Question

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. (a) \(\mathrm{AgBr}\) (b) \(\mathrm{CaCO}_{3}\) (c) \(\mathrm{Ni}(\mathrm{OH})_{2}\) (d) \(\mathrm{Ca}_{3}\left(\mathrm{PO}_{4}\right)_{2}\)

Short answer

\(\mathrm{CaCO}_3\), \(\mathrm{Ni(OH)}_2\), and \(\mathrm{Ca}_3(\mathrm{PO}_4)_2\) dissolve better in acidic conditions.

Step-by-step solution

Consider the solubility rule for acidic conditions

Some compounds that contain anions capable of reacting with acids will dissolve better in acidic solutions than in pure water. These anions can form weak acids or gaseous products.

Analyze each compound for reactivity in acidic conditions

(a) **Silver Bromide (\( \mathrm{AgBr} \))** doesn't contain an anion that reacts with acid. Therefore, it doesn't become more soluble in acidic solutions. (b) **Calcium Carbonate (\( \mathrm{CaCO}_3 \))** contains the carbonate ion, which reacts with acids to produce carbon dioxide gas: \[ \mathrm{CO}_3^{2-} + 2 \mathrm{H}^+ \rightarrow \mathrm{CO}_2(g) + \mathrm{H}_2O \]This reaction improves its solubility in acidic conditions.(c) **Nickel Hydroxide (\( \mathrm{Ni(OH)}_2 \))** contains the hydroxide ion, which reacts with acids to form water:\[ \mathrm{OH}^- + \mathrm{H}^+ \rightarrow \mathrm{H}_2O \]This increases solubility in acidic solutions.(d) **Calcium Phosphate (\( \mathrm{Ca}_3\left(\mathrm{PO}_4\right)_2 \))** contains the phosphate ion, which can react with acids to form different protonated species or products:\[ \mathrm{PO}_4^{3-} + \mathrm{H}^+ \rightarrow ~intermediate \left(\textrm{e.g.,}~ \mathrm{HPO}_4^{2-}, \mathrm{H}_2PO_4^-, \mathrm{H}_3PO_4 \right) \]This also improves its solubility in acidic conditions.

Write the balanced net ionic equations for acidic dissolution

(b) **Calcium Carbonate dissolving in acid:**\[ \mathrm{CaCO}_3(s) + 2\mathrm{H}^+ \rightarrow \mathrm{Ca}^{2+} + \mathrm{CO}_2(g) + \mathrm{H}_2O \](c) **Nickel Hydroxide dissolving in acid:**\[ \mathrm{Ni(OH)}_2(s) + 2\mathrm{H}^+ \rightarrow \mathrm{Ni}^{2+} + 2\mathrm{H}_2O \](d) **Calcium Phosphate dissolving in acid:**\[ \mathrm{Ca}_3(\mathrm{PO}_4)_2(s) + 6\mathrm{H}^+ \rightarrow 3\mathrm{Ca}^{2+} + 2\mathrm{H}_3\mathrm{PO}_4 \]

Conclusion on solubility in acidic solutions

Compounds (b) \(\mathrm{CaCO}_3\), (c) \(\mathrm{Ni(OH)}_2\), and (d) \(\mathrm{Ca}_3(\mathrm{PO}_4)_2\) are more soluble in acidic solutions than in pure water.

Key concepts

Net Ionic Equations

Net ionic equations help us understand what's really happening in a chemical reaction. They focus only on the particles that undergo a change during the reaction.
This approach simplifies the equation by removing spectator ions, which remain unaffected throughout the process. Here's how you can think about it:

• Identify the compounds that dissolve or undergo a chemical change in solution.
• Write down the ions present.
• Cancel out the ions that appear on both sides of the equation.

Using a net ionic equation allows us to focus on the key players in a reaction, like when calcium carbonate (\( \mathrm{CaCO}_3(s) + 2\mathrm{H}^+ \rightarrow \mathrm{Ca}^{2+} + \mathrm{CO}_2(g) + \mathrm{H}_2O \)) dissolves in an acidic solution.

Carbonate Ion Reaction

The carbonate ion (\( \mathrm{CO}_3^{2-} \)) is a prominent player in many chemical reactions involving acids. When it reacts with hydrogen ions (\( \mathrm{H}^+ \)), it forms new products. This reaction can create carbon dioxide gas (\( \mathrm{CO}_2(g) \)) and water.Consider calcium carbonate, a common substance that interacts with acids:

• As it breaks down, it releases carbon dioxide, a gas that escapes from the solution.
• This process enhances the solubility of calcium carbonate in acidic environments.

Reactions like these help to explain why certain minerals dissolve better in acidic rather than neutral or basic solutions.

Phosphate Ion Reaction

Phosphate ions (\( \mathrm{PO}_4^{3-} \)) provide an interesting example of how acids can increase solubility. When you introduce an acid, new species like \( \mathrm{HPO}_4^{2-} \) and \( \mathrm{H}_2PO_4^- \) form.This reaction transforms calcium phosphate, making it more soluble.

• The process involves the sequential protonation of the phosphate ion.
• This not only changes the ion's charge but also creates particles with different properties.

Such chemical dynamics explain why phosphate-containing minerals like \( \mathrm{Ca}_3(\mathrm{PO}_4)_2 \) dissolve more readily in acidic solutions.

Acid-Base Reactions

Acid-base reactions are essential to understand, particularly in the context of solubility in acidic solutions.
They involve the transfer of protons (hydrogen ions), turning basic anions into more neutral or protonated forms.Two results typically arise:

• Water formation when hydroxide ions (\( \mathrm{OH}^- \)) and hydrogen ions combine.
• Gas formation, such as carbon dioxide in carbonate reactions.

These transformations are key:

• When basic compounds like nickel hydroxide (\( \mathrm{Ni(OH)}_2 \)) meet an acid, they are neutralized, resulting in water and a dissolved metal ion.

Understanding acid-base reactions demystifies how some compounds dissolve better in acidic environments than in pure water.