Methanoic acid (HCOOH) is the simplest carboxylic acid. It is found in ant venom and reacts with a range of substances. Describe the chemical properties of methanoic acid that show it is a carboxylic acid, and explain why its reactions with carbonates produce a gas.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Methanoic acid has the molecular formula HCOOH. It dissolves in water to produce an acidic solution. When methanoic acid is added to sodium carbonate (Na₂CO₃), effervescence is observed.
Model answer (5 marks)
Methanoic acid is a weak acid; it only partially dissociates in water:
HCOOH ⇌ H⁺ + HCOO⁻
When it reacts with sodium carbonate it forms the salt sodium methanoate:
2 HCOOH + Na₂CO₃ → 2 NaHCOO + CO₂ + H₂O
The reaction releases carbon dioxide gas and water. The H⁺ ions from the acid attack the carbonate ion, causing the carbonate to decompose and liberate CO₂.
These features show that methanoic acid behaves as a carboxylic acid.
HCOOH ⇌ H⁺ + HCOO⁻
When it reacts with sodium carbonate it forms the salt sodium methanoate:
2 HCOOH + Na₂CO₃ → 2 NaHCOO + CO₂ + H₂O
The reaction releases carbon dioxide gas and water. The H⁺ ions from the acid attack the carbonate ion, causing the carbonate to decompose and liberate CO₂.
These features show that methanoic acid behaves as a carboxylic acid.
Examiner tips
- Use the reaction equation to show all products, including CO₂ and H₂O; this demonstrates the gas formation.
- Mention that the acid is weak and only partially dissociates – this is a key property of carboxylic acids.
Common mistakes
- Forgetting to include the water produced in the carbonate reaction.
- Confusing the salt formed (sodium methanoate) with a different product or omitting the CO₂ gas.
Mark scheme (5 marks)
- Methanoic acid is a weak acid / partially dissociates / does not fully ionise in water
- Methanoic acid reacts with carbonates / sodium carbonate to produce a salt (sodium methanoate)
- Carbon dioxide gas is produced / released in the reaction with carbonate
- Water is also produced in the reaction with carbonate
- The acid provides hydrogen ions / H⁺ ions which react with the carbonate ions, causing the carbonate to decompose and release CO₂
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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