AQA Chemistry A-level: Required Practical 10

Preparation of a Pure Organic Solid and Liquid

Reflux

• Definition: Continuous boiling and condensing of a reaction mixture.
• Purpose: Allows heating of organic reaction mixture without losing reactants/products.
• Technique: Add anti-bumping granules to prevent vigorous/uneven boiling.

Purifying an Organic Liquid

• Method:
  • Use a separating funnel to divide the mixture into two layers:
    • Higher density liquid (aqueous) is the bottom layer.
    • Organic product layer forms above.
  • Washing:
    • Use sodium hydrogencarbonate solution for acidic impurities (produces H2O, CO2, Na2SO4).
    • Use saturated sodium chloride solution to help separate layers.
  • Separation:
    • Allow layers to separate, discard the aqueous layer.
    • Add drying agent (anhydrous sodium sulphate) to the organic layer.
    • Decant into distillation flask and distil to obtain pure product.
  • Drying:
    • Use anhydrous calcium chloride to remove remaining water.

Testing Purity

Melting Point

• Pure Sample: Sharp melting point matching data books.
• Impure Sample: Lowered and broader melting range.
• Measurement: Use electronic melting point machine or capillary tube with thermometer.
• Potential Error: Discrepancy between thermometer and sample tube temperatures.

Boiling Point

• Purpose: Assess liquid purity.
• Method: Distillation setup or boiling in an oil bath.
• Considerations:
  • Pressure affects boiling point.
  • Accuracy can be limited as different substances may share boiling points.
• Correct Measurement: Ensure thermometer measures temperature of saturated vapour.

Recrystallisation

• Steps:
  1. Dissolve impure compound in minimal hot solvent.
     • Solvent should dissolve compound and impurities when hot but not the compound when cold.
  2. Hot filter solution to remove insoluble impurities.
  3. Cool solution slowly in ice to maximize crystal yield.
  4. Use suction filtration (Buchner flask) to separate crystals.
  5. Wash crystals with distilled water.
  6. Dry crystals to remove excess water.
• Yield Loss:
  • Crystals lost during filtering or washing.
  • Product remaining in solution post-recrystallization.
  • Potential side reactions.

These notes cover the essential techniques and considerations for preparing and purifying organic solids and liquids, including reflux, purification using a separating funnel, and determining purity through melting and boiling point analysis, as well as the recrystallization process.