"Ethanoic acid" is searched about 4,800 times a month in the UK, almost all by GCSE and A-level chemistry students, because it is the carboxylic acid every exam board uses to teach the whole functional group: the weak acid, the four reactions, the ester, the homologous series. This page gives the formula and structure, the properties table from the NIST Chemistry WebBook and PubChem, the weak-acid explanation with pH and rate, the four reactions with balanced equations, vinegar and the industrial route, the uses, and the comparison with hydrochloric acid, with a printable sheet and practice set. Students outside the UK know it as acetic acid; the companion ketone page is acetone.
Download the ethanoic acid sheet with practice (PDF) (A4, the structure, the properties and the four reactions with equations on page 1, the carboxylic acid family, the weak-acid comparison and the practice set with answers on page 2, free).
The formula and the structure
Ethanoic acid is the second carboxylic acid: a two-carbon molecule (ethan-) carrying the carboxyl group (-oic acid).
- Structural formula: CH₃COOH (also written CH₃CO₂H or CH₃–COOH).
- Molecular formula: C₂H₄O₂; molar mass 60.05 g/mol (PubChem CID 176).
- Displayed formula: a carbon with three hydrogens, joined to a second carbon that has a double bond to one oxygen and a single bond to an O–H.
- Skeletal formula: a single line (the C–C bond) ending in a carbon drawn with =O above and –OH to the side.
- The functional group: –COOH, the carboxyl group: a carbonyl (C=O) and a hydroxyl (O–H) on the same carbon. It is the hydrogen on that O–H that is released as H⁺.
The carboxyl carbon is trigonal planar (about 120°). The group is strongly polar and hydrogen-bonds readily, which is why ethanoic acid mixes with water in all proportions and boils far higher (118 °C) than acetone (56 °C) or ethanol (78 °C) of similar size: in the pure liquid and in the vapour, pairs of molecules hold together as hydrogen-bonded dimers.
The properties table
| Property | Value | Source |
|---|---|---|
| Formula; molar mass | CH₃COOH, C₂H₄O₂; 60.05 g/mol | PubChem |
| Systematic name; common name | ethanoic acid; acetic acid | |
| Appearance | colourless liquid, sharp vinegar smell | |
| Boiling point | 391.2 K = 118.1 °C | NIST WebBook |
| Melting point | 289.6 K = 16.5 °C (pure acid freezes in a cold room: "glacial") | NIST WebBook |
| Density | about 1.05 g/cm³ at 20 °C (slightly denser than water) | handbook value |
| Solubility in water | miscible in all proportions | |
| Acid dissociation constant | pKa 4.76 (Ka 1.7 × 10⁻⁵ mol/dm³) at 25 °C | handbook value |
| pH of 0.1 mol/dm³ solution | about 2.9 | from Ka |
| Concentration in vinegar | 4 to 8 percent by volume | food standards |
| Hazard | pure acid corrosive (skin burns, eye damage), flammable (flash point about 39 °C); vinegar harmless | safety data sheets |
Why it is a weak acid
Put ethanoic acid in water and only a small fraction of the molecules give up their hydrogen ion:
CH₃COOH ⇌ CH₃COO⁻ + H⁺
The double arrow is the point. The equilibrium lies far to the left; in a 0.1 mol/dm³ solution about 1.3 percent of the molecules are ionised at any moment. Hydrochloric acid at the same concentration is fully ionised (HCl → H⁺ + Cl⁻, a one-way arrow), so it has about 100 times the hydrogen ion concentration. Consequences the exams ask about:
| Ethanoic acid 0.1 mol/dm³ | Hydrochloric acid 0.1 mol/dm³ | |
|---|---|---|
| Ionisation | partial (about 1%) | complete |
| pH | about 3 | about 1 |
| Reaction with magnesium | slow fizzing | fast fizzing |
| Volume of alkali to neutralise 25 cm³ | the same (every molecule eventually gives up its H⁺ as the equilibrium shifts) | the same |
| Electrical conductivity | low | high |
Two words that students mix up: strong and weak describe how fully an acid ionises; concentrated and dilute describe how much acid is dissolved. Glacial ethanoic acid is concentrated and weak; 0.001 mol/dm³ hydrochloric acid is dilute and strong. The strong acids list has the seven that are fully ionised.
The four reactions every paper asks
Ethanoic acid does what every acid does, slower, and makes salts called ethanoates (acetates).
- With a metal → salt + hydrogen. Mg + 2CH₃COOH → (CH₃COO)₂Mg + H₂. Magnesium ethanoate and hydrogen; the test for the gas is a squeaky pop. Slow bubbling compared with hydrochloric acid.
- With a carbonate → salt + water + carbon dioxide. Na₂CO₃ + 2CH₃COOH → 2CH₃COONa + H₂O + CO₂. Sodium ethanoate; the gas turns limewater milky. This is the vinegar-and-baking-soda volcano: NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂.
- With a base or alkali → salt + water (neutralisation). NaOH + CH₃COOH → CH₃COONa + H₂O. Also with metal oxides: CuO + 2CH₃COOH → (CH₃COO)₂Cu + H₂O, giving blue-green copper ethanoate.
