2. Intermolecular potentials
We will now consider intermolecular potentials, particularly intermolecular pair
potentials, the interaction between two isolated molecules. These pair potentials can be
used as a starting point to consider multibody interactions, such as in liquids and solids.
Interactions between closed shell molecules – that is, not chemical bonding – are termed
Van der Waals forces after the 19th c. Dutch physicist Johannes v.d. Waals.
Van der Waals forces include:
How strong are intermolecular interactions? These interactions are generally much
weaker than covalent or ionic bonds. Compare the strength of interaction, V, with the
thermal energy of the molecules.
2.1 Coulomb forces or charge-charge interactions
We have already seen that the Coulombic potential energy, and the force, between two
charged particles varies as a function of the separation r:
F
Q1Q2
40 r r 2
and
V
Q1Q2
40 r r
Exercise 2.1
Compare the interaction energy of Na+ and Cl- , when separated by 2.76 Å, to the thermal
energy at 27°C.
Ionic compounds usually occur as ionic crystals in which there are many interactions
between ions in the crystal lattice. How can we determine the total energy of interaction
in an ionic crystal?
We need to know:
E.g., NaCl