Carboxylic acids are a homologous series with the carboxyl group, -COOH, as their functional group and the general formula CnH2n+1COOH. Ethanoic acid, CH3COOH, is the most important member.
It is a weak acid that shows the typical reactions of an acid with reactive metals, bases and carbonates, forming ethanoate salts, and it is made by oxidising ethanol.
This page covers content standard 10.7 of the Form 5 Carbon Compounds chapter: carboxylic acids. This family gives ethanol its most examined product and links the chapter to everything you learned about acids in Form 4. Master the functional group, the three typical acid reactions, and the idea of a weak acid, and this standard is straightforward marks.
What are carboxylic acids?
Carboxylic acids are a homologous series whose functional group is the carboxyl group, –COOH. The general formula is CₙH₂ₙ₊₁COOH. The –COOH group is what makes these compounds acidic, because in water it releases a hydrogen ion (H⁺).
The members. methanoic acid (HCOOH), ethanoic acid (CH₃COOH) and propanoic acid (C₂H₅COOH). Ethanoic acid is the one you must know in full, it is the acid in vinegar.
| Name | Molecular formula |
|---|---|
| Methanoic acid | HCOOH |
| Ethanoic acid | CH₃COOH |
| Propanoic acid | C₂H₅COOH |
Ethanoic acid is a weak acid
Ethanoic acid is a weak acid: in water it only partially ionises, so a solution contains mostly un-ionised CH₃COOH molecules and only a few hydrogen ions. This is different from a strong acid such as hydrochloric acid, which ionises completely. Because it produces fewer hydrogen ions at the same concentration, ethanoic acid has a higher pH (less acidic) and reacts more slowly than a strong acid of the same concentration. It still shows all the typical reactions of an acid, just less vigorously.
Making ethanoic acid
Ethanoic acid is made by the oxidation of ethanol. Warming ethanol with acidified potassium dichromate(VI) (which turns from orange to green) or acidified potassium manganate(VII) (which is decolourised from purple) oxidises the alcohol to the acid: CH₃CH₂OH + 2[O] → CH₃COOH + H₂O. Bacteria in the air can do the same thing slowly, which is why wine left open turns sour.
The three typical reactions of ethanoic acid
Like any acid, ethanoic acid reacts in three characteristic ways, and each one produces an ethanoate salt.
- With a reactive metal → salt + hydrogen. For example, with magnesium: Mg + 2CH₃COOH → (CH₃COO)₂Mg + H₂. You see effervescence, and the gas gives a “pop” with a lighted splint.
- With a base or alkali → salt + water. For example, with sodium hydroxide: CH₃COOH + NaOH → CH₃COONa + H₂O. This is a neutralisation.
- With a carbonate → salt + water + carbon dioxide. For example, with sodium carbonate: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂. You see effervescence, and the gas turns limewater milky.
Worked example
Question. Ethanoic acid is added to sodium carbonate. Write the balanced chemical equation, state one observation, and name the test that confirms the gas produced.
Step 1, Identify the reaction type. An acid reacting with a carbonate gives salt + water + carbon dioxide. The salt is sodium ethanoate, CH₃COONa.
Step 2, Write the balanced equation. Balance the sodium and the ethanoate groups:
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂Step 3, State an observation. Effervescence (bubbling) is seen as a colourless gas is released.
Step 4, Confirm the gas. Bubble the gas through limewater: it turns the limewater milky (cloudy), confirming the gas is carbon dioxide.
Answer. The equation is 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂; the observation is effervescence; the gas turns limewater milky, confirming carbon dioxide.
Practice question
Question. A piece of magnesium ribbon is added to dilute ethanoic acid. Write the equation for the reaction, and state what you would observe and the test for the gas produced.
Answer. The equation is Mg + 2CH₃COOH → (CH₃COO)₂Mg + H₂. The magnesium dissolves and there is effervescence as a colourless gas (hydrogen) is given off. The gas is tested by bringing a lighted splint to it: the gas burns with a “pop” sound, confirming it is hydrogen. Note that the reaction is slower than with a strong acid of the same concentration, because ethanoic acid is a weak acid that only partially ionises.
Exam tip
The single idea that earns and loses the most marks here is weak versus strong acid: ethanoic acid partially ionises, hydrochloric acid fully ionises, so at the same concentration ethanoic acid gives fewer H⁺ ions, a higher pH, and a slower reaction. Learn the three acid reactions as a set and always name the salt correctly (ending in -ethanoate). Do not forget to balance the metal and carbonate equations, the “2” in front of CH₃COOH is a frequent slip. For gas tests, keep them paired: hydrogen → lighted splint gives a pop; carbon dioxide → turns limewater milky.
Naming the salts and everyday uses
The salt of a carboxylic acid takes its name from the acid, with the ending changed from -ic acid to -oate: ethanoic acid gives ethanoate salts, and propanoic acid gives propanoate salts. So sodium hydroxide and ethanoic acid give sodium ethanoate, while magnesium and ethanoic acid give magnesium ethanoate. Getting this ending right is worth a mark in every salt equation. Ethanoic acid itself has familiar uses: it is the acid in vinegar (used to flavour and preserve food), and it is a raw material for making esters and other chemicals in industry. Being able to link the acid to vinegar, and to name the salt correctly, shows the examiner you understand the family rather than just memorising equations.
Where this fits
This is content standard 10.7 of the Carbon Compounds chapter. It follows directly from alcohols (oxidising ethanol makes ethanoic acid) and leads into esters (an acid plus an alcohol makes an ester). It also connects back to the Form 4 Acids, Bases and Salts chapter, where the same three acid reactions first appear. Practise the salt equations in the chapter worked examples and check the weak-acid reasoning on the common mistakes page. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers drill the acid reactions and the weak-acid explanation until they are automatic. Because these acid reactions recur across SPM Chemistry, the marks they carry add up over the whole paper.
Quick recap
- Carboxylic acids: homologous series, functional group –COOH, general formula CₙH₂ₙ₊₁COOH.
- Members: methanoic, ethanoic, propanoic acid; ethanoic acid is the acid in vinegar.
- Ethanoic acid is a weak acid, it only partially ionises.
- Made by oxidising ethanol (dichromate orange → green).
- Three reactions → an ethanoate salt: metal (+ H₂), base (+ H₂O), carbonate (+ H₂O + CO₂).
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