In a student's distillation setup the thermometer bulb is below the side arm, condenser water enters at the upper connector, and the receiver is sealed. State the correction and reason for each fault.
[6 marks]
Total for this question: 6
A-level Chemistry required practicals
Distilling a product from a reaction
AQA 3.3.5.3 · RP5
A-level Chemistry (7405) · Required practical 5 — method, techniques, safety, analysis and uncertainty. Includes errors documented in examiner reports.
Board and spec code confirmed against AQA 7405 · registry checked 2026-07-11How this checking works
Prepare cyclohexene by acid-catalysed dehydration of cyclohexanol, purify it using a separating funnel and distillation, then assess its yield and purity.
Apparatus
- Round-bottom or pear-shaped flask, still head and receiver adaptor
- Thermometer, water condenser and secure clamps
- Heating mantle, hotplate or controlled water bath
- Anti-bumping granules added before heating
- Measuring glassware, balance and collection vessel
- Separating funnel, sodium carbonate solution, anhydrous drying agent and clean redistillation apparatus
Apparatus & techniques (AT)
AT b · controlled heating
Heat the reaction mixture steadily with a controlled electric or water-bath source so the intended volatile fraction distils safely.
AT d · titration/distillation/reflux/qualitative tests/filtration
Assemble and operate a simple distillation train, then separate, dry or redistil the collected liquid when the context requires it.
AT k · safe handling
Manage flammable vapour, corrosive or oxidising reagents and hot glassware without sealing the apparatus.
Safety
Hazard
Volatile organic liquids and vapours may be flammable and harmful by inhalation.
Control
Work in suitable ventilation, keep ignition sources away, use controlled electric heating and cool the receiver where appropriate.
Hazard
The concentrated phosphoric(V) or sulfuric acid catalyst for the cyclohexanol dehydration is corrosive; with sulfuric acid the mixture can char and release sulfur dioxide.
Control
Add the concentrated acid slowly to the alcohol with cooling and swirling, wear eye protection and gloves, and work with ventilation.
Hazard
Potassium dichromate(VI), used in the secondary oxidation context, is a category 1B carcinogen and mutagen, as well as corrosive and oxidising.
Control
Wear eye protection and gloves as required by the risk assessment, add reagents at a controlled rate and collect chromium(VI) waste separately.
Hazard
A sealed or poorly clamped distillation apparatus can pressurise or break while hot.
Control
Keep the system open to the receiver, clamp each heavy component independently and let the glassware cool before dismantling.
Hazard
Distilling to dryness can overheat concentrated residues and cause violent boiling or decomposition.
Control
Stop heating while liquid remains in the distillation flask; never distil to dryness.
Method
- 1For the primary context, measure the cyclohexanol into the distillation flask, then add the concentrated phosphoric(V) acid catalyst (or concentrated sulfuric acid) slowly with cooling and swirling; aldehyde preparation by controlled oxidation is a secondary distillation context.
- 2Add anti-bumping granules to the cold mixture in the flask, then assemble dry, securely clamped apparatus with no sealed outlet.
- 3Position the thermometer bulb level with the entrance to the condenser so it measures vapour passing into the condenser.
- 4Run condenser water in through the lower connection and out through the upper connection, then heat steadily rather than to dryness.
- 5Collect the fraction around the expected boiling range in a labelled receiver, cooling it where volatility makes loss significant.
- 6Transfer the distillate to a separating funnel and wash with sodium carbonate solution, venting the funnel frequently to release carbon dioxide pressure; separate the organic layer, dry it over anhydrous calcium chloride or magnesium sulfate, then redistil, collecting the fraction around 83 °C for cyclohexene.
CPAC focus (editorial)
This is an editorial study focus, not an AQA mapping of fixed CPAC competencies to this practical.
- CPAC 1: Editorial focus: follow the reaction, assembly and collection sequence without adding granules to a hot liquid or closing the system.
- CPAC 2: Editorial focus: choose collection temperature and work-up steps that isolate the intended volatile product rather than merely any condensate.
- CPAC 3: Editorial focus: control flammable vapour, hazardous reagent, pressure and hot-glass risks throughout the preparation.
