Equilibria Involving Sparingly Soluble Salts
- Account for your observations.
- Did the precipitated dissolve? Explain.
- What effect did the addition of have on the contents of the test tube? Explain.
The Correct Answer and Explanation is :
When a sparingly soluble salt is added to water, it establishes an equilibrium between its solid form and its constituent ions in solution. For example, consider the dissolution of silver chloride (AgCl):
AgCl (s)⇌Ag+(aq)+Cl−(aq)\text{AgCl (s)} \rightleftharpoons \text{Ag}^+ (\text{aq}) + \text{Cl}^- (\text{aq})
The solubility product constant (Kₛₚ) for AgCl at 25°C is 1.8 × 10⁻¹⁰, indicating its limited solubility in water. citeturn0search2
1. Account for your observations.
Upon adding a sparingly soluble salt like AgCl to water, it partially dissolves, releasing its ions into the solution until the concentration of these ions reaches a point where the rate of dissolution equals the rate of precipitation. This dynamic equilibrium results in a saturated solution where the concentration of dissolved ions remains constant.
2. Did the precipitated salt dissolve? Explain.
Yes, the precipitated salt did dissolve to some extent. The extent of dissolution depends on the salt’s Kₛₚ value. For AgCl, with a Kₛₚ of 1.8 × 10⁻¹⁰, only a small amount dissolves, leading to a low concentration of Ag⁺ and Cl⁻ ions in the solution.
3. What effect did the addition of a common ion have on the contents of the test tube? Explain.
Adding a common ion, such as NaCl, to the solution introduces additional Cl⁻ ions. According to Le Châtelier’s principle, the system will adjust to counteract this change by shifting the equilibrium to the left, favoring the formation of more solid AgCl. This results in increased precipitation of AgCl, thereby decreasing its solubility in the solution. This phenomenon is known as the common-ion effect. citeturn0search21
In summary, the dissolution of sparingly soluble salts is governed by their Kₛₚ values, and the addition of a common ion decreases their solubility by shifting the dissolution equilibrium toward the solid phase.