Page 163 - MODERN ELECTROCHEMISTRY

ION–SOLVENT INTERACTIONS 103

Fig. 2.33. The electrical equivalence between a water molecule and a
quadrupole.

to the lone pairs near the oxygen atom. In fact, from this intimate viewpoint, the charge
distribution in the water molecule can be represented (Fig. 2.33) by a model with four
charges of equal magnitude q: a charge of near each hydrogen atom, and two
charges each of value near the oxygen atom. Thus, rather than consider that the
water molecule can be represented by a dipole (an assembly of two charges), a better
approximation, suggested by Buckingham (1957), is to view it as a quadrupole, i.e.,
an assembly of four charges. What may this increase in realism of the model do to the
remaining discrepancies in the theory of ion-solvent interactions?

2.15.5. The lon–Quadrupole Model of Ion–Solvent Interactions

The structural calculation of the heat of ion-solvent interactions involves the
following cycle of hypothetical steps: (1) A cluster of water molecules is removed
from the solvent to form a cavity; (2) the cluster is dissociated into independent
water molecules; (3) n out of water molecules are associated with an ion in the
gas phase through the agency of ion–dipole forces; (4) the primary solvated ion thus
formed in the gas phase is plunged into the cavity; (5) the introduction of the primary
solvated ion into the cavity leads to some structure breaking in the solvent outside the
cavity; and (6) finally, the water molecule left behind in the gas phase is condensed
into the solvent. The heat changes involved in these six steps are
and respectively, where, for