AQA A-Level Chemistry Paper 2, 2024: Question 4
13 marks · Medium difficulty · Practical Techniques & Data Analysis
Preparation of an Ester: Reflux, Purification and Purity Testing
Practise this questionQuestion
Question text
04 This question is about the preparation of an ester.
04.1 Ester F can be prepared from propan-2-ol and ethanoic acid.
Give an equation for this reaction.
Name ester F.
[2 marks]
Equation
Name
This method is used to prepare a sample of ester F.
Step 1 Mix 10 cm3 of propan-2-ol with 10 cm3 of ethanoic acid.
Add 5 drops of concentrated sulfuric acid.
Reflux this reaction mixture for 20 minutes.
Step 2 Transfer the cooled reaction mixture to a separating funnel.
Add 20 cm3 of aqueous sodium carbonate and shake the mixture.
Step 3 Transfer the organic layer to a beaker and add 5 g of
anhydrous magnesium sulfate.
Decant off the organic liquid.
Step 4 Collect the ester using simple distillation.
04.2 Describe how Step 1 should be done.
In your description you should
• give details of suitable equipment used to add each reagent to the reflux apparatus
• draw a labelled diagram of the apparatus used for refluxing the reaction mixture
• explain any safety precautions needed other than eye protection.
[6 marks]
04.3 In Step 2 the reaction mixture from Step 1 is shaken with aqueous sodium carbonate.
*12* State the purpose of the sodium carbonate.
Suggest a precaution that should be taken while this mixture is shaken in the
separating funnel.
Give a reason for your suggested precaution.
[3 marks]
Purpose of sodium carbonate
Precaution
Reason
04.4 Give the reason for the use of anhydrous magnesium sulfate in Step 3.
[1 mark]
04.5 Suggest how the purity of the ester can be confirmed during the distillation in Step 4.
[1 mark]
Mark scheme
Show the mark scheme
Question Answers Additional Comments/Guidelines Mark
M1 CH3CH(OH)CH3 + CH3COOH ⇌ CH3COOCH(CH3)2 + H2O
O O
OH
M2 Methyl ethyl ethanoate. + + H2O
O
OH
Allow ECF from incorrect 5 carbon ester 2
04.1 (1 x AO1,
Allow other valid names 1 x AO2)
1-methylethyl ethanoate
Isopropyl ethanoate
2-propyl ethanoate
Propan-2-yl ethanoate
– A-LEVEL CHEMISTRY – – JUNE 2024
This question is marked using Levels of Response. Refer to the Mark Indicative Chemistry Content
Scheme Instructions for Examiners for guidance.
Stage 1
Level 3 All stages are virtually complete (virtually complete means one
5–6 marks from stage 1 and two from stages 2 and 3) 1a Measuring cylinder(s) for the propan-2-ol and
Answer communicates the whole explanation, including ethanoic acid (size not required but if specified
should be between 10 - 100 cm3)
equations, coherently and shows a logical progression through 3
all three stages Allow 10cm / graduated pipette or burette
Level 2 All stages are covered but the explanation of each stage may 1b (Dropping/teat) pipette for sulfuric acid (NOT
3–4 marks be incomplete or may contain inaccuracies (covered means graduated or other qualification for pipette)
one from a stage)
OR two stages virtually complete (virtually complete means Stage 2 Diagram eg below to include
one from stage 1 or two from stages 2 and 3) 19
2a Labelled condenser shown vertical and open at
Answer is coherent and shows some progression through all 6
04.2 top and bottom i.e. in cross section
three stages. Some steps in each stage may be incomplete (6 x AO3)
2b Labelled water in at the bottom and water out at
Level 1 Two stages are covered (covered means one from a stage) the top of the vertical condenser
1–2 marks but the explanation of each stage may be incomplete or may
contain inaccuracies
2c Labelled reaction flask recognisable as either
OR only one stage is virtually complete (virtually complete pear shaped or round bottomed
means one from stage 1 or two from stages 2 and 3)
Answer shows some progression between two stages
– A-LEVEL CHEMISTRY – – JUNE 2024
0 mark Insufficient correct chemistry to gain a mark.
