RP10

• Minimum volume & hot water

  To obtain a saturated solution & enable crystallisation on cooling
• Filtered hot


  To remove insoluble impurities OR to prevent crystals forming during filtration
• Cooled in ice


  To increase the amount of crystals formed
• Washed with cold water


  To remove soluble impurities (and hot water would dissolve crystals)
• Reasons for not 100% yield

  • Impure salicylic acid
  • Reaction did not go to completion
  • Mass lost during purification
• Why might a pure sample appear to not melt at the correct temperature?

  
  

  The temperature on the thermometer is not the same as the sample OR the sample heats up at a faster/slower rate than the thermometer
• Heating under reflux


  • The mixture of liquids is heated to boiling point for a prolonged period of time
  • Vapour is formed which escapes from the liquid mixture, is cooled by the condenser and changes back to a liquid returning to the flask
• Distillation


  The purpose is to isolate the impure product from the rest of the reaction mixture
• Washing and drying


  1. Transfer the distillate to a separating funnel and add sodium carbonate solution

  2. Allow the two layers to separate and carefully run off the aqueous lower layer

  3. Add saturated calcium chloride and repeat

  4. Pour the organic layer in to a beaker and add anhydrous calcium chloride to dry
• Excess acidified potassium dichromate
• Reflux (for a period of time)
• Immediately distil (once the reagents are mixed)

  To produce propanal in high yield rather than propanoic acid
• Anti-bumping granules


  To prevent vigorous boiling by preventing larger bubbles from forming
• Keep away from naked flames (heat with a water bath)
• Method


  Weigh solid by difference and transfer to reaction vessel

  Measure out any liquid reagents using glassware

  If required, heat reaction vessel until complete

  Filter under reduced pressure
• Purifying solid (recrystallisation)


  Dissolve in minimum hot solvent- usually water of ethanol

  Allow to cool until crystals form using ice bath if needed

  Filter off product with Buchner funnel and place in desiccator for drying

  Record mass accurately
• Test for purity of product 


  Place crushed sample in capilliary tube

  Place tube in melting point apparatus (Thiele Tube)

  Heat sample

  Heat very slowly near melting point and record melting point temperature

  Compare value to data source value

  Large range of melting point, melting before melting point indicates impure product