- CPAC 4: Editorial focus: record quantities, vapour temperature, collected boiling range, product mass or volume and physical appearance.
Results & processing
- Record starting quantities, the observed boiling range, product mass or volume and any separation or drying observations.
- For cyclohexene preparation, record the separating-funnel washes and drying step before collecting the purified distillation fraction.
- Calculate the theoretical amount from the limiting reagent, then percentage yield = actual amount / theoretical amount x 100.
- A narrow boiling range close to reliable reference data supports purity, but boiling point alone does not prove identity.
Analysis skills
- Explain the acid-catalysed elimination of water from cyclohexanol to form cyclohexene and connect separating-funnel purification to the final distillation.
- Distinguish distillation, which removes volatile product as it forms, from reflux, which returns condensed vapour to the reaction flask.
- Use limiting-reagent stoichiometry and molar masses to calculate theoretical mass and percentage yield.
- Interpret boiling range alongside chemical tests or spectroscopic data rather than treating one physical measurement as conclusive.
Uncertainty
Sources
- Balance or volume resolution affects measured quantities, while transfer, evaporation and material left in the apparatus reduce recovered product.
- Thermometer position, heating rate and mixed fractions can broaden or shift the observed boiling range.
Calculations
- Convert the limiting-reagent measurement into moles before applying the reaction ratio and calculating theoretical product mass.
- Calculate percentage yield from actual divided by theoretical amount; include measurement uncertainties separately from chemical or transfer losses.
Interpretation
- A low yield may reflect incomplete reaction, side reactions, transfer loss or volatile loss; repeating the same method does not remove those systematic losses.
- A yield above 100% suggests retained solvent, water or another impurity, or a measurement error, rather than extra product formation.
Exam angles
- Draw or correct a safe distillation setup, including thermometer position, condenser-water direction and an unsealed outlet.
- Describe the preparation of cyclohexene from cyclohexanol by acid-catalysed dehydration, separating-funnel purification and distillation.
- Explain why distillation is chosen instead of reflux when forming an aldehyde from a primary alcohol.
- Calculate theoretical yield and percentage yield, then distinguish measurement uncertainty from physical product loss.
- Explain separating-funnel layers, drying and redistillation without attributing immiscibility to density.
Where students lose marks
Drawing apparatus as disconnected symbols or as an external view that hides the vapour path.
Fix: Use a clear cross-sectional arrangement showing the flask, still head, correctly placed thermometer, condenser, receiver and open outlet.
- Examiner report: C2-19 · PDF p. 3
Confusing reflux with distillation, or placing the thermometer where it measures the liquid rather than the outgoing vapour.
Fix: For distillation, direct condensate to a separate receiver and place the bulb at the condenser entrance; omit apparatus that serves no purpose in the stated setup.
- Examiner report: C3-21 · PDF p. 5
Saying two liquid layers form because their densities differ.
Fix: Layers form because the liquids are immiscible; density determines which immiscible layer sits on top.
- Examiner report: C3-23 · PDF p. 3
Improve the method
- Cool the receiver and minimise transfers when the desired product is volatile.
- Collect only the intended boiling fraction and redistil after drying when a purer sample is required.
- Rinse transfer vessels with a small compatible portion where the procedure permits, rather than leaving product behind.
- Use a calibrated thermometer correctly positioned in the vapour path and heat at a steady rate.
Source references
- Specification: CSpec 3.3.5.3 · PDF p. 57
Try it — exam-style
Medium
ORIGINAL
Medium
ORIGINAL
15.0 g of cyclohexanol, Mr = 100.0, is dehydrated to cyclohexene, Mr = 82.0, in a 1:1 mole ratio. After washing, drying and redistillation, 8.61 g of cyclohexene is collected. Calculate the percentage yield.
[3 marks]
Total for this question: 3
Hard
ORIGINAL
7.40 g of propan-1-ol, Mr = 60.0, is oxidised to propanal, Mr = 58.0, in a 1:1 mole ratio. The dry propanal collected has mass 4.80 g. Calculate the percentage yield.
[4 marks]
Total for this question: 4
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