Revision:Intermolecular Forces

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Intermolecular forces are ones which act between molecules. There are 3 types of intermolecular forces:

Instantaneous and Induced Dipoles

The electrons in a molecule are constantly moving around at random. At a single moment in time, there is likely to be a higher concentration of electrons at one end of a molecule than at another. This makes the specific molecule polar for that moment in time. The negative end of that molecule will repel electrons in an adjacent molecule, and the result is a very small negative charge on one end of a molecule next to a very small positive charge on another molecule - this is induced dipole interaction, which occurs in everything.

The forces involved are very weak. With more electrons, the forces get stronger, as the instantaneous diples are likely to be stronger. This is why longer chains of alkanes have higher boiling and melting points. Also, more contact a molecule has with it's neighbours, the stronger the forces, as they are closer together on average. Therefore, branched chain alkanes have lower melting and boiling points than lineral alkanes of the same carbon chain length.

Permanent Dipoles

The chemistry behind permanent dipole interactions is similar to that of induced dipole interactions, except this only occurs in molecules which have a permanent dipole.

Hydrogen Bonds

Sometimes a hydrogen atom will lose control of the electrons to the atom it's bonded to so much it's essentially a proton. This means lone pairs of electrons on atoms on another molecule will be more strongly attracted to the hydrogen atom. This only happens for H-O, H-N and H-F bonds, as O, N, and F are the only atoms electronegative enough.

Hydrogen bonds in alcohols explain their low volatility compared to alkanes with similar numbers of electrons. They also explain the trend in melting and boiling temperatures of hydrogen halides. HF, with its H-F bond, has a high melting and boiling temperature compared to the others. The other molecules can't form hydrogen bonds, so the melting and boiling temepratures increase down the group, due to the increased number of electrons and the stronger induced dipole interactions.

Solvents and Solubility

Some ionic compounds dissolve in water - this is because of the attraction between the freed positive and negative ions and the polar water molecules. For alcohols, which contain a non-polar chain of carbons and a polar OH group, their solubility in water depends on the length of the non-polar chain. The longer it is, the less soluble it is in water; while water will dissolve the polar OH part, the non-polar chain of carbons will oppose this. Compared to haloalkanes however, alcohols are very soluble in water, due to the hydrogen bonding which is possible between it and the water molecules. Haloalkanes are polar, but the energy required to break the hydrogen bonding in water is too much for the energy released when weak dipole interactions form. teehee!

(The solubility in non-aqueous solvents of compounds which have similar intermolecular forces to those in the solvent - I'd assume they are soluble if forces are similar, though I'm not sure)