more positive and the reaction is entropy favoured. Hence the chelate effect may be regarded primarily
as an entropy effect with some additional stability conferred by delocalization of π electrons into the
ring system. This latter effect is particularly likely if the ligands are highly conjugated.
3.5—
Solubility Equilibria
The solubility of solids in liquids is an important process for the analyst, who frequently uses
dissolution as a primary step in an analysis or uses precipitation as a separation procedure. The
dissolution of a solid in a liquid is favoured by the entropy change as explained by the principle of
maximum disorder discussed earlier. However it is necessary to supply energy in order to break up the
lattice and for ionic solids this may be several hundred kilojoules per mole. Even so many of these
compounds are soluble in water. After break up of the lattice the solute species are dispersed within the
solvent, requiring further energy and producing some weakening of the solvent-solvent interactions.
The energy needed to bring about this change can only be supplied by the solvation of the solute species
utilizing van der Waals' type ion-dipole interactions and to a lesser extent dipole-dipole reactions, etc.
The total energy available from this source, the solvation energy, may be in the range –400 to –4000 kj
mol–^1 for aqueous systems containing ionic solids. On the other hand non-ionizing solvents will have a
much lower ability to produce solvation and insufficient energy is available to break up ionic lattices
and produce a solution. Solvent-solute interactions between an ionizing solvent and a covalent solid are
small but may be large enough to overcome the low lattice energy for many solids although their
solubility will still be very low as a result of the associated nature of the solvent. Covalent solvent-
solute systems have minimal interactions and may approximate in behaviour to ideal mixtures. The
dissolution equilibrium for an ionic compound AB may be summarized by
Solubility Products
The solubility of a sparingly soluble salt is expressed by the equilibrium constant for the reaction,
equation (3.34), e.g.
Both [AB] and [S] are unchanged in solubility reactions of sparingly soluble salts, hence the above
equation may be rewritten thus
Ksp, known as the solubility product, is widely used as a measure of the solubility of sparingly soluble
salts. It should be noted that the dimensions of this constant will change according to the stoichiometry
of the reaction.