Chapter 28.2

Question 1

What is filtration under reduced pressure used for?

Answer 1

For separating a solid product from a solvent or liquid reaction mixture.

Question 2

What apparatus is required to carry out the technique of filtration under reduced pressure?

Answer 2

Buchner flask
Buchner funnel
pressure tubing 
filter paper
access to filter or vacuum pump

Question 3

When is recrystallisation used?

Answer 3

To remove impurities from the solid product which has been obtained after filtration.

Question 4

Why do chemists determine the melting point of solids?

Answer 4

To see whether a solid is pure.

Question 5

How will the melting point be affected if a substance contains impurities?

Answer 5

The solid will melt over a wide range of temperatures. An impure sample also has a lower melting point than a pure sample.

Question 6

Recrystallisation process

Answer 6

1. dissolve impure solid in minimum amount of hot solvent
2. let solution cool and crystals will form
3. filer the crystals by vacuum filtration
4. wash pure crystals with minimum amount of cold solvent
5. dry the crystals

Question 7

Give 6 ways the purity of a dry solid can be analysed.

Answer 7

melting point, TLC, GC, IR, MS, NMR

Question 8

How can an acidic impurity be removed from an organic liquid?

Answer 8

add Na2CO3 (aq)

Question 9

How can an alkaline impurity be removed from an organic liquid?

Answer 9

add dilute acid

Question 10

How can an organinc liquid be separated into an organic (non-polar) layer and an aqueous layer?

Answer 10

using a separating funnel

Question 11

How can an organic product be dried?

Answer 11

1. remove traces of water by using a separating funnel to separate the liquid into an organic (non-polar) layer and an aqueous layer.
2. add an anhydrous salt (e.g. anhydrous Na2so4)

Question 12

What does distillation do?

Answer 12

Separates pure liquid from other impurities still present.

Question 13

Give 5 ways the purity of a dry liquid can be analysed.

Answer 13

TLC, GC, IR, MS, NMR

Question 14

What is reflux used for?

Answer 14

For boiling volatile, flammable reactants and/or products over a prolonged period of time.

Question 15

How does reflux prevent the loss of volatile reactants / products?

Answer 15

A vertical condenser is used.

Question 16

What could we have upon cooling the refluxed reaction mixture?

Answer 16

1. a mixture of immiscible liquids that separates into 2 layers (organic and aqueous) - use a separating funnel
   OR
2. a mixture of miscible liquids - use distillation as each component has a unique boiling point.

Question 17

What would we see in thin layer chromatography of a pure substance?

Answer 17

Only one spot. Extra spots would indicate impurities.

Question 18

What is another name for vacuum filtration?

Answer 18

Filtration under reduced pressure?

Question 19

What is the process in vacuum under reduced pressure?

Answer 19

1. Wet filter paper with the solvent used in preparing your solid.
2. Ensure good tight fit between the Buchner funnel and the side arm flask by using a rubber bung.
3. Slowly pour your reaction mixture from a beaker into the CENTRE of the filter paper.
4. Rinse out the beaker with the solvent so that all of the solid crystals collect in the Buchner funnel.
5. Rinse the crystals in the Buchner funnel with more solvent and leave them under suction for a few minutes td dry.

Question 20

What properites must the desired product and the impurities have for purification by recrystallisation to work?

Answer 20

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.