Ethanoic Acid
CH3COOH: the standard weak acid — partially ionised, slower with magnesium than HCl, made by oxidising ethanol, and the acid used in esterification.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-20.
Ethanoic acid (CH3COOH) is the most examined carboxylic acid in 0620. It is the textbook example of a weak acid and the acid half of the esterification reaction.
Key facts
| Property | Value |
|---|---|
| Formula | CH3COOH (molecular C2H4O2) |
| Mr | 2(12) + 4(1) + 2(16) = 60 |
| Functional group | -COOH (carboxyl) |
| Acid strength | Weak — partially ionised (CH3COOH ⇌ CH3COO- + H+) |
| Key property | Colourless liquid, sharp vinegar smell, miscible with water |
| Main uses | Vinegar; making esters |
Where ethanoic acid appears in 0620
- Carboxylic acids: the well-known member of the series.
- Acids, bases and indicators and the pH scale: the standard weak acid for strong-vs-weak comparisons.
- Esters: reacts with alcohols to form esters.
Weak acid behaviour
Ethanoic acid is the reference weak acid for 0620. Compared with hydrochloric acid of the same concentration:
| Property | Ethanoic acid (weak) | Hydrochloric acid (strong) |
|---|---|---|
| Ionisation | Partial (⇌) | Complete (->) |
| pH at equal concentration | Higher (e.g. 3) | Lower (e.g. 1) |
| Rate with magnesium | Slower (fewer H+ ions) | Faster |
| Electrical conductivity | Lower (fewer ions) | Higher |
Both acids give the same set of reactions; ethanoic acid is simply slower because fewer H+ ions are present at any moment. “Weak” describes the degree of ionisation, not the concentration.
Key reactions
With reactive metals, carbonates and alkalis
2CH3COOH(aq) + Mg(s) -> (CH3COO)2Mg(aq) + H2(g)
2CH3COOH(aq) + Na2CO3(aq) -> 2CH3COONa(aq) + H2O(l) + CO2(g)
CH3COOH(aq) + NaOH(aq) -> CH3COONa(aq) + H2O(l)
The salt formed is an ethanoate — sodium ethanoate (CH3COONa), magnesium ethanoate, and so on.
Esterification
CH3COOH(l) + C2H5OH(l) ⇌ CH3COOC2H5(l) + H2O(l)
Warm with a concentrated sulfuric acid catalyst. The product, ethyl ethanoate, is a sweet-smelling ester.
Production by oxidation of ethanol
C2H5OH + 2[O] -> CH3COOH + H2O
Using warm acidified potassium dichromate(VI) (orange to green) in the laboratory, or bacteria in air to make vinegar.
Telling a weak acid from a strong one
Given two acids of the same concentration, any of these show which is weak:
- Measure the pH: the weak acid has the higher pH (fewer H+ ions).
- Add magnesium: the weak acid fizzes more slowly.
- Test electrical conductivity: the weak acid conducts less well (fewer ions).
The comparison only works at equal concentration, because a concentrated weak acid can still contain more H+ ions than a very dilute strong acid.
Ethanoic acid is warmed with ethanol and a few drops of concentrated sulfuric acid. Name the organic product, give the conditions, and state one observation. (3 marks)
Mark scheme
- Product: ethyl ethanoate [1]
- Conditions: concentrated sulfuric acid as a catalyst, and warm/heat the mixture [1]
- Observation: a sweet or fruity smell (accept: an oily layer forms on top) [1]
Examiner note: name the ester with the alcohol part first — ethyl (from ethanol) then ethanoate (from ethanoic acid). “Ethanoate ethyl” or “ethyl ethanoic acid” scores zero for the name.
Common exam mistakes
- Treating “weak” as “dilute”. Weak means only partially ionised; concentrated (glacial) ethanoic acid is still weak but is corrosive.
- Writing full ionisation with an arrow. Ethanoic acid ionises reversibly, so use ⇌: CH3COOH ⇌ CH3COO- + H+.
- Saying a weak acid does not react with carbonates or metals. It gives all the usual acid reactions, just more slowly.
- Reversing the ester name or forgetting the -COOH product of oxidation is ethanoic acid, not ethanal.
Studying this yourself? Tutoring arrangements are normally made by a parent or guardian. Message us for the details to share with them, or send them this page.