T&P Purification

Question 1

Draw and label the apparatus used to purify organic liquids after synthesis.

Answer 1

Question 2

Describe how to purify an organic liquid which is mixed with an immiscible, aqueous liquid, and which contains acidic / alkaline impurities.

Answer 2

• Add mixture to separating funnel
• Allow layers to separate + settle (denser layer is below)
• To identify aqueous layer, add water + allow to separate. Layer which grew is aqueous
• Dispose of aqueous layer
• Run organic product into conical flask
• To remove acidic / alkaline impurities, add sodium hydrogencarbonate solution / dilute acid until pH neutral
• Dry crude product by adding anhydrous salt, e.g. Na₂SO₄ / MgCO₃ (these absorb water)
• Separate pure product by distillation

Question 3

What is the purpose of simple distillation?

Answer 3

Separating a mixture of miscible liquids with different boiling points.

Question 4

Draw a diagram of the apparatus used for simple distillation.

Answer 4

Question 5

Describe how simple distillation would be used to separate pure liquids from a mixture.

Answer 5

• Add mixture + anti-bumping granules to pear-shaped flask
• Attach condenser to flask
• Put collecting beaker at end of condenser
• Insert bung into pear-shaped flask, with thermometer attached, ensuring its end is in line with condenser tube
• Boil gently using heating mantle
• Most volatile liquid collects in beaker first. Once thermometer reads above its boiling point, stop heating
• Remove and replace beaker
• Heat again and repeat until all liquids have collected

Question 6

What is recrystallisation used for?

Answer 6

Purifying solid, crude organic products.

Question 7

Describe the process of recrystallisation.

Answer 7

• Choose solvent in which desired product is soluble at high temp and sparingly soluble at low temp
• Dissolve mixture in minimum quantity of hot solvent (maximises yield)
• Hot-filter off insoluble impurities (preheating filter funnel + conical flask prevents crystallisation)
• Cool filtrate + allow to crystallise
• Collect crystals by vacuum filtration
• Wash with cold solvent
• Allow to dry (in oven)

Question 8

Vacuum filtration is used to rapidly separate a solid from a filtrate.

Describe how the process is carried out.

Answer 8

• Connect a conical flask (with a side arm) to a vacuum pump
• Place Büchner funnel on flask
• Put a piece of damp filter paper in funnel
• Add solid
• Wash with water
• Suck to remove water (+ solvent)
• Disconnect flask from pump before turning off (prevents suck-back)
• Crude solid is left on filter paper

Question 9

Describe how the melting points (and thus the purity) of organic solids are determined. Include a diagram of the apparatus.

Answer 9

• Seal one end of glass capillary tube by melting in Bunsen flame
• Dip open end into solid to pick up small amount, then tap solid down to sealed end
• Fix tube in melting point apparatus
• Gently heat + stir surrounding liquid
• Record temperature at which solid starts + finishes melting
• Difference = melting range. The larger it is, the more impure the solid

Question 10

Draw a diagram of the apparatus used for the electrolysis of an aqueous solution, during which no gas is collected.

Answer 10

Question 11

Draw a diagram of the apparatus used for the electrolysis of an aqueous solution, during which gaseous products are collected from both electrodes.

Answer 11

Rubber bung*

Question 12

Describe how you would carry out the electrolysis of an aqueous solution.

Answer 12

• Place inert (graphite/platinum) anode + cathode in beaker containing electrolyte
• Connect electrodes with dc supply (e.g. battery)
• Turn on power

Question 13

Describe how electrolysis could be used to purify a sample of magnesium. State where the pure metal will be after the electrolysis has occurred.

Answer 13

• Anode should be impure Mg
• Cathode should be pure Mg
• Electrolyte should contain Mg²⁺ ions
• Place electrodes in beaker containing electrolyte
• Connect electrodes with dc supply (e.g. battery)
• Turn on power
• Pure Mg enters solution from anode

Question 14

Describe how you would carry out the electrolysis of an aqueous solution, and how you would collect the gaseous products from each electrode.

Answer 14

• Place inert (graphite/platinum) anode + cathode in beaker containing electrolyte
• Place inverted test tubes, filled with water, over electrodes
• Connect electrodes with dc supply (e.g. battery)
• Turn on power
• Gaseous products displace water + collect in test tubes