Carboxylic Acids

The carboxyl group (–COOH) is a carbon bonded to a hydroxyl AND a double-bonded oxygen: –C(=O)–O–H. It's the source of acidity.

The first three:

n	Name	Formula	Where found / use
1	Methanoic acid	HCOOH	Ant venom, nettle stings (also called formic acid)
2	Ethanoic acid	CH₃COOH	Vinegar (~5-8% solution); food preservative
3	Propanoic acid	CH₃CH₂COOH	Used in preservatives (calcium propanoate prevents mould in bread)

Higher carboxylic acids (longer chains) become greasy and less soluble — fatty acids in oils and fats are carboxylic acids with 12-20+ carbons.

Carboxylic acids show the THREE standard reactions of acids:

1. + Metal → salt + H₂ $2\,\text{CH}_3\text{COOH} + \text{Mg} \rightarrow (\text{CH}_3\text{COO})_2\text{Mg} + \text{H}_2$ Fizzes; releases hydrogen.

2. + Base / alkali → salt + water (neutralisation) $\text{CH}_3\text{COOH} + \text{NaOH} \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O}$ Sodium ethanoate (CH₃COONa) is the salt produced.

3. + Carbonate → salt + water + CO₂ $2\,\text{CH}_3\text{COOH} + \text{Na}_2\text{CO}_3 \rightarrow 2\,\text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2$ The fizz is CO₂ (turns limewater milky).

Salts are named with the -anoate suffix:

• HCOO⁻ = methanoate.
• CH₃COO⁻ = ethanoate.
• CH₃CH₂COO⁻ = propanoate.

So you get sodium ethanoate, calcium propanoate, magnesium methanoate, etc.

Strong acids (HCl, HNO₃, H₂SO₄) fully ionise in water: $\text{HCl (aq)} \rightarrow \text{H}^+ + \text{Cl}^-$

Essentially 100% of the HCl molecules release an H⁺.

Weak acids (carboxylic acids, carbonic acid H₂CO₃) only partially ionise — an equilibrium: $\text{CH}_3\text{COOH (aq)} \rightleftharpoons \text{H}^+ + \text{CH}_3\text{COO}^-$

Only a SMALL fraction (often <1%) ionise at any moment. The double-arrow ⇌ shows the equilibrium.

Consequences of being WEAK:

• Higher pH (closer to neutral) at the same concentration as a strong acid.
• Reactions still happen but more slowly.
• Conduct electricity less well in solution.

That's why vinegar (~0.5-1.0 M ethanoic acid) has pH ~2-3, while 1 M HCl has pH 0. Same concentration, very different pH — because vinegar barely ionises.

MYP criterion A typical question: 'Why is ethanoic acid weak compared to hydrochloric acid?' Answer: ethanoic acid only PARTIALLY ionises in water (most molecules remain as undissociated CH₃COOH); HCl fully ionises.