Which of the following reactions does NOT represent a neutralisation reaction?

Question

Examoogle · Accepted Answer

The correct answer is Option D: Na ₂ CO ₃ + 2HCl → 2NaCl + H ₂ O + CO ₂. A neutralisation reaction is a chemical process in which an acid and a base react quantitatively with each other to form salt and water as the primary products. Option D describes the reaction of a carbonate salt with an acid, which produces a salt, water, and carbon dioxide gas with effervescence, making it a gas-evolution acid-carbonate reaction rather than a standard acid-base neutralisation.

Step-by-Step Solution & Concepts:
A neutralisation reaction is a chemical process in which an acid and a base react quantitatively with each other to form salt and water as the primary products. Option D describes the reaction of a carbonate salt with an acid, which produces a salt, water, and carbon dioxide gas with effervescence, making it a gas-evolution acid-carbonate reaction rather than a standard acid-base neutralisation.

Working Principle: In standard neutralisation reactions (Options A, B, and C), protons ($H^+$) from an acid combine with hydroxide ions ($OH^-$) from a base to form water ($H_2O$), accompanied by the formation of a salt. In Option D, sodium carbonate ($Na_2CO_3$), a basic salt, reacts with hydrochloric acid ($HCl$) to produce sodium chloride ($NaCl$), water ($H_2O$), and carbon dioxide ($CO_2$) gas due to the decomposition of unstable carbonic acid ($H_2CO_3$) intermediate.

Key Formulas: $H^+ + OH^- ightarrow H_2O$ — Net ionic equation for neutralisation; $CO_3^{2-} + 2H^+ ightarrow H_2O + CO_2 \uparrow$ — Reaction of carbonate with acid

Key Points: Neutralisation is an exothermic reaction between an acid and a base.; Metal carbonates and metal hydrogen carbonates react with acids to produce salt, water, and brisk effervescence of carbon dioxide.; Strong acid and strong base neutralisation has a standard enthalpy change of approximately $-57.1 	ext{ kJ/mol}$.; Option D is an acid-carbonate reaction, not a pure acid-base neutralisation.

Advantages: Helps in understanding different reaction types in stoichiometry.; Crucial for identifying gas evolution tests in analytical chemistry.

Applications: Antacids neutralising stomach acidity.; Treatment of acidic soils using agricultural lime.; Testing for carbonate salts using dilute hydrochloric acid.

Additional Info: Option A: Strong acid ($H_2SO_4$) + Strong base ($Ca(OH)_2$) $ightarrow$ Neutralisation.. Option B: Weak acid ($CH_3COOH$) + Strong base ($NaOH$) $ightarrow$ Neutralisation.. Option C: Strong acid ($HCl$) + Strong base ($NaOH$) $ightarrow$ Neutralisation.

Answer: Option D is correct — Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂ represents an acid-carbonate reaction releasing carbon dioxide gas rather than a standard neutralisation reaction.

Exam Tip: Always check for gas evolution ($CO_2$) when metal carbonates react with acids; such reactions are classified as decomposition/displacement reactions alongside neutralisation.

Examoogle · Answer

Option A: H ₂ SO ₄ + Ca(OH) ₂ → CaSO ₄ + 2H ₂ O

Examoogle · Answer

Option B: CH ₃ COOH + NaOH → CH ₃ COONa + H ₂ O

Examoogle · Answer

Option C: HCl + NaOH → NaCl + H ₂ O

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