Carboxylic acids are a homologous series of organic compounds. Explain the typical acidic properties of carboxylic acids and describe how the members of this homologous series differ from one another in terms of their structure.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ethanoic acid is the carboxylic acid found in vinegar. It belongs to a homologous series that includes methanoic acid, propanoic acid and butanoic acid.
Model answer (4 marks)
Carboxylic acids are weak acids that give H⁺ ions in aqueous solution, so a 0.01 M solution has a pH below 7, turns litmus paper red and reacts with metals, carbonates or alkalis to give CO₂, water and a salt.
All members of the series contain the same –COOH functional group. The only structural difference is the length of the alkyl chain: each successive acid has one extra CH₂ group (methanoic, ethanoic, propanoic, butanoic, …).
All members of the series contain the same –COOH functional group. The only structural difference is the length of the alkyl chain: each successive acid has one extra CH₂ group (methanoic, ethanoic, propanoic, butanoic, …).
Examiner tips
- Show the acidic reaction (e.g. with NaOH) to earn the reaction point.
- State the pH/colour change to demonstrate acidity. Mention the common –COOH group. Explain the CH₂ increment for each series member.
Common mistakes
- Failing to mention the –COOH functional group. Using a non‑acidic reaction such as with a base that doesn’t produce CO₂. Not specifying that the chain length increases by one CH₂ for each successive acid.
Mark scheme (4 marks)
- Carboxylic acids react with metals / carbonates / alkalis (accept any one valid acidic reaction)
- Carboxylic acids have a pH below 7 / turn litmus red / produce hydrogen ions (H⁺) in solution
- Each successive member of the homologous series has one additional CH₂ group (in the carbon chain)
- All members share the same functional group –COOH
Key terms in this question
carboxylic acids · homologous series
Related
- All OCR GCSE Chemistry A: Gateway Science (J248) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Organic chemistry questions
- Describe how steam cracking is used to produce smaller, more useful molecules fr…
- Ethene is an alkene produced by the cracking of crude oil fractions. Ethene can …
- Ethanol can be produced by the fermentation of glucose using yeast. Ethanol belo…
- Explain why cracking is an important industrial process for the petrochemical in…