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10.4 Carboxylic Acids and Esters

GCSE Chemistry · AQA · Chapter 10: Organic Reactions

A free GCSE Chemistry lesson for AQA (8462) from OBA School: reactions of carboxylic acids, why they are weak acids, how esters are made, and what esters are used for. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe how carboxylic acids react with carbonates and name the salts formed
  • Explain why carboxylic acids are weak acids in terms of ionisation and pH (Higher tier)
  • Describe how an ester is made from a carboxylic acid and an alcohol
  • Name simple esters and give their properties and uses
  • Calculate volumes of gas and percentage yields, and use tests to tell alcohols, acids and esters apart

Key words

  • Weak acid: An acid that is only partially ionised in water, e.g. ethanoic acid.
  • Strong acid: An acid that is completely ionised in water, e.g. hydrochloric acid.
  • Ionise: Split up to form ions. Acids ionise in water to release hydrogen ions, H⁺(aq).
  • Ester: A compound with the –COO– functional group, made from a carboxylic acid and an alcohol.
  • Esterification: The reaction of a carboxylic acid with an alcohol to make an ester and water.
  • Volatile: Evaporates easily at room temperature.

Reactions of carboxylic acids

Carboxylic acids dissolve in water to form acidic solutions (pH below 7). They have the typical reactions of acids, but react more slowly than hydrochloric acid at the same concentration. With a carbonate they fizz, giving off carbon dioxide:

carboxylic acid + metal carbonate → salt + water + carbon dioxide

2CH₃COOH(aq) + Na₂CO₃(aq) → 2CH₃COONa(aq) + H₂O(l) + CO₂(g)

The salts of ethanoic acid are called ethanoates, e.g. sodium ethanoate, CH₃COONa. Salts of propanoic acid are propanoates, and so on. Carboxylic acids also react with reactive metals such as magnesium to give a salt and hydrogen. Their solutions conduct electricity because they contain ions, and hydrogen is given off at the negative electrode when they are electrolysed.

Worked example: volume of gas (C = 12, H = 1, O = 16; 1 mol of gas = 24 dm³ at RTP)

6.0 g of ethanoic acid reacts with excess sodium carbonate. What volume of CO₂ forms?
Mr(CH₃COOH) = 24 + 4 + 32 = 60, so moles = 6.0 ÷ 60 = 0.10 mol.
Ratio acid : CO₂ = 2 : 1, so 0.050 mol CO₂. Volume = 0.050 × 24 = 1.2 dm³.

Answer: 1.2 dm³ of carbon dioxide

Why carboxylic acids are weak acids (Higher tier)

An acid releases H⁺ ions in water. A strong acid such as hydrochloric acid ionises completely. A weak acid such as ethanoic acid ionises only partially: most molecules stay whole, and the reaction is reversible.

HCl(aq) → H⁺(aq) + Cl−(aq) (complete)

CH₃COOH(aq) ⇌ CH₃COO−(aq) + H⁺(aq) (partial, reversible)

So, at the same concentration, a solution of ethanoic acid has a lower concentration of H⁺ ions than hydrochloric acid. Its pH is higher (e.g. about 3 instead of 1 at 0.1 mol/dm³), and it reacts more slowly with carbonates and metals. It also conducts electricity less well, because it contains fewer ions.

Exam tip: “Weak” and “dilute” are different ideas. Weak means only partly ionised. Dilute means a small amount of acid in a given volume of water. You can have a concentrated solution of a weak acid.

Making esters

A carboxylic acid reacts with an alcohol to make an ester and water. A few drops of concentrated sulfuric acid are added as a catalyst, and the mixture is warmed. The reaction is reversible.

  • Method: mix ethanol and ethanoic acid in a boiling tube, add a few drops of concentrated sulfuric acid, and warm in a hot water bath for a few minutes.
  • Safety: ethanol and esters are flammable, so use a water bath, not a naked flame. Concentrated sulfuric acid is corrosive: wear eye protection and gloves.
  • Result: pour the mixture into sodium carbonate solution to neutralise the acids. The ester floats as an oily layer with a sweet, fruity smell.

