Common Ion Effect

HSC Chemistry Syllabus 

• Conduct an investigation to determine solubility rules, and predict and analyse the composition of substances when two ionic solutions are mixed, for example:

– potassium chloride and silver nitrate

– potassium iodide and lead nitrate

– sodium sulfate and barium nitrate (ACSCH065)
 

• Predict the formation of a precipitate given the standard reference values for K_sp

What is the common ion effect?

This video will explore the common ion effect which states that the addition of an ionic compound containing ions present in the equilibrium system, will have the same result as changing the concentration of that chemical species.  

 Common Ion Effect

• According to Le Châtelier’s principle, changing the concentration of chemical species of an equilibrium system will disturb and move the position of the equilibrium. Addition of an ionic compound that contains an ion present in the equilibrium system will achieve the same result. This is known as the common ion effect.

 

Example 1 – Barium sulfate solution

• Addition of sodium sulfate to a saturated solution of barium sulfate increases the amount of barium sulfate precipitate.

 

• This is because, according to Le Chatelier’s principle, an increase in [SO₄^2–] causes the equilibrium position to shift to the left in order to reduce [SO₄^2–]. As a result, [Ba²⁺] decreases and the amount of BaSO₄(s) increases.

 

 

• Addition of sodium sulfate reduces the solubility of barium sulfate
• Concentrations of ions change in a specific way such that the solubility product constant, K_sp, remains constant. This means the final [SO₄^2–] > initial [SO₄^2–] and final [Ba²⁺] < initial [Ba²⁺]

 

Example 2 – Lead(II) chromate solution

• Consider a saturated solution of lead(II) chromate:

$$PbCrO_{4(s)}\rightleftharpoons {\rm Pb}_{(aq)}^{2+}+CrO_{4(aq)}^{2-}\ \ \ \ K_{sp}=\left[{\rm Pb}_{\left(aq\right)}^{2+}\right]\left[CrO_{4\left(aq\right)}^{2-}\right]=2.3\times{10}^{-13}$$ 

• Suppose Na₂CrO₄, a soluble salt is added to the saturated PbCrO₄ The concentration of the common ion, CrO₄^2-, increases, and some of it combines with Pb²⁺ ion to form more solid PbCrO₄ (figure below). In effect, the addition of Na₂CrO₄ will shift the equilibrium to the left.

 

 

• As a result of the addition, [Pb²⁺] is reduced. And, since [Pb²⁺] defines the solubility of PbCrO₄, in effect, the solubility of PbCrO₄ has decreased.
• The same result would occur if Na₂CrO₄ solution was the solvent; that is, PbCrO₄ is more soluble in water than in aqueous Na₂CrO₄.

 

In general:

• Addition of a common ion reduces the solubility of the ionic compound.
• Removal of a common ion increases the solubility of the ionic compound.
• Adding or removal common ions does not change the value of the solubility product, K_sp.

 

Practice Questions

Question 1

Given the K_sp for calcium sulfate is `9.1 \times 10^{–6}`, determine whether a solution which contains calcium ions at a concentration of 1.00 `mol L^{–1}` and sulfate ions at a concentration of 2.56 `mol L^{–1}` is saturated with calcium sulfate.

 

Question 2

Given that K_sp BaSO₄ = [Ba²⁺] [SO₄^2-] = `1.1 \times 10^{–10}`, determine the solubility of barium sulfate in a solution of sodium sulfate of a concentration 1.00 `mol L^{–1}`

 

 

Previous Section: Calculating Solubility Product Quotient Q

Next Section: Effect of Temperature on Solubility

 

BACK TO MODULE 5: EQUILIBRIUM AND ACID REACTIONS