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Ch.17 - Applications of Aqueous Equilibria
All textbooksMcMurry 8th EditionCh.17 - Applications of Aqueous EquilibriaProblem 114b
Chapter 17, Problem 114b

Which of the following compounds are more soluble in acidic solution than in pure water? Write a balanced net ionic equation for each dissolution reaction. (b) Fe(OH)3

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Identify the nature of the compound Fe(OH)3. It is a sparingly soluble iron(III) hydroxide, which tends to dissolve better in acidic solutions due to the reaction of its hydroxide ions with the hydrogen ions in the acid.
Write the dissolution reaction of Fe(OH)3 in water: Fe(OH)3(s) ⇌ Fe^{3+}(aq) + 3OH^{-}(aq). This reaction shows that Fe(OH)3 dissociates into iron(III) ions and hydroxide ions in water.
Consider the common ion effect and the solubility product (Ksp) of Fe(OH)3. In pure water, the solubility is limited and governed by its Ksp.
Add an acid (source of H^+ ions) to the system. The added H^+ ions will react with the OH^{-} ions to form water: H^+(aq) + OH^{-}(aq) → H2O(l). This reaction decreases the concentration of OH^{-} ions in the solution.
Write the net ionic equation for the dissolution of Fe(OH)3 in an acidic solution, showing how the reaction shifts to dissolve more Fe(OH)3: Fe(OH)3(s) + 3H^+(aq) → Fe^{3+}(aq) + 3H2O(l). This equation demonstrates that in the presence of excess H^+, the solubility of Fe(OH)3 increases as the hydroxide ions are neutralized.

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Key Concepts

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

Solubility and pH

The solubility of certain compounds can be affected by the pH of the solution. In acidic conditions, some metal hydroxides, like Fe(OH)3, can dissolve more readily due to the formation of soluble metal complexes or the reaction of hydroxide ions with protons, leading to increased solubility.
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Net Ionic Equations

A net ionic equation represents the chemical species that are involved in a reaction, excluding spectator ions. It highlights the actual chemical change occurring in the solution, which is particularly useful for understanding the dissolution process of compounds in different pH environments.
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Dissolution of Metal Hydroxides

Metal hydroxides, such as Fe(OH)3, typically have low solubility in water. However, in acidic solutions, they can dissolve by reacting with hydrogen ions to form soluble metal ions, such as Fe^3+, thus demonstrating a shift in solubility based on the solution's acidity.
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Related Practice
Textbook Question

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.) (a) HCl