The attractive forces between molecules are called intermolecular forces. Intermolecular attractions hold two or more molecules together. Intermolecular attractions can be of the following types:
1) Dipole-dipole interactions
2) London forces (Dispersion forces)
3) Hydrogen bonds.
Intermolecular attractions are often known as Van der Waal's forces. These forces are much weaker than intramolecular forces (covalent bonds). The strength of intermolecular forces is in the range1 to 40 kJ mol
-1 while intramolecular forces are in the range 150 to 800
kJ mol-1. The strength of intermolecular forces decreases rapidly with increase in distance.
Dipole-Dipole Interaction
These are the interparticle forces among the polar molecules. These forces arise due to electrostatic interactions between the oppositely charged ends of permanent dipoles. The positive end of one molecule attracts the negative end of the other molecule and vice versa as shown in fig.1.32.
fig 1.32 - Dipole-dipole forces between polar molecules
