Solvation and Precipitaion Equilibria

Ionic solids (salts) typically dissolve in water to give separated aqueous cations and anions. The solubility of any substance is the amount (g or mols) present at equilibrium in a given quantity of solvent (L).

Learn Table 7.1 on page 253.

Ionic Equilibria

Most salts dissociate into ions when they dissolve. Equilibrium exist between the solid salt and its aqueous ions. For example take the solvation of a salt composed of cation, C⁺ and anion, A^-:
CA(s) <==> C⁺(aq) + A^-(aq)

Since solids don't go into the mass action equation we can define an equilibrium solubility product as,

K_sp = [C⁺][A^-]

Some notes on K_sp:

1) K_sp expressions are not written for highly soluable salts because such solutions are too nonideal. Use K_sp only for slightly soluable salts.

2) K_sp values come from experiment and widely vary. Numerical values are usually good to only two significant figures.

3) K_sp depend on temperature. Usually it increases as temperature increases.

Precipitation and K_sp

Salts tend to precipitate from aqueous solution whent he concentrations of its ions cause Q to exceed K_sp. To compute the equilibrium concentrations of ions after mixing two solutions we use the standard ICE Box method.

Complex Ions and Solubility

In coordination complexes, a central metal ion binds to one or more ligands. The ligands are molecules or ions. The interaction between the metal ion and the ligand is called coordination. Consider the coordination of 4 ligands to a typical +2 metal ion:
M²⁺(aq) + 4 L(aq) <==> ML₄²⁺(aq)

Since all species are aqueous they enter into the formation constant,



Metal ions gain ligands one by one. In the above example the metal gains its ligand one after the other, which each gain represented by a K. We can approximate this if the ligan is present in excess and the K's are fairly large, then

1) Most of the metal ion will be tied up in the complex with the most ligands.

2) The concentration of free ligands will approximately equal the concentration of excess ligand that is calculated assuming complete coordination.