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.) (c) NaCN

Verified step by step guidance

1. Identify the solubility equilibrium of Zn(OH)_2: Zn(OH)_2(s) \rightleftharpoons Zn^{2+}(aq) + 2OH^{-}(aq).
2. Recognize that NaCN dissociates in water to form Na^+ and CN^- ions.
3. Consider the potential reaction between Zn^{2+} and CN^- to form a complex ion, such as Zn(CN)_4^{2-}.
4. Write the balanced net ionic equation for the formation of the complex ion: Zn^{2+}(aq) + 4CN^{-}(aq) \rightarrow Zn(CN)_4^{2-}(aq).
5. Conclude that the formation of the complex ion reduces the concentration of Zn^{2+} ions, shifting the equilibrium to dissolve more Zn(OH)_2, thus increasing its solubility.

Key Concepts

Solubility Product Constant (Ksp)

The solubility product constant (Ksp) is a numerical value that represents the extent to which a sparingly soluble ionic compound can dissolve in water. It is defined as the product of the molar concentrations of the ions, each raised to the power of their coefficients in the balanced dissolution equation. For Zn(OH)2, the Ksp expression would involve the concentrations of Zn²⁺ and OH⁻ ions, and changes in these concentrations can affect solubility.

Common Ion Effect

The common ion effect refers to the decrease in solubility of a salt when a common ion is added to the solution. In the case of Zn(OH)2, if NaCN is added, it introduces CN⁻ ions, which can affect the equilibrium of the dissolution reaction. Understanding this effect is crucial for predicting how the addition of certain ions influences the solubility of sparingly soluble compounds.

Complex Ion Formation

Complex ion formation occurs when metal ions bind with ligands to form a larger, charged species. In this scenario, the addition of NaCN can lead to the formation of the complex ion [Zn(CN)4]²⁻, which effectively removes Zn²⁺ ions from the solution. This shift in equilibrium can increase the solubility of Zn(OH)2, as the dissolution reaction will shift to produce more Zn²⁺ ions to counteract the decrease caused by complexation.