A student carries out a multi-step organic synthesis. In the first step, ethene reacts with steam to produce ethanol. In the second step, ethanol is oxidised to produce ethanoic acid. The student wants to confirm the identity of the ethanoic acid product using chemical tests. Describe the chemical tests the student could carry out to show that the product contains a carboxylic acid group, and state the observations that would confirm a positive result.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ethanoic acid is a carboxylic acid commonly used in the food industry. It can be made in the laboratory from ethanol by oxidation.
Model answer (5 marks)
1. Add a small amount of the product to a solution of sodium carbonate (or sodium hydrogencarbonate). 2. Observe effervescence – bubbles of gas are released. 3. Pass the gas through limewater; the limewater turns milky, indicating the gas is CO₂. 4. Test the product with litmus or a universal indicator; the solution turns blue litmus red or shows an acidic pH. 5. These two observations together confirm the presence of a carboxylic acid group.
Examiner tips
- Use the correct reagent (sodium carbonate or bicarbonate) and note the effervescence clearly.
- Show that the gas is CO₂ by passing it through limewater and observing the milky precipitate.
- Include the pH or colour change of litmus/universal indicator to demonstrate acidity.
- Mention that both tests together confirm a carboxylic acid.
Common mistakes
- Using a weak base that does not give clear effervescence.
- Failing to pass the gas through limewater, so CO₂ is not confirmed.
- Not specifying the colour change of litmus or the pH value.
Mark scheme (5 marks)
- Add the product to sodium carbonate (or sodium hydrogencarbonate) solution
- Observation: effervescence / bubbles produced (carbon dioxide gas given off)
- Pass the gas produced through limewater to confirm it is carbon dioxide
- Test with litmus / universal indicator — the product turns blue litmus red / gives an acidic pH
- State that both tests together confirm the presence of a carboxylic acid (acidic AND produces CO₂ with carbonate)
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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