Butanoic acid is a carboxylic acid responsible for the unpleasant smell of rancid butter. Explain what is meant by the term 'carboxylic acid', and describe what would be observed when butanoic acid solution is added to magnesium ribbon, naming one product formed in this reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Butanoic acid has the molecular formula C₃H₇COOH and is a weak acid found in dairy products that have spoiled.
Model answer (5 marks)
A carboxylic acid is an organic compound that contains a carboxyl functional group –COOH. The –COOH group consists of a carbonyl (C=O) bonded to a hydroxyl (–OH) and gives the compound acidic properties.
When a solution of butanoic acid is added to magnesium ribbon, the acid reacts with the metal. The magnesium dissolves, producing a vigorous effervescence.
The effervescence is due to the evolution of hydrogen gas.
The product formed is hydrogen gas (H₂).
When a solution of butanoic acid is added to magnesium ribbon, the acid reacts with the metal. The magnesium dissolves, producing a vigorous effervescence.
The effervescence is due to the evolution of hydrogen gas.
The product formed is hydrogen gas (H₂).
Examiner tips
- Define the –COOH group and state its acidic nature; mention partial dissociation in water.
- Describe the reaction with magnesium: dissolution of the ribbon and effervescence.
- Name the gas produced – hydrogen – as the key product.
- Use correct chemical terminology and avoid unnecessary words.
Common mistakes
- Confusing the carboxyl group with a carbonyl group alone.
- Claiming the acid is a strong acid or that it fully dissociates.
- Failing to mention the magnesium dissolving or the gas produced.
Mark scheme (5 marks)
- Carboxylic acids contain the carboxyl / –COOH functional group
- Carboxylic acids are weak acids (only partially dissociate / ionise in water)
- Bubbles / fizzing / effervescence observed
- Magnesium dissolves / disappears (gets smaller / ribbon dissolves)
- Hydrogen gas is produced (named product)
Key terms in this question
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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