Seneca
Sign up

Common-Ion Effect

Chemistry: Cambridge International A Level, topic 1.7.11

Common-Ion Effect

Compared with pure water, the solubility of an ionic compound is less in aqueous solutions containing a common ion. This is an example of a phenomenon known as the common ion effect.

Common-Ion Effect

Common-Ion Effect

  • Compared with pure water, the solubility of an ionic compound is less in aqueous solutions containing a common ion.
  • A common ion is one also produced by dissolution of the ionic compound.
  • This is an example of a phenomenon known as the common-ion effect.
  • The common-ion effect is a consequence of the law of mass action that may be explained using Le Châtelier’s principle.
Example

Example

  • Consider the dissolution of silver iodide:
  • AgI(s) ⇌ Ag+(aq)+I−(aq)
  • This solubility equilibrium may be shifted left by the addition of either silver(I) or iodide ions.
  • This would resulti n the precipitation of AgI and lowered concentrations of dissolved Ag+ and I–.
  • In solutions that already contain either of these ions, less AgI may be dissolved than in solutions without these ions.
𝐾sp & common-ion effect

𝐾sp & common-ion effect

  • This effect may also be explained in terms of mass action as represented in the solubility product expression:
  • 𝐾sp = [Ag+][I−]
  • The mathematical product of silver(I) and iodide ion molarities is constant in an equilibrium mixture regardless of the source of the ions.
  • So, an increase in one ion’s concentration must be balanced by a proportional decrease in the other.

Test yourself on Common-Ion Effect

  • Mg(OH)2 exists the below solubility equilbrium:

    Mg(OH)2(s)⇌Mg2+(aq)+2OH−(aq)

    What is the effect on the amount of solid Mg(OH)2 when MgCl2 is added to a saturated solution of Mg(OH)2?

  • Mg(OH)2 exists the below solubility equilbrium:

    Mg(OH)2(s)⇌Mg2+(aq)+2OH−(aq)

    What is the effect on the amount of solid Mg(OH)2 when NaNO3 is added to a saturated solution of Mg(OH)2?

  • Mg(OH)2 exists the below solubility equilbrium:

    Mg(OH)2(s)⇌Mg2+(aq)+2OH−(aq)

    What is the effect on the amount of solid Mg(OH)2 when KOH is added to a saturated solution of Mg(OH)2?

  • Mg(OH)2 exists the below solubility equilbrium:

    Mg(OH)2(s)⇌Mg2+(aq)+2OH−(aq)

    What is the effect on the amount of solid Mg(OH)2 when more Mg(OH)2 is added to a saturated solution of Mg(OH)2?

Practise this topic for free