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4.2.3Physical Chemistry

Distillation of a Product from a Reaction — AQA AS Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 101 of 102

📚 AQA AS Chemistry Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Section 1 of 6Distillation Apparatus

Heating mantle
  • Heating mantle An electric heater gives controlled heating, which is safer than a Bunsen burner for flammable liquids.
Pear-shaped flask
  • Pear-shaped flask Holds the reaction mixture, together with anti-bumping granules to allow smooth boiling.
Still head and thermometer
  • Still head and thermometer The thermometer bulb sits level with the side arm to measure the temperature of vapour entering the condenser.
Condenser
  • Condenser Cooling water enters at the bottom and leaves at the top so the jacket stays full and cooling is efficient.
Receiving flask
  • Receiving flask Collects the distillate that drips from the condenser.

Section 2 of 6Preparing Cyclohexene

C₆H₁₁OH → C₆H₁₀ + H₂O, dehydration with concentrated phosphoric acid as catalyst.

1
Place cyclohexanol in a weighed pear-shaped flask, add concentrated phosphoric acid carefully and add anti-bumping granules.
2
Assemble the distillation apparatus and heat the flask gently with a heating mantle.
3
Collect the distillate that comes over within about 2 °C of the expected boiling point of cyclohexene.
4
Discard the first drops that distil at a lower temperature, using a clean receiver for the main fraction.
5
Stop collecting if the temperature rises more than a few degrees above the boiling point of the product.

Section 3 of 6Purifying the Product

Purifying the Product
Impurities
  • Impurities The crude distillate contains water, unreacted alcohol, traces of acid and side products.
Washing
  • Washing Shake with sodium hydrogencarbonate solution in a separating funnel, releasing the pressure, to remove acidic impurities.
Separating
  • Separating Run off the lower aqueous layer and keep the upper organic layer containing the cyclohexene.
Drying
  • Drying Add anhydrous calcium chloride to remove water, then decant or filter the clear liquid into a clean flask.

Section 4 of 6Reflux or Distillation?

Reflux or Distillation?
Distillation
  • Distillation Used to remove a product as it forms, as when an aldehyde is distilled from an oxidising mixture.
Reflux
  • Reflux Used to heat a mixture for a long time without loss of volatile reagents, such as the full oxidation to a carboxylic acid.
Test of purity
  • Test of purity Shaking a little of the product with bromine water, which turns from orange to colourless, confirming an alkene.
Yield
  • Yield Percentage yield compares the mass actually obtained with the theoretical maximum.

Section 5 of 6Worked Example

Question

  • 20.0 cm³ of cyclohexanol (density 0.962 g cm⁻³, Mr = 100.2) was converted to cyclohexene (Mr = 82.1).
  • The mass of pure cyclohexene obtained was 11.0 g.
  • Calculate the percentage yield.

  1. Mass of cyclohexanol = 20.0 × 0.962 = 19.24 g.
  2. Moles of cyclohexanol = 19.24 ÷ 100.2 = 0.192 mol.
  3. The equation is 1:1, so the theoretical moles of cyclohexene = 0.192 mol and the mass = 0.192 × 82.1 = 15.8 g.
  4. Percentage yield = (11.0 ÷ 15.8) × 100 = 69.6%.

Answer The percentage yield is about 70%.

Section 6 of 6Exam Checklist & Tips

4.2.3 – Key Points for the Exam

  • Distillation Heating a mixture and condensing the vapour to separate a component of lower boiling point.
  • Reflux Continuous boiling and condensing of vapour back into the flask.
  • Distillate The liquid collected after condensation of vapour.
  • Anti-bumping granules Small pieces of porous material that promote smooth boiling.
  • Separating funnel Glassware used to separate two immiscible liquids by running off the lower layer.

Common Mistakes

  • Connecting the cooling water in at the top of the condenser, so the jacket does not fill.
  • Sealing the end of the apparatus; the system must stay open to the air.
  • Using the volume rather than the mass of cyclohexanol to find moles.
  • Forgetting to release the pressure when shaking the separating funnel.

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