8 min readConcept sheetPart III (4 marks) and Part IV (7 marks): reactions with equations; also Part I (1 mark)

Ethanoic acid: preparation, properties and uses

Ethanoic acid (CH₃COOH) is made by oxidising ethanol. A weak acid, it reacts with metals, carbonates and bases, and forms esters with alcohols.

Common nameAcetic acid; vinegar is a 5 to 8% solution

Functional group-COOH (carboxyl)

Glacial acetic acidPure ethanoic acid; freezes at 290 K into ice-like crystals

Boiling point391 K (118 °C)

Preparation

Ethanoic acid is made, even on a large scale, by the oxidation of ethanol with alkaline potassium permanganate (KMnO₄) or acidified potassium dichromate (K₂Cr₂O₇): CH₃CH₂OH + 2[O] → CH₃COOH + H₂O. Here [O] stands for the oxygen given by the oxidising agent.

Physical properties

A colourless liquid with a sour taste and the sharp smell of vinegar.
Boils at 391 K.
Pure ethanoic acid freezes at 290 K into ice-like crystals, so it is called glacial acetic acid.
Mixes with water in all proportions.
Turns blue litmus red: it is a weak acid.

Chemical properties (each equation is balanced)

1
2CH₃COOH + 2Na → 2CH₃COONa + H₂↑
With active metals such as sodium: sodium ethanoate and hydrogen
2★
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂↑
With carbonates: brisk effervescence of CO₂
3
CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂↑
With bicarbonates (baking soda): CO₂, which turns lime water milky
4
CH₃COOH + NaOH → CH₃COONa + H₂O
Neutralisation with a base: a salt and water
5★
CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
Conc. H₂SO₄ · Esterification: fruity-smelling ethyl ethanoate
6
CH₃COONa + NaOH → CH₄ + Na₂CO₃
CaO, heat (soda lime) · Decarboxylation of sodium ethanoate gives methane

★ key step

Uses of ethanoic acid

As vinegar for cooking and for preserving pickles and sauces.
To coagulate rubber from latex.
In making dyes, perfumes, plastics (such as cellulose acetate) and medicines.
As a laboratory reagent.

Ethanol or ethanoic acid? A quick test

EthanolNo effervescence with sodium bicarbonate. Neutral to litmus. Pleasant smell.

Ethanoic acidBrisk effervescence of CO₂ with sodium bicarbonate. Turns blue litmus red. Smell of vinegar.

Both react with sodium to give hydrogen, so sodium cannot tell them apart. Use sodium bicarbonate or litmus.

In real life

The vinegar in Indian pickles, chutneys and sauces is dilute ethanoic acid; bacteria make it by slowly oxidising dilute alcohol in air. Its acidity stops microbes from spoiling the food.

Exam angle

1 marks: “What is glacial acetic acid?” Pure ethanoic acid, which freezes at 290 K into ice-like crystals.

2 marks: “What happens when ethanoic acid reacts with sodium bicarbonate? Write the equation.” Brisk effervescence of CO₂: CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂↑.

4 marks: “Write any four chemical properties of ethanoic acid with equations.” Reaction with sodium (H₂), sodium carbonate (CO₂), sodium hydroxide (neutralisation), ethanol (ester); decarboxylation to methane.

Remember it

Ethanoic acid's partners: Metal gives H₂, Carbonate gives CO₂, Base gives water, Alcohol gives an ester: 'My Cat Bites Apples'.