- With an alcohol → ester + water (esterification). CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O, with a few drops of concentrated sulfuric acid as catalyst and gentle warming. The product, ethyl ethanoate, is a sweet-smelling liquid (pear drops, nail varnish) used as a solvent. Esters are named alcohol part first (ethyl) then acid part (ethanoate).
A fifth, for A-level: reduction with lithium aluminium hydride gives ethanol; and the acid is itself the oxidation product of ethanol (ethanol → ethanal → ethanoic acid with acidified potassium dichromate, orange to green), which closes the loop with the alcohols topic.
How it is made
Vinegar is made by letting bacteria (Acetobacter) oxidise the ethanol in wine, cider, beer or diluted spirit with oxygen from the air: C₂H₅OH + O₂ → CH₃COOH + H₂O. A bottle of wine left open sours for the same reason. Malt vinegar, wine vinegar and cider vinegar are named for what was fermented.
Industrial ethanoic acid (several million tonnes a year) is made by methanol carbonylation: CH₃OH + CO → CH₃COOH over a rhodium (Monsanto process) or iridium (Cativa process) catalyst. Smaller amounts come from oxidising ethanal or butane.
Uses
| Use | Notes |
|---|---|
| Vinegar | food, pickling (the acid stops bacterial growth), descaling kettles (reacts with the limescale, calcium carbonate, reaction 2 above) |
| Making vinyl acetate | the monomer for PVA glue and paints, the largest single use |
| Making cellulose acetate | photographic film, cigarette filters, fibres |
| Making esters | ethyl ethanoate and butyl ethanoate as solvents for inks, paints and nail varnish |
| Making other chemicals | ethanoic anhydride (for aspirin), chloroethanoic acid, terephthalic acid processes |
| Laboratory | a weak acid for buffers (ethanoic acid and sodium ethanoate), a solvent, a reagent |
The carboxylic acid family
| Name | Formula | Common name | Boiling point |
|---|---|---|---|
| Methanoic acid | HCOOH | formic acid (ant stings, nettles) | 101 °C |
| Ethanoic acid | CH₃COOH | acetic acid (vinegar) | 118 °C |
| Propanoic acid | C₂H₅COOH | propionic acid (a bread preservative) | 141 °C |
| Butanoic acid | C₃H₇COOH | butyric acid (rancid butter) | 164 °C |
General formula CₙH₂ₙ₊₁COOH. Each extra CH₂ raises the boiling point by about 20 °C. All are weak acids with pKa around 4 to 5; all make salts ending in -oate and esters with alcohols.
Where it sits in each curriculum
| Curriculum | When | What is expected |
|---|---|---|
| England | GCSE Chemistry (Organic chemistry: carboxylic acids, their reactions with carbonates, their weak-acid behaviour, esters; triple science only on most boards), A-level Chemistry (carboxylic acids and esters, acid strength and Ka, oxidation of alcohols) | the four reactions, the weak-acid explanation, esterification; pKa calculations at A-level |
| Australia | Year 11 and 12 Chemistry (organic chemistry: functional groups, carboxylic acids, esterification; acids and bases: Ka) | structure, reactions, strength |
| India (CBSE) | Class 10 (Carbon and its Compounds: ethanoic acid's properties and reactions with metals, carbonates, bases and ethanol), Class 12 (Aldehydes, Ketones and Carboxylic Acids) | the Class 10 chapter uses exactly these four reactions |
| US | high-school chemistry (functional groups), AP Chemistry (weak acids, Ka, buffers made from acetic acid and sodium acetate) | weak-acid equilibrium and buffer calculations |
| Canada (Ontario) | Grade 12 Chemistry SCH4U (organic: carboxylic acids and esters; equilibrium: weak acids) | structure, reactions, Ka |
Practice (answers in the PDF)
- Draw the displayed formula of ethanoic acid and circle the functional group.
- Write the balanced equation for ethanoic acid reacting with sodium carbonate, and name the gas and its test.
- Explain why 0.1 mol/dm³ ethanoic acid has a higher pH than 0.1 mol/dm³ hydrochloric acid.
- Name the ester formed from ethanoic acid and methanol, and give the catalyst.
- Why is pure ethanoic acid called glacial?
- 25 cm³ of ethanoic acid needs 20 cm³ of 0.1 mol/dm³ sodium hydroxide to neutralise it. What is the acid's concentration? (CH₃COOH + NaOH → CH₃COONa + H₂O)
Download the sheet and the practice set (PDF)
Where to get help
A 1-on-1 online chemistry class in UK after-school hours with a tutor who teaches GCSE and A-level chemistry (AQA, Edexcel, OCR), CBSE Class 10 to 12 and the Australian Years 11 and 12 every week: organic chemistry taught from the functional group, with every equation written out and balanced by hand until it is automatic. The GCSE chemistry page explains how the classes run; the free 30-minute first class is a real lesson on this week's topic. Related pages: acetone, the strong acids and bases list and the 100 common compounds chart.
Questions parents ask
1What is the formula of ethanoic acid?
2Is ethanoic acid a strong or weak acid?
3What does ethanoic acid react with?
4What is glacial acetic acid?
5How is vinegar made?
See how we teach this, 1-on-1 online →
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