SOLUBILITY EQUILIBRIA
Solubility, precipitation reactions and Ks
Consider solid calcium carbonate in equilibrium with a saturated solution of
calcium carbonate;
CaCO3(s)  Ca2+(aq) + CO32-(aq)
and an equilibrium expression
K = [Ca2+] [CO32-]
[CaCO3]
The concentration of a solid is a constant, related to its density, so we simplify
the expression to:
Ks = [Ca2+] [CO32-]
where Ks is a special equilibrium constant related to solutions. It is called the
solubility constant or solubility product.
A large Ks indicates a large solubility. Substances called “insoluble” have
small Ks values.
IONIC PRODUCTS AND PREDICTING PRECIPITATION
In any solution the product of the ions raised to the power of their mole ratio is
known as the ionic product (IP) or the reaction quotient (Q).
At equilibrium, the value of Q is constant and the ionic product is known as
the solubility constant or solubility product, Ks. If the ionic product:
 is greater than the solubility product, precipitation occurs
IP > Ks, ppt occurs
 equals the solubility product, there is a saturated solution
IP = Ks, saturated solution (equilibrium)
 is less than the solubility product, no precipitation occurs
IP < Ks, no ppt
Consider the mixing of two solutions AB(aq) and CD(aq).
AB(aq) + CD(aq)
A+(aq) + B-(aq) + C+(aq) D-(aq)
In this mixture there are two new possible combination of salts:
and
The ionic product of AD(aq) is:
The ionic product of CB(aq) is:
If the ionic product for either of these new possible combinations of salts is
greater than their corresponding solubility product (Ks), precipitation of one or
both of these salts must occur.
When the two salts are added together, dilution occurs and the new
concentrations of ions must be taken into account when calculating IP values.
Example: Equal volumes of 0.02 mol L-1 calcium chloride and 0.04 mol L-1
sodium sulfate solutions are added together. Will any calcium sulfate be
precipitated?
Ks(CaSO4) = 2 x 10-5 at 25C
The equilibrium expression for calcium sulfate dissolving is:
The Ks expression for calcium sulfate is:
On mixing, the concentration of ions halves because the volume doubles:
[Ca2+] =
(where does the [Ca2+] come from?)
[SO42-] =
(where does the [SO42-] come from?)
Using the Ks expression to calculate the ionic product (IP):
IP = [Ca2+][SO42-] =
Since IP(
)
Ks (2 x 10-5), precipitation occurs/does not occur.
THE COMMON ION EFFECT
If we take a saturated solution of sodium chloride and try to dissolve more
sodium chloride we end up with solid sodium chloride at the bottom of the
beaker. If we take the same saturated solution and add a few drops of
concentrated hydrochloric acid we also end up with solid sodium chloride.
Why?
In a saturated solution the ionic product equals the solubility product:
Ks = [Na+][Cl -]
By adding HCl the concentration of the common ion, Cl -, is increased. With
the ionic product now exceeding the solubility product precipitation of sodium
chloride must occur.
The common ion effect involves two sources for one of the ions involved in a
dissolution reaction. Mixing solutions of calcium sulfate and sodium sulfate
provides two sources of sulfate ions. The sulfate ion is the ion common to
both these solutions. The effect of a common ion on mixing is to alter the ionic
product, which may cause precipitation if the ionic product becomes greater
than the solubility product. Generally, the presence of a common ion will
decrease the solubility of a substance.