What is filtration under reduced pressure used for?
For separating a solid product from a solvent or liquid reaction mixture.
What apparatus is required to carry out the technique of filtration under reduced pressure?
Buchner flask Buchner funnel pressure tubing filter paper access to filter or vacuum pump
When is recrystallisation used?
To remove impurities from the solid product which has been obtained after filtration.
Why do chemists determine the melting point of solids?
To see whether a solid is pure.
How will the melting point be affected if a substance contains impurities?
The solid will melt over a wide range of temperatures. An impure sample also has a lower melting point than a pure sample.
Recrystallisation process
- dissolve impure solid in minimum amount of hot solvent
- let solution cool and crystals will form
- filer the crystals by vacuum filtration
- wash pure crystals with minimum amount of cold solvent
- dry the crystals
Give 6 ways the purity of a dry solid can be analysed.
melting point, TLC, GC, IR, MS, NMR
How can an acidic impurity be removed from an organic liquid?
add Na2CO3 (aq)
How can an alkaline impurity be removed from an organic liquid?
add dilute acid
How can an organinc liquid be separated into an organic (non-polar) layer and an aqueous layer?
using a separating funnel
How can an organic product be dried?
- remove traces of water by using a separating funnel to separate the liquid into an organic (non-polar) layer and an aqueous layer.
- add an anhydrous salt (e.g. anhydrous Na2so4)
What does distillation do?
Separates pure liquid from other impurities still present.
Give 5 ways the purity of a dry liquid can be analysed.
TLC, GC, IR, MS, NMR
What is reflux used for?
For boiling volatile, flammable reactants and/or products over a prolonged period of time.
How does reflux prevent the loss of volatile reactants / products?
A vertical condenser is used.
What could we have upon cooling the refluxed reaction mixture?
- a mixture of immiscible liquids that separates into 2 layers (organic and aqueous) - use a separating funnel
OR - a mixture of miscible liquids - use distillation as each component has a unique boiling point.
What would we see in thin layer chromatography of a pure substance?
Only one spot. Extra spots would indicate impurities.
What is another name for vacuum filtration?
Filtration under reduced pressure?
What is the process in vacuum under reduced pressure?
- Wet filter paper with the solvent used in preparing your solid.
- Ensure good tight fit between the Buchner funnel and the side arm flask by using a rubber bung.
- Slowly pour your reaction mixture from a beaker into the CENTRE of the filter paper.
- Rinse out the beaker with the solvent so that all of the solid crystals collect in the Buchner funnel.
- Rinse the crystals in the Buchner funnel with more solvent and leave them under suction for a few minutes td dry.
What properites must the desired product and the impurities have for purification by recrystallisation to work?
They must have different solubilities in the chosen solvent. The desired product must be soluble in the solvent when the solvent is hot but insoluble when it is cold.
