Question

Which of the following ionic compounds will be more soluble in acid solution than in water: (a) \(\mathrm{BaSO}_{4}\), (b) \(\mathrm{PbCl}_{2}\) (c) \(\mathrm{Fe}(\mathrm{OH})\) (d) \(\mathrm{CaCO}_{3} ?\)

Short answer

Fe(OH)₃ and CaCO₃ are more soluble in acid than in water.

Step-by-step solution

Understand the Solubility in Acid

Ionic compounds can be more soluble in acidic solutions if their anions react with hydrogen ions (H⁺) from the acid. By forming new products, these reactions remove the anion from solution, thus increasing the compound's solubility.

Evaluate Compound (a) – BaSO₄

Barium sulfate ( BaSO₄ ) has the sulfate ion, SO₄^{2-} , as its anion. Sulfates generally do not react with H⁺ in acids to form more soluble species, so BaSO₄ is not more soluble in acid.

Evaluate Compound (b) – PbCl₂

Lead(II) chloride ( PbCl₂ ) has the chloride ion, Cl^{-} , which does not react significantly with H⁺ in acid. Therefore, PbCl₂ will not be more soluble in acid compared to water.

Evaluate Compound (c) – Fe(OH)

Iron(III) hydroxide ( Fe(OH)₃ ) contains hydroxide ions, OH^{-} , which can react with H⁺ to form water: OH⁻ + H⁺ → H₂O . This removes the hydroxide ions from the solution, increasing the solubility of Fe(OH)₃ in acid.

Evaluate Compound (d) – CaCO₃

Calcium carbonate ( CaCO₃ ) contains carbonate ions, CO₃^{2-} , which react with H⁺ to form HCO₃^{-} or CO₂ gas: CO₃^{2-} + 2H⁺ → CO₂ + H₂O . This reaction increases solubility in acid.

Identify the Most Soluble in Acid

From the analysis: Fe(OH)₃ and CaCO₃ are more soluble in acidic solutions due to their anion's reaction with H⁺. However, BaSO₄ and PbCl₂ do not show increased solubility in acid.

Key concepts

Ionic Compounds

Ionic compounds are substances made up of positive and negative ions. These ions are held together by strong electrostatic forces known as ionic bonds. When dissolved in water, these compounds separate into their respective ions.

• Ionic compounds are commonly formed from metals and nonmetals.
• Examples include substances like sodium chloride ( NaCl ) and calcium carbonate ( CaCO₃ ).

In some cases, the solubility of these compounds can increase when placed in different environments, such as acidic solutions. This is mainly due to the interactions between the ions and the elements in the solution which can lead to new, more soluble compounds.

Anion Reactivity

Anions are negatively charged ions that play a crucial role in the solubility of ionic compounds in acidic solutions. The reactivity of anions with hydrogen ions ( H⁺ ) can significantly affect the solubility of a compound.

• Some anions, like the carbonate ion ( CO₃^{2-} ), react readily with H⁺ , forming new products that help dissolve compounds.
• Others, like the sulfate ion ( SO₄^{2-} ), do not react significantly with acids, resulting in little to no increase in solubility.

This difference in reactivity influences whether an ionic compound will be more soluble in an acidic environment.

Hydrogen Ions Interaction

In acidic solutions, the presence of hydrogen ions ( H⁺ ) can modify the behavior of ionic compounds. When H⁺ ions interact with certain anions, new products are formed, which can enhance the solubility of these compounds.

• For instance, OH^{-} (hydroxide ions) react with H⁺ to create water, effectively removing hydroxide ions from the solution.
• In the case of CO₃^{2-} (carbonate ions), the reaction with H⁺ can produce carbon dioxide and water, aiding in the solubility increase of compounds like calcium carbonate.

These interactions are pivotal in determining how compounds such as iron(III) hydroxide and calcium carbonate behave in acidic environments.

Iron(III) Hydroxide Solubility

Iron(III) hydroxide (Fe(OH)₃) tends to have low solubility in water, but when introduced to an acidic solution, its solubility can increase. This is due to the interaction between hydroxide ions and hydrogen ions:\[ ext{OH}^{-} + ext{H}^{+} ightarrow ext{H}_2 ext{O}\]

• This reaction effectively removes OH^{-} ions from the solution, reducing the tendency for these ions to recombine with iron to form solid Fe(OH)₃.
• As OH^{-} is removed, more Fe(OH)₃ can dissolve to restore equilibrium, leading to increased solubility.

This process demonstrates how Fe(OH)₃ transitions from a minimal solubility state in water to enhanced solubility in acidic conditions.

Calcium Carbonate Solubility

Calcium carbonate (CaCO₃) also has limited solubility in water. However, it becomes more soluble in acidic conditions. This is due to the reaction of carbonate ions (CO₃^{2-}) with hydrogen ions:\[ ext{CO}_3^{2-} + 2 ext{H}^{+} ightarrow ext{CO}_2 + ext{H}_2 ext{O}\]

• Carbon dioxide gas (CO₂) is produced as a byproduct and can escape from the solution, driving the reaction forward.
• As a result, CaCO₃ dissolves more readily to replace the lost CO₃^{2-} ions.

This process is why calcium carbonate materials, like limestone or chalk, erode more rapidly in acidic environments.