Carboxylic Acid – IGCSE Chemistry Definition

Carboxylic acids are tested for their acid reactions (with metals, bases, carbonates), their role in ester formation, and their classification as weak acids. They bridge organic chemistry with the acids-and-bases topic, giving examiners versatile question opportunities.

The 0620 definition

Carboxylic acids are a homologous series of organic compounds containing the carboxyl functional group, −COOH. Their general formula is CₙH₂ₙ₊₁COOH.

The first three carboxylic acids

Name	Formula	Found in
Methanoic acid	HCOOH	Ant stings
Ethanoic acid	CH₃COOH	Vinegar
Propanoic acid	C₂H₅COOH	Bread preservative

Carboxylic acids as weak acids

Ethanoic acid partially ionises in water:

CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq)

Notice the reversible arrow ⇌ — this shows partial ionisation, confirming it is a weak acid.

Because they are acids, carboxylic acids undergo the standard acid reactions:

1. With metals (above hydrogen in the reactivity series)

2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂

2. With bases

2CH₃COOH + CuO → (CH₃COO)₂Cu + H₂O

3. With carbonates

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂

4. With alkalis

CH₃COOH + NaOH → CH₃COONa + H₂O

Ester formation

Carboxylic acid + alcohol → ester + water

CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O

Conditions: concentrated sulfuric acid catalyst, gentle warming. This is a condensation reaction because water is lost.

Worked exam question

Ethanoic acid reacts with sodium carbonate. (a) State two observations. [2] (b) Write the balanced equation. [1]

Mark scheme

(a) Effervescence / fizzing / bubbles [1]; solid dissolves / solution forms [1]

(b) 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂ [1]

Common exam mistakes

• Forgetting that carboxylic acids are weak acids — using a forward arrow instead of ⇌ for the ionisation equation.
• Confusing the −OH in −COOH with the −OH in alcohols. The −COOH group behaves as an acid; the −OH in alcohols does not.
• Omitting the acid catalyst (H₂SO₄) when describing ester formation.