THE COMMON ION EFFECT

Contents for this page Related topics
Equilibrium in solution
The solubility product
The common ion effect
Additional questions		 The strength of acids and bases
The pH scale
The hydrolysis of ions		 Data
Glossary

Learning Outcomes
After studying this section, you will (a) understand what is meant by the "solubility product" of a sparingly soluble salt and (b) understand the basis for the phenomenon known as the "common ion effect".

Equilibrium in solution:

Consider the dissolution of a sparingly soluble salt (such as silver iodide, AgI) in water:

Dissociation of silver iodide

Since this is a dynamic equilibrium, there will be an equilibrium constant K associated with it:

Equilibrium constant for silver iodide dissociation

The solubility product:

By convention, the concentration of a solid is 1 mol.dm-3. The concentration of water in pure water
is 55.6 mol.dm-3, we have

Solubility product

where Ksp is the SOLUBILITY PRODUCT for AgI.

For any sparingly soluble BINARY SALT (of the form M+Y-), the solubility product is

Solubility product of binary salt

For a TERNARY SALT M2+Y2-, the solubility product is given by

Solubility product of ternary salt

This tells us that in a saturated solution of a sparingly soluble salt, the product of the ion concentrations (raised to a suitable power depending on the composition of the salt) will be a constant at a particular temperature.

This enables us to determine whether a salt will precipitate out of solution under a given set of conditions.

The common ion effect:

A salt is generally less soluble in a solution containing an ion which is the same as one of the constituent ions of that salt. This is known as the COMMON ION EFFECT.

We can see that this must necessarily occur if we apply Le Chatelier's Principle to an equilibrium such as

Dissociation of sodium chloride

Adding an excess Cl- (or Na+) to a saturated solution of NaCl imposes a stress on the equilibrium, which will adjust in order to oppose the stress. A shift to the left will use up Na+ or Cl- to form solid NaCl.

Additional questions

The concentration of water in water:

The mass of 1 dm3 water is 1000 g, Mr for water, H2O, is 18.0, so, [H2O] = 1000/18.0 = 55.6 mol.dm-3.

Le Chatelier's principle:

Le Chatelier's Principle
"If a system in equilibrium is subjected to a stress the equilibrium will shift in the direction which tends to relieve that stress."