions, and the energy released in forming these is sufficient to overcome the existing intermolecular bonds in each of the pure liquids. This is why they mix.
There is a lot of truth in the phrase “like dissolves like” (but this wouldn’t be enough of an answer in an exam paper!)
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updated: 16 February 2005
