Page 310 - Instant notes

Physical chemistry 296

energy of the bonding orbitals between the two electronegative
elements, by the inclusion of an extra term resulting from the
hydrogen 1s orbital, in the linear combination of molecular
orbitals.
Related topic Strong solid phase interactions (H5)

Weak intermolecular forces

The distinction between weak and strong binding energies is an arbitrary one, but
−1
binding energies of >200 kJ mol , limited to covalent or ionic bonds, are generally
regarded as strong. The forces between molecules are regarded as weak with typical
−1
energies significantly less than 50 kJ mol .

Dipolar interactions

When a material consists of molecules with permanent electric dipoles (see Topic H5),
the electrostatic force between molecules changes their potential energy. The energy,
U(r), of the electrostatic interaction between two parallel dipoles with dipole moments µ 1
and µ 2, a distance r apart, is given by:

where θ is the angle between the dipole axes and the vector linking the centers of the two
dipoles. This potential is a maximum if θ=0°, when the positive pole of one dipole
experiences only the negative pole of the second. The potential is zero if θ=90°, as both
negative and positive poles of one dipole are experienced equally, and so cancel out, at
the second dipole.
In real systems, the strength of the interaction is usually less than, or comparable with,
the thermal energy, and the dipoles are able to rotate in three dimensions. At low
temperatures, the thermal energy is insufficient to overcome some alignment of the
dipoles, whereas higher temperatures have the effect of averaging out the potentials of the
surrounding dipoles. Under these conditions, the expression for the potential experienced
by a dipole, at temperature T, alters to:

Polarizability

Regardless of whether or not a molecule or atom possesses a permanent dipole moment, a
dipole moment, resulting from charge separation, may be induced in the molecule