Page 135 - Bruno Linder Elementary Physical Chemistry

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

Chapter 12

Elements of Intermolecular Forces

So far, the emphasis has been on bonds between atoms and between ions.
The forces (interaction energies) can be dubbed valence forces. How do
these simple molecules interact to form larger aggregates? This occurs via
intermolecular forces. These forces account for the deviation from ideal
behavior of gases, for formation of liquids and solids, for surface tension,
viscosity and a host of other phenomena.

12.1. Types of Intermolecular Forces
12.1.1. Electrostatic Forces
12.1.1.1. Ion–Ion Forces
These take place between ions in solution. For example, NaCl dissolved in
+
water contains Na and Cl ions, which attract and repel each other. (The
−
interaction energy varies as Q 1 Q 2 /R,where the Q’s are the charges of the
ions and R is the distance between them.)

12.1.1.2. Ion-Dipole Forces
These exist between an ion and the (partial) charge of a polar molecule (see
2
Fig. 12.1). (The interaction energy varies as Qµ/R ,where Q is the charge
and µ thedipolemoment.)

12.1.1.3. Dipole–Dipole Forces
The molecules have dipole moments but are neutral. However, there is
attraction between the positive end of one molecule and the negative end
3
of another (Fig. 12.2). (The interaction energy varies as µ 1 µ 2 /R .)

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