Is the solubility of Zn(OH)₂ increased, decreased, or unchanged on addition of each of the following substances? Write a balanced net ionic equation for each dissolution reaction. (See Appendix C.6 for formulas of complex ions.) (b) KOH

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Identify the solubility equilibrium of Zn(OH)₂. The dissolution of Zn(OH)₂ in water can be represented by the equation: Zn(OH)₂(s) ⇌ Zn²⁺(aq) + 2OH^-(aq).

Consider the effect of adding KOH to the solution. KOH is a strong base that dissociates completely in water to form K⁺ and OH^- ions.

Apply Le Chatelier's Principle to predict the shift in equilibrium. The addition of OH^- ions from KOH will increase the concentration of OH^- in the solution, causing the equilibrium to shift to the left, thus decreasing the solubility of Zn(OH)₂.

Write the net ionic equation for the dissolution of Zn(OH)₂ in the presence of excess OH^-: Zn(OH)₂(s) + 2OH^-(aq) ⇌ Zn(OH)₄^2-(aq).

Conclude that the solubility of Zn(OH)₂ decreases upon the addition of KOH due to the common ion effect, where the increased concentration of OH^- shifts the equilibrium towards the formation of the solid Zn(OH)₂.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Solubility Product Constant (Ksp)

The solubility product constant (Ksp) is a numerical value that represents the equilibrium between a solid and its ions in a saturated solution. For a sparingly soluble compound like Zn(OH)2, Ksp helps predict how changes in concentration of ions affect solubility. When a substance that shares a common ion is added, the solubility typically decreases due to the common ion effect.

Common Ion Effect

The common ion effect refers to the decrease in solubility of a salt when a solution already contains one of the ions present in the salt. In the case of Zn(OH)2, adding KOH introduces OH- ions, which shifts the equilibrium to favor the formation of solid Zn(OH)2, thus reducing its solubility. This principle is crucial for understanding how additional ions influence solubility.

Net Ionic Equations

A net ionic equation represents the actual chemical species that are involved in a reaction, excluding spectator ions. For the dissolution of Zn(OH)2 in the presence of KOH, the net ionic equation highlights the formation of Zn(OH)4^2- ions and the precipitation of Zn(OH)2. Writing net ionic equations is essential for clearly illustrating the chemical changes occurring in solution.