Page 136 - Bruno Linder Elementary Physical Chemistry

August 19, 2010 10:40 9in x 6in b985-ch12 Elementary Physical Chemistry

Elements of Intermolecular Forces 121

Ion–dipole interaction.
Fig. 12.1

Dipole–dipole interaction.
Fig. 12.2

12.1.2. van der Waals Forces
12.1.2.1. Orientation Forces
These exist between rotating dipolar molecules that arise from the correl-
ative rotational motion of the permanent dipole moments in neighboring
molecules. The interaction energy has,

2 2 2 6
E r = − µ µ /(R kT ) (12.1)
1 2
3
6
2
The inverse R accounts for the a/V term in the van der Waals equation
of state. Note that this interaction energy is temperature-dependent. The
negative sign indicates that the interaction energy is attractive.

12.1.2.2. Induction Forces
These forces exist between a polar and another (polar or nonpolar)
molecule. A polar molecule can induce a dipole moment in another molecule
by distorting its charge distribution.