Required Practical 5 - Distillation of a Product from a Reaction (AQA A-Level Chemistry): Revision Notes

4.2.3 Required Practical 5 - Distillation of a Product from a Reaction

Aim

To distil a product from a chemical reaction to separate it based on boiling point differences. This practical focuses on the preparation of cyclohexene from cyclohexanol through dehydration.

Equipment

Chemicals

• Cyclohexanol (20 cm³)
• Concentrated phosphoric acid ($H₃PO₄$, approximately 8 cm³)
• Saturated sodium chloride solution (50 cm³)
• Anhydrous calcium chloride (drying agent)
• Bromine water (for alkene test)

Apparatus

• Pear-shaped flask (50 cm³)
• Graduated dropping pipette
• Anti-bumping granules
• Distillation apparatus:
  • Condenser
  • Thermometer
  • Still head
• Separating funnel
• Conical flask
• Measuring cylinder (25 cm³)
• Beakers (for separating layers)
• Stopper
• Balance (2 decimal places)
• Heating mantle or water bath (for heating)

Risk Assessment

• Cyclohexanol is flammable:
  • Use a water bath or electric heater instead of an open flame.
• Concentrated phosphoric acid is corrosive:
  • Wear safety goggles, gloves, and a lab coat.
  • Handle carefully, avoiding contact with skin and eyes.
• Cyclohexene is flammable:
  • Keep away from sparks and open flames.
• Ensure proper ventilation when handling volatile substances, or perform in a fume cupboard.

Method

1. Weigh cyclohexanol:

• Pour 20 cm³ of cyclohexanol into a 50 cm³ pear-shaped flask.
• Record the mass.

2. Add phosphoric acid:

• Carefully add 8 cm³ of concentrated phosphoric acid using a graduated dropping pipette. Shake the mixture.

3. Set up distillation apparatus:

• Add anti-bumping granules to the flask to ensure smooth boiling.
• Assemble the distillation apparatus with the flask, condenser, and thermometer.
• Ensure the thermometer bulb is at the junction with the condenser.

4. Distillation:

• Heat the flask gently.
• Collect any liquid distilling below 100°C in a separate container.

5. Separate layers:

• Transfer the distillate to a separating funnel.
• Add 50 cm³ of saturated sodium chloride solution, shake, and allow layers to separate.
• Discard the lower aqueous layer and keep the upper organic layer containing crude cyclohexene.

6. Dry the product:

• Add a few lumps of anhydrous calcium chloride to the cyclohexene to remove water.
• Shake and let stand until clear.

7. Final collection:

• Decant the dry cyclohexene into a clean, pre-weighed sample container.
• Calculate the mass of cyclohexene produced by weighing the container and subtracting the empty container's mass.

8. Test for cyclohexene:

• Add Bromine water to a sample of cyclohexane.
• A positive test (for alkenes) will turn the solution from orange to colourless.

Results

• Record the initial and final masses to determine the yield of cyclohexene.
• Observe and note the colour change in the Bromine water test to confirm the presence of an alkene. Example calculation of percentage yield:

$$\text{Percentage yield} = \left( \frac{\text{actual yield}}{\text{theoretical yield}} \right) \times 100\%$$

Errors and Improvements

• Errors:
  • Loss of product due to evaporation or poor separation.
  • Impurities from incomplete dehydration.
• Improvements:
  • Use a water-cooled condenser to minimise evaporation losses.
  • Ensure tight seals in the apparatus to prevent leaks.
  • Use a fume cupboard for better control of volatile compounds.