Worked example: percentage yield (C = 12, H = 1, O = 16)

A student reacts 6.0 g of ethanoic acid with excess ethanol and collects 5.5 g of ethyl ethanoate.
Moles of acid = 6.0 ÷ 60 = 0.10 mol, so the maximum is 0.10 mol of ester (ratio 1 : 1).
Theoretical mass = 0.10 × 88 = 8.8 g. Percentage yield = 5.5 ÷ 8.8 × 100 = 62.5%

Answer: 62.5%

Telling the families apart

Alcohols, carboxylic acids and esters are often colourless liquids. Simple tests tell them apart:

  • Universal indicator; Alcohol (e.g. propanol) green, pH 7 (neutral); Carboxylic acid (e.g. propanoic acid) orange, about pH 3–4; Ester (e.g. ethyl ethanoate) green, pH 7
  • Add sodium carbonate; Alcohol (e.g. propanol) no fizzing; Carboxylic acid (e.g. propanoic acid) fizzes: CO₂ turns limewater milky; Ester (e.g. ethyl ethanoate) no fizzing
  • Smell (waft carefully); Alcohol (e.g. propanol) sharp, like surgical spirit; Carboxylic acid (e.g. propanoic acid) sharp, vinegary; Ester (e.g. ethyl ethanoate) sweet, fruity
  • Mix with water; Alcohol (e.g. propanol) mixes (small alcohols); Carboxylic acid (e.g. propanoic acid) mixes (small acids); Ester (e.g. ethyl ethanoate) forms a separate oily layer

Exam tip: To tell a weak acid from a strong acid, compare solutions of the same concentration: the strong acid has the lower pH and fizzes faster with a carbonate. (Higher tier)

Naming and using esters

  • ethanol; Carboxylic acid ethanoic acid; Ester ethyl ethanoate
  • methanol; Carboxylic acid ethanoic acid; Ester methyl ethanoate
  • ethanol; Carboxylic acid propanoic acid; Ester ethyl propanoate
  • propanol; Carboxylic acid methanoic acid; Ester propyl methanoate

Esters are volatile and many have distinctive, fruity smells. They are used as flavourings in sweets and drinks and in perfumes, where they evaporate easily so the smell reaches your nose. Ethyl ethanoate is also used as a solvent, for example in some glues and nail varnish removers.

Common mistakes

  • Saying ethanoic acid is weak because it is dilute. Weak means partially ionised.
  • Forgetting the water when an ester forms, or forgetting the acid catalyst.
  • Naming the ester the wrong way round. The alcohol gives the first word (“ethyl”), the acid the second (“ethanoate”).
  • Leaving carbon dioxide out of the carbonate reaction. Acid + carbonate gives salt + water + carbon dioxide.

Revision questions

Try these, then check your answers below each one.

Question 1. What forms when ethanoic acid reacts with sodium carbonate? (Choose from: Sodium ethanoate, water and carbon dioxide, Sodium ethanoate and hydrogen, Ethyl ethanoate and water, Sodium hydroxide and carbon dioxide.)

Answer: Sodium ethanoate, water and carbon dioxide

Working: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂. Hydrogen forms with metals; an ester forms with an alcohol.

Question 2. Name the ester made from ethanol and ethanoic acid.

Answer: ethyl ethanoate

Working: Ethyl ethanoate, CH₃COOC₂H₅.

Question 3. Higher tier. Why is ethanoic acid described as a weak acid? (Choose from: It is very dilute, It only partially ionises in water, It does not dissolve in water, It has a pH above 7.)

Answer: It only partially ionises in water

Working: Weak acids are only partially ionised: CH₃COOH ⇌ CH₃COO− + H⁺. “Dilute” describes concentration, which is a different idea.

Related lessons

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