CB15 Vanderwaals’ forces: dependent on the surface area contact between the molecules.

Aim: To illustrate that intermolecular forces are dependent, among other things, on the surface area of the contact between the molecules.

In an apolar molecule small charge changes can arise as a result of coincidental movements of the bonding electrons. As a result very small and very temporary dipoles are induced. As a result similar charge changes can occur on neighbouring molecules.

 

   This gives rise to an attraction between the two molecules. Because the induced dipoles are weak the attraction is small. These forces are called dispersion or London forces. The term Vanderwaals’ forces is frequently used solely for these forces. In some books the term Vanderwaals’ forces covers, when present, dipole – dipole (and other multi-pole) interactions too (see CB 13).

The size of the Vanderwaals’ forces between particles is determined among other things by the contact surface area. We can verify this using the boiling point of the three isomers of pentane as our example.
In n-pentane the molecules can come into contact with each other over their whole length. Therefore, various charge shifts are possible. The Vanderwaals’ forces are thus larger here than in the other two examples.

In di-methyl propane, which is almost spherical, the contact possibilities are much smaller. There are therefore fewer charge shifts. The attraction between the particles is smaller.

The Vanderwaals’ forces between the methybutane molecules lie somewhere between those of the two previous examples.
As a result of these forces the boiling point of dimethyl propane is the lowest (TE = 10° C). The attraction between the molecules is here the least. The boiling point of methylbutane TE = 28° C and for n–pentane TE = 38° C.