Practical 10

• What is recrystallisation?

  > A method used to purify a solid organic compound by removing the insoluble and soluble impurities
• What properties must the solvent used for recrystallisation have?

  > The insoluble impurities must not dissolve in hot and cold solvent

  > The soluble impurities must dissolve in hot and cold solvent

  > The product must be soluble in hot solvent and insoluble in cold solvent
• Explain how you could experimentally find an appropriate solvent for recrystallisation.

  > Take 4 test tubes and add a small spatula of the solid product

  > Add a small amount of a different solvent to each test tube

  > Place the test tubes in a water bath and the solid product should dissolve. Discard any test tubes where the solid product does not dissolve
• Explain how you could experimentally find an appropriate solvent for recrystallisation.

  > Place the remaining test tubes in an ice bath. The product should form crystals. Discard any test tubes where the product does not crystalise

  > Collect the crystals using filtration, allow them to dry and determine the melting point

  > The most appropriate solvent is the one which produces the purest product
• Outline a method for the preparation of pure aspirin.

  > Add 2 g of salicylic acid and 10 cm3 of ethanoic anhydride measured using a measuring cylinder to a conical flask

  > Warm the mixture in a water bath for 20 minutes whilst stirring. Solid product should form

  > Collect the crude aspirin using reduced pressure filtration and wash with a little cold water

  > Allow the crude aspirin to dry
• Outline a method for the preparation of pure aspirin. pt2

  > To purify the crude aspirin, recrystallisation should be used

  > Add the crude aspirin to a boiling tube and add the minimum volume of solvent.

  > Warm the solvent using a water bath. The product should dissolve
• Outline a method for the preparation of pure aspirin. pt3

  > Filter the mixture using reduced pressure filtration to remove the insoluble impurities

  > Collect the filtrate and place in an ice bath to allow the aspirin to crystallise

  > Collect the crystals of aspirin using reduced pressure filtration and wash with a little cold water

  > Allow the crystals to dry by leaving them in a warm place

  > Determine the melting point of the purified aspirin and compare to a data book
• Why is the solid organic product dissolved in the minimum volume of hot solvent?

  To produce a hot saturated solution
• Why is the solid organic product dissolved in the minimum volume of hot solvent?

  To produce a hot saturated solution
• Why are the crystals washed with a small volume of cold solvent?

  To remove any soluble impurities which may be on the crystals
• Why are the crystals compressed in the Buchner flask using a spatula?

  To remove any air spaces to ensure better drying
• Explain how you would determine the melting point of the pure aspirin.

  > Place a small amount of the solid into a capillary tube

  > Place the capillary tube into a melting point apparatus

  > Slowly increase the temperature until the solid melts
• What would be the difference in observation between the crude aspirin and the purified aspirin?

  The purified aspirin would be whiter and more crystalline
• Give two reasons why the yield would be less than 100%.

  > Loss in mass during transfer

  > Incomplete reaction
• Give one reason why the yield would be greater than 100%.

  The product was wet
• Why do industries prefer to use ethanoic anhydride rather than ethanoyl chloride?

  > Ethanoic anhydride does not produce corrosive fumes of hydrogen chloride

  > Ethanoic anhydride is less vulnerable to hydrolysis