Ethanoic acid reacts with ethanol to form an ester and water. Describe the properties of ethanoic acid that show it is a weak acid, and explain what is meant by the term 'ester' in the context of this reaction. Include the name of the functional group present in ethanoic acid and state one use of the ester formed.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ethanoic acid (CH₃COOH) belongs to the carboxylic acid homologous series. When it reacts with ethanol (CH₃CH₂OH) in the presence of an acid catalyst, an ester called ethyl ethanoate is formed.
Model answer (5 marks)
Ethanoic acid is a weak acid because it only partially dissociates in water, giving a lower concentration of H⁺ ions than a strong acid of the same concentration.
The functional group present in ethanoic acid is the carboxyl group (–COOH).
An ester is the compound formed when a carboxylic acid reacts with an alcohol; the –OH from the acid and the –OH from the alcohol combine, eliminating water.
Ethyl ethanoate is commonly used as a solvent, in perfumes, flavourings or food flavouring.
The functional group present in ethanoic acid is the carboxyl group (–COOH).
An ester is the compound formed when a carboxylic acid reacts with an alcohol; the –OH from the acid and the –OH from the alcohol combine, eliminating water.
Ethyl ethanoate is commonly used as a solvent, in perfumes, flavourings or food flavouring.
Examiner tips
- Use the word ‘partially dissociates’ to show weak‑acid behaviour. Mention the carboxyl group explicitly. Explain ester formation as a condensation reaction. Give a single, clear example of an ester use.
Common mistakes
- Saying the acid is ‘completely dissociated’ or ‘strong’. Forgetting to name the carboxyl group. Describing the ester as a ‘hydrolysis product’ instead of a condensation product. Giving a non‑chemical use such as ‘food preservative’ which is not a typical ester application.
Mark scheme (5 marks)
- Ethanoic acid is a weak acid because it only partially dissociates / ionises in water (not fully dissociated)
- Ethanoic acid produces a lower concentration of H⁺ ions / has a higher pH than a strong acid of the same concentration
- The functional group in ethanoic acid is the carboxyl group (–COOH)
- An ester is the compound formed when a carboxylic acid reacts with an alcohol / the –OH from the acid and –OH from the alcohol combine and water is eliminated
- One correct use of ethyl ethanoate / esters — e.g. as a solvent, in perfumes, in flavourings, or in food flavouring
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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