Common-Ion Effect: Chemical Equilibrium and Solubility Examples
In chemistry, the common-ion effect occurs when an ion already present in a solution is added from another source. This addition shifts the chemical equilibrium of a system to counteract the change, typically resulting in a decrease in the solubility of a sparingly soluble salt or a decrease in the ionization of a weak electrolyte.
Key Facts
- The common-ion effect is a practical application of Le Chatelier's principle.
- Adding a common ion shifts the equilibrium position toward the un-ionized or solid form.
- It reduces the dissociation of weak electrolytes.
- It lowers the solubility of salts in the presence of a shared ion.
Dissociation of Hydrogen Sulfide in Hydrochloric Acid
Hydrogen sulfide (H2S) is classified as a weak electrolyte, meaning it only partially ionizes when dissolved in water. This creates a dynamic equilibrium between the un-ionized molecules and the constituent ions:
H2S ⇌ H+ + HS−
According to the law of mass action—which states that the rate of a chemical reaction is proportional to the product of the activities of the reactants—the acid dissociation constant (Ka) is expressed as:
Ka = [H+][HS−] / [H2S]
When hydrochloric acid (HCl), a strong electrolyte that ionizes completely (HCl → H+ + Cl−), is added to the solution, it introduces a high concentration of H+ ions. Because H+ is a common ion to both substances, the equilibrium of the H2S dissociation shifts to the left to maintain a constant Ka value.
As a result of this shift, the dissociation of H2S decreases, the concentration of un-ionized H2S molecules increases, and the concentration of sulfide ions decreases.
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Solubility of Barium Iodate in Barium Nitrate
The common-ion effect also significantly impacts the solubility of salts. Consider barium iodate, Ba(IO3)2, which has a solubility product (Ksp)—the equilibrium constant for a solid substance dissolving in an aqueous solution—defined as Ksp = [Ba2+][IO3−]2 = 1.57 × 10−6.
In pure water, the solubility of barium iodate is 7.32 × 10−3 M. However, when barium iodate is placed in a solution containing 0.0200 M of barium nitrate, Ba(NO3)2, the concentration of the common barium ion (Ba2+) increases.
This increase in barium ions forces the equilibrium to shift, reducing the concentration of iodate ions. Consequently, the solubility of barium iodate drops to 1.40 × 10−3 M, making it approximately five times less soluble than it is in pure water.
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Summary of Common-Ion Effect Examples
| System | Common Ion | Added Substance | Primary Result |
|---|---|---|---|
| H2S (Weak Electrolyte) | H+ | HCl | Decreased dissociation of H2S |
| Ba(IO3)2 (Sparingly Soluble Salt) | Ba2+ | Ba(NO3)2 | Decreased solubility (from 7.32 to 1.40 × 10−3 M) |
Frequently Asked Questions
What is a common ion?
A common ion is an ion that is already present in a solution and is then added to that solution from another compound.
How does the common-ion effect influence weak electrolytes?
It suppresses the ionization of the weak electrolyte, shifting the equilibrium toward the un-ionized molecular form.
What happens to the solubility of a salt when a common ion is added?
The solubility of the salt decreases because the equilibrium shifts toward the solid precipitate to maintain the solubility product constant.
Why is HCl considered a strong electrolyte in the H2S example?
HCl is a strong electrolyte because it ionizes nearly completely into its constituent ions (H+ and Cl−) when dissolved in water.
What is the relationship between the common-ion effect and the law of mass action?
The law of mass action provides the mathematical basis (via equilibrium constants like Ka and Ksp) to predict how the system will shift when the concentration of a common ion is changed.