London Dispersion forces
A German physicist Fritz London introduced these forces in 1930.
“The forces of attraction between two temporary dipoles created due to electronic cloud displacement or forces of attraction between temporary i.e. instantaneous and induced dipoles are called London dispersion forces”.
These forces generate due to two reasons:
- When two atoms or molecules come closer, their electronic clouds influence each other i.e. they repel each other. So electronic clouds of the two, move away and as a result, instantaneous dipoles are created.
When an instantaneous dipole comes closer to a symmetrical molecule its electronic cloud repels or its nucleus attracts the electronic cloud of the symmetrical molecule and as a result, a dipole is induced in it. This dipole is called an induced dipole.
A force of attraction is generated between instantaneous dipole and induced dipole, which is known as the London dispersion force or dipole-induced dipole forces.
- London forces are stronger in n-butane than that in isobutane. Long straight chain molecules can come closer to each other as compared to branched-chain molecules so London forces are more operative in straight chain compounds. In branched chain molecules, there are fewer points of contact of molecules with each other so London forces in them are less operative and are weak. Thus the boiling points of straight chain molecules are higher than that of branched-chain molecules.
- These forces are short-range and short-lived forces.
Factors affecting strength of London dispersion forces:
Number of electrons:
More is the number of electrons stronger will be London forces e.g. order of strength of London forces in the following is
Cl2 > O2 > N2 > H2
Number of electrons 34 18 14 2
Size of molecules:
Bigger the molecules stronger will be London force e.g. London forces among hydrogen molecules (H2) is stronger than that in helium molecules (He). Both molecules have two electrons but the size of the H2 molecule is larger than that of He molecule so H2 have stronger forces than He.
More is the number of electrons or the larger the molecular size stronger will be the London forces and vice versa. The following table shows that going down a non-metal group, molecular size, as well as the number of electrons, increases so London forces increase and in turn boiling points increase.
|Zero Group||B.P ( oC )||Group VII-A||B.P ( oC )|
London forces are stronger in long and straight chain molecules as compared to that in short and branched chain molecules.
n-pentane is a straight chain molecule so dispersion forces in it are stronger than the other two isomers, so its boiling point is comparatively high. The other two isomers are branched chain molecules so their boiling points are less, furthermore, neo pentane is more branched so its boiling point is least.
e.g. (ii) n-butane also called butane has boiling point higher than that of isobutane (also called 2-methyl propane). The boiling points of the two are –0.5oC and –11.7 oC respectively.
Long straight chain molecules can come closer to each other as compared to branched-chain molecules so London forces are more operative in straight chain compounds. In branched chain molecules, there are fewer points of contact of molecules with each other so London forces in them are less operative and are weak. Thus the boiling points of branched-chain molecules are lower than that of straight chain molecules.
Presence of London Dispersion forces:
London forces are the only forces that exist in all molecules. Although they are present in all kinds of molecules but are more prominent in molecules, which have no other forces e.g. Cl2, H2, O2, noble gases etc.
Comparison of Strength of Intermolecular forces:
- In the case of hydrides of Group-V, VI& VII elements, the hydride of the first element in each group has extraordinarily high boiling point. Hydride of the first element in these groups has hydrogen bonding which is a strong intermolecular force so the boiling point is high. Other hydrides have dipole-dipole forces, which are comparatively weak so their boiling points are low.
- Boiling point of water is higher than ethyl alcohol. Although both have hydrogen bonding that of water is stronger than that of ethyl alcohol due to more number of “O-H” groups in water. Hence the boiling point of water is higher than that of ethyl alcohol.
- Fluoromethane and ethane both have an equal number of electrons so London forces in the two compounds are equal but fluoromethane in addition to London forces also has dipole-dipole forces so its boiling point is slightly higher.
- Trichloromethane (Chloroform) is a polar molecule so it in addition to London forces also has dipole-dipole interactions whereas tetrachloromethane is non-polar so it has only London forces. But tetrachloromethane (Carbon tetra chloride) has more number of electrons than trichloromethane so the later has stronger London forces than the former. London forces of tetrachloromethane dominate the diploe-diploe forces of trichloromethane It is this reason that tetrachloromethane has a higher boiling point than trichloromethane.
Van der Waal’s Forces: Dipole-dipole interactions and London dispersion forces collectively called Van der Waal’s forces. These are weaker than hydrogen bonding.