Edexcel A Level Chemistry:复习笔记3.5.3 Organic Liquid Preparation & Purification

Organic Liquid Preparation & Purification

 

Simple Distillation

  • This is used to separate a liquid and soluble solid from a solution (e.g. water from a solution of salt water) or a pure liquid from a mixture of liquids
  • The solution is heated, and pure water evaporates producing a vapour which rises through the neck of the round bottomed flask
  • The vapour passes through the condenser, where it cools and condenses, turning into the pure liquid that is collected in a beaker
  • After all the water is evaporated from the solution, only the solid solute will be left behind

Simple distillation of saltwater, IGCSE & GCSE Chemistry revision notes

Diagram showing the distillation of a mixture of salt and water

Fractional Distillation

  • This is used to separate two or more liquids that are miscible with one another (e.g., ethanol and water from a mixture of the two)
  • The solution is heated to the temperature of the substance with the lowest boiling point
  • This substance will rise and evaporate first, and vapours will pass through a condenser, where they cool and condense, turning into a liquid that will be collected in a beaker
  • All of the substance is evaporated and collected, leaving behind the other components(s) of the mixture
  • For water and ethanol
    • Ethanol has a boiling point of 78 ºC and water of 100 ºC
    • The mixture is heated until it reaches 78 ºC, at which point the ethanol boils and distills out of the mixture and condenses into the beaker

     

  • When the temperature starts to increase to 100 ºC heating should be stopped. Water and ethanol are now separated

Fractional-distillation-of-a-mixture-of-ethanol-and-water

Fractional distillation of a mixture of ethanol and water

Solvent Extraction

  • This method uses a solvent to remove a desired organic product from a reaction mixture
  • Various solvents can be used but the solvent should have the following features:
    • Immiscible (does not mix) with the solvent containing the desired organic product
    • The desired organic product should be much more soluble than the added solvent
  • The process is summarised as follows
    • Place the reaction mixture in a separating funnel and add the chosen solvent forming a separate layer
    • Place a stopper in the neck of the funnel and gently shake the contents of the funnel for a while
    • Allow the contents of the funnel to settle into two layers
    • Remove the stopper and open the tap to allow the lower layer to drain into a flask
    • Pour the remaining layer into a separate flask

8.3.2-Separation-of-ethyl-ethanoate

Process of solvent extraction

  • Using smaller volumes of solvent in stages is more efficient at removing the desired organic product compared to using the same volume in one portion

Crystallisation

  • Used to separate a dissolved solid from a solution, when the solid is much more soluble in hot solvent than in cold (e.g., copper sulphate from a solution of copper (II) sulphate in water)
  • The solution is heated, allowing the solvent to evaporate, leaving a saturated solution behind
  • Test if the solution is saturated by dipping a clean, dry, cold glass rod into the solution
    • If the solution is saturated, crystals will form on the glass rod

     

  • The saturated solution is allowed to cool slowly
  • Crystals begin to grow as solids will come out of solution due to decreasing solubility
  • The crystals are collected by filtering the solution, they are washed with cold distilled water to remove impurities and are then allowed to dry

Process-Of-Crystallisation

Diagram showing the process of crystallisation

Boiling point determination

  • A liquids purity can be determined by measuring its boiling point
  • Impurities present in the liquid will raise its boiling temperature
  • The boiling points of pure organic compounds have been carefully measured and are widely available in data books and online
  • The measured boiling point can be compared to the data book value to determine how pure the compound is

 

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