04.2
(cont)
– A-LEVEL CHEMISTRY – –
M1 To neutralise / remove / react with (excess) acid
M2 Remove stopper/bung OR tip the funnel upside down and open
the tap
22 M3 There will be a build up of pressure / gas / carbon dioxide M3 must be linked to their precaution in M2
04.3 (1 x AO1,
2 x AO3)
OR
M2 Allow add stopper
M3 To prevent spillage
Drying agent / To remove water Not dehydrating agent 1
04.4
(1 x AO1)
Compare boiling point to a data book/known value Boils at sharp boiling point / over a narrow temp 1
04.5
range (1 x AO1)
How to answer it
Making an Ester by Reflux, Separation, Drying & Distillation
What this question tests
Writing and naming the ester formed from propan-2-ol and ethanoic acid, describing a correct reflux setup and technique (with equipment and safety), explaining the roles of Na₂CO₃ (wash) and anhydrous MgSO₄ (drying), and stating how to assess purity during distillation from boiling point behaviour.
Examiner notes: many lost marks by drawing the ester wrongly (but could still gain the name), by not giving cross-sectional reflux diagrams, and by giving safety steps without the matching reason. Remember: purity ↔ sharp b.p. close to data value.
Equation and name of ester F
✅ Correct answers
CH₃CH(OH)CH₃ + CH₃COOH ⇌ CH₃COOCH(CH₃)₂ + H₂O
Name: isopropyl ethanoate (also accepted: methyl ethyl ethanoate / 2-propyl ethanoate / propan-2-yl ethanoate).
❌ Common errors
- Wrong connectivity of the ester (drawn with 5 carbons but not as an isopropyl ethanoate).
- Forgetting that the acid part gives the –COO– group attached to the alkyl from the alcohol.
💡 Key knowledge
Esterification is acid-catalysed (conc. H₂SO₄) and reversible.
How to do Step 1 (reflux) clearly and safely
📐 Procedure & apparatus
- Measure 10 cm³ propan-2-ol and 10 cm³ ethanoic acid with measuring cylinders / pipettes. Add to a pear-shaped (or round-bottom) flask.
- Add ~5 drops conc. H₂SO₄ using a dropping/teat pipette. Add anti-bumping granules.
- Attach a vertical Liebig/Allihn condenser for reflux: water in at bottom, out at top.
- Heat the flask gently for ~20 min using an electric heater / water bath (not a naked flame).
🧠 Safety (give precaution + reason)
- Fume cupboard / well-ventilated area — to avoid inhaling acidic/volatile fumes.
- Wear gloves — conc. H₂SO₄ and ethanoic acid are corrosive.
- Add anti-bumping granules before heating — to ensure smooth boiling and prevent violent bumping.
- Use water bath/electric heater — reagents and product are flammable.
💡 Diagram checklist for full marks
- Vertical condenser drawn as a cross-section, open at both ends.
- Water flow labelled “in (bottom)” and “out (top)”.
- Flask clearly shown with contents and heat source; anti-bumping granules labelled.
Why add aqueous sodium carbonate in the separating funnel?
✅ Purpose
To neutralise/remove excess acid (reacts with ethanoic acid and residual H₂SO₄).
🧠 Precaution & reason
Precaution: Vent the funnel frequently / remove the stopper and open the tap while shaking (or tip it and open the tap).
Reason: CO₂ is produced, causing a pressure build-up that could force liquid out or pop the stopper.
Why use anhydrous magnesium sulfate in Step 3?
✅ Answer
It is a drying agent — removes water from the organic layer by forming hydrates.
How to confirm purity during simple distillation (Step 4)
✅ What to look for
- Collect the fraction that distils at a sharp boiling point (narrow range).
- Compare to the literature b.p. for isopropyl ethanoate — close agreement indicates high purity.
❌ Common mistakes
- Quoting “melting point” or saying “clear liquid” — neither proves purity here.
- Giving a safety step without the matching reason in part (b).
Final takeaways
💡 Key knowledge
- Ester name = alkyl (from alcohol) + carboxylate (from acid).
- Reflux prevents loss of volatile reagents while heating to speed up reaction.
- Work-up = neutralise, separate, dry, then distil to purify.
🧠 Exam technique
- For levelled practical questions, give: apparatus + method + labelled diagram + safety with reasons.
- Use the word vent in separating-funnel steps to secure the pressure mark.
Topics
Required Practicals · Organic Chemistry · Required Practical 10: Preparation of a pure organic liquid · 3.3.1 Introduction to Organic Chemistry · 3.3.9 Carboxylic Acids and Derivatives
Question and mark scheme from the AQA A-Level Chemistry examination, Paper 2, 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.