Van der Waal's forces -- Electrons will not be evenly spread around an atom/molecule at any given time, meaning the molecule will have a slight +ve charge on one end, and a -ve at the other. this temporary state may cause attraction between two molecules, pulling them together (also known as London dispersion forces). The magnitude pof Van der Waals force depends on the relative molecular mass, high mass produces a larger force.
Boiling points of the alkanes data and alkenes data
These very clearly illustrate the effect of increasing Van der Waals attractions as the relative molecular mass increases. The influence of branching in the alkanes can also illustrate the effect of different surface areas on the Van der Waala forces (the more branching the lower the b.p)
Dipole-dipole forces -- Polar molecules, when properly oriented, will attract each other as a result of this. Stronger than van der Waal's forces.
Hydrogen bonding -- When hydrogen is bonded to nitrogen, oxygen or fluorine, a very strong dipole is formed, making the hydrogen very strongly positive. This hydrogen is then attracted to the lone pairs on other similar molecules (nitrogen, oxygen and fluorine all have lone pairs) forming a hydrogen bond, which is stronger than van der Waal's or dipole-dipole, but weaker than covalent bonding.
The effect of hydrogen bonding on intermolecular forces can be demonstrated very well by studying the boiling points of the group 6 hydrides
b.p. comparison of main group hydrides
Order of priority
Hydrogen bonding strongest
Dipole -dipole interactions
Van der Waals forces
Hydrogen bonds result from hydrogen bonded as described above. This results in molecules with hydrogen bonding exhibiting stronger intermolecular forces, ie higher boiling/melting points etc. eg H2O has a higher bp then H2S due to hydrogen bonding, and so on down the strength list.
Water has a very high melting and boline point due to extensive hydrogen bonding....
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