Butanoic acid is responsible for the distinctive smell of rancid butter. Explain the chemical nature of butanoic acid, including its functional group, its position within a homologous series, and how its acidic properties are typically demonstrated in a reaction with sodium carbonate.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Butanoic acid is a carboxylic acid found in rancid butter. It has a pungent, unpleasant odour and is soluble in water.
Model answer (5 marks)
Butanoic acid contains the functional group –COOH.
It is a member of the carboxylic acids homologous series.
When it reacts with sodium carbonate it produces carbon dioxide gas (effervescence/bubbles).
The reaction also gives a sodium salt of butanoate and water.
This demonstrates the typical acidic properties of carboxylic acids, which react with carbonates in the same way as other acids.
It is a member of the carboxylic acids homologous series.
When it reacts with sodium carbonate it produces carbon dioxide gas (effervescence/bubbles).
The reaction also gives a sodium salt of butanoate and water.
This demonstrates the typical acidic properties of carboxylic acids, which react with carbonates in the same way as other acids.
Examiner tips
- Use the exact phrase ‘–COOH’ for the functional group. Mention the homologous series explicitly. State the effervescence and the formation of a salt and water. Show the reaction is a typical acid–carbonate test.
Common mistakes
- Writing the functional group as ‘COOH’ without the dash. Forgetting to mention the formation of a salt. Describing the reaction as ‘produces CO2’ only, omitting water and the salt.
Mark scheme (5 marks)
- Butanoic acid contains the functional group –COOH
- Butanoic acid is a member of the carboxylic acids homologous series
- Butanoic acid reacts with sodium carbonate to produce carbon dioxide gas (effervescence / bubbles)
- A salt and water are also produced in the reaction with sodium carbonate
- This reaction demonstrates the typical acidic properties of carboxylic acids / carboxylic acids react with carbonates in the same way as other acids
Key terms in this question
functional group · homologous series · sodium carbonate
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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