Page 137 - Bruno Linder Elementary Physical Chemistry

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

122 Elementary Physical Chemistry

Comment: The ease with which a charge distribution can be distorted
is called the polarizability, denoted as α. The induction energy has the
form, E i = −α 1 µ 2 /R −6 . The negative sign indicates that energy is
attractive.

12.1.2.3. London Dispersion Forces
These forces exist between all kinds of molecular species (polar, nonpolar
molecules, atoms, ions, etc.)
Even though nonpolar neutral molecules or atoms do not possess
permanent dipole moments, they have instantaneous dipoles. The van der
Waals or London dispersion forces arise from the correlative motion of these
instantaneous dipole moments in neighboring molecules. For a diatomic
molecule, the dispersion energy is
3 −6
E dis = − α 1α 2 R I 1 I 2 /(I 1 + I 2 ) (12.2)
2
where I 1 and I 2 are the ionization energies of the two atoms. Note that the
dispersion energy is temperature-independent.

Comments:
1) The van der Waals forces vary as R −6 , as noted before, resulting
in V −2 dependent term of the equation of state.
2) The London forces are generally greater (stronger) than the
orientation or induction forces, but weaker than the electrostatic
forces.
3) Dispersion forces could be explained only after the advance of
quantum mechanics. Classically, all motion ceases at T =0 K. In
quantum mechanics there is motion, even at absolute zero.

12.2. Hydrogen Bonding
This exists between a H atom bonded covalently to either F, O or N and
an unshared pair of electrons on a neighboring F, O or N. (Note: Hydrogen
bondings are always the strongest of the intermolecular forces.)