Write balanced ionic equations for the following reactions: (a) Aqueous perchloric acid is neutralized by aqueous calcium hydroxide. (b) Aqueous sodium hydroxide is neutralized by aqueous acetic acid.
The Correct Answer and Explanation is:
Balanced Ionic Equations:
(a) Aqueous perchloric acid is neutralized by aqueous calcium hydroxide:
Ionic Equation: 2H+(aq)+2ClO4−(aq)+Ca2+(aq)+2OH−(aq)→Ca2+(aq)+2ClO4−(aq)+2H2O(l)2\text{H}^+ (aq) + 2\text{ClO}_4^- (aq) + \text{Ca}^{2+} (aq) + 2\text{OH}^- (aq) \rightarrow \text{Ca}^{2+} (aq) + 2\text{ClO}_4^- (aq) + 2\text{H}_2\text{O} (l)
Net Ionic Equation: 2H+(aq)+2OH−(aq)→2H2O(l)2\text{H}^+ (aq) + 2\text{OH}^- (aq) \rightarrow 2\text{H}_2\text{O} (l)
or simplified: H+(aq)+OH−(aq)→H2O(l)\text{H}^+ (aq) + \text{OH}^- (aq) \rightarrow \text{H}_2\text{O} (l)
(b) Aqueous sodium hydroxide is neutralized by aqueous acetic acid:
Ionic Equation: CH3COOH(aq)+Na+(aq)+OH−(aq)→CH3COO−(aq)+Na+(aq)+H2O(l)\text{CH}_3\text{COOH} (aq) + \text{Na}^+ (aq) + \text{OH}^- (aq) \rightarrow \text{CH}_3\text{COO}^- (aq) + \text{Na}^+ (aq) + \text{H}_2\text{O} (l)
Net Ionic Equation: CH3COOH(aq)+OH−(aq)→CH3COO−(aq)+H2O(l)\text{CH}_3\text{COOH} (aq) + \text{OH}^- (aq) \rightarrow \text{CH}_3\text{COO}^- (aq) + \text{H}_2\text{O} (l)
Explanation
In acid-base chemistry, a neutralization reaction occurs when an acid reacts with a base to form water and a salt. Ionic equations help illustrate the species actually involved in the reaction by showing how soluble compounds dissociate in aqueous solution.
For part (a): Perchloric acid (HClO₄) is a strong acid, meaning it completely dissociates in water into H⁺ and ClO₄⁻ ions. Calcium hydroxide (Ca(OH)₂) is a strong base and dissociates completely into Ca²⁺ and OH⁻ ions. The full ionic equation includes all ions in the solution. When combining, H⁺ reacts with OH⁻ to form water. The calcium and perchlorate ions are spectator ions—they remain unchanged and don’t participate directly in the reaction. Removing them gives the net ionic equation, which simplifies to the neutralization of H⁺ by OH⁻ to form H₂O.
For part (b): Acetic acid (CH₃COOH) is a weak acid and does not fully dissociate in solution. Sodium hydroxide (NaOH) is a strong base and dissociates completely into Na⁺ and OH⁻. The hydroxide ion reacts directly with intact acetic acid molecules to produce acetate (CH₃COO⁻) and water. Since sodium ions remain unchanged, they are omitted in the net ionic equation. This shows a weak acid reacting with a strong base, forming a salt (acetate ion) and water.
Understanding the nature of acids and bases—strong vs. weak—is essential when writing ionic equations, as it determines whether species are written in dissociated form or not. Net ionic equations emphasize the actual chemical change by eliminating ions that do not participate directly.
