Required Practical 10: Aspirin

Question 1

Draw a diagram to show the apparatus you would use to filter the crude product under reduced pressure (2 marks)

Answer 1

• Side-arm flask

- Flat-bottomed filter funnel + filter paper

Question 2

Describe briefly how you would determine an accurate value for the melting point of aspirin (2 marks)

Answer 2

• Heat the melting point tube in an oil bath

- Slowly near the melting point

Question 3

State two observations, during the melting point determination of aspirin, that would indicate that the sample of aspirin is not pure (2 marks)

Answer 3

• Melting point range would be wide

- And below the true melting point

Question 4

Suggest why a pure sample of aspirin may sometimes appear to melt at a temperature different from 135 °C (1 mark)

Answer 4

The temperature on the thermometer may not actually be the same as the sample

Question 5

During the production of aspirin, salicylic acid is purified by recrystallisation. Explain why first, the sample is:
- dissolved in a minimum volume of hot water
- dissolved in hot water
- filtered out hot
- cooled in ice
- washed with cold water
(5 marks)

Answer 5

• To obtain saturated solution
• To increase the yield of salicylic acid in the solution
• To prevent crystals from forming during filtration
• To increase the amount of crystals formed
• To remove soluble impurities

Question 6

Explain why anti-bumping granules are used during reflux (2 marks)

Answer 6

• Provide a larger surface area

- Allowing smaller bubbles to form

Question 7

Explain why reflux is beneficial (2 marks)

Answer 7

• Allows strong heating without losing volatile reactants and products
• They evaporate and condense and fall back into the flask

Question 8

Describe what happens when a reaction mixture is refluxed (2 marks)

Answer 8

• A mixture of liquids is heated to boiling point for a prolonged time
• Vapour is formed which escapes from the liquid mixture, is changed back into liquid and returned to the liquid mixture

Question 9

State the catalyst used in the production of aspirin (1 mark)

Answer 9

Concentrated sulfuric acid

Question 10

Outline the method of recrystallisation (4 marks)

Answer 10

• Dissolve the product in the minimum volume of water
• Hot water / solvent
• Allow the solution to cool and allow crystals to form
• Filter off the pure product under reduced pressure / using a Buchner funnel and side arm flask

Question 11

Suggest 3 reasons why there could be a loss of yield during the recrystallisation of aspirin [3 marks]

Answer 11

• Crystals lost when filtering or washing
• Some products stay in the solution after recrystallisation
• Other side reactions could have occured

Question 12

Suggest why there could be a larger than 100% percentage yield of aspirin [1 mark]

Answer 12

• The crystals may not have been dried properly

Question 13

Describe how you would prepare aspirin [5 marks]

Answer 13

• Add 2g of salicylic acid + 4cm³ of ethanoic anhydride to pear shaped flask
• Add a few drops of phosphoric acid, swirl and mix
• Fit flask to reflux condenser and heat the mixture on a boiling water bath for about 5 minutes
• Then add the mixture to 40cm³ of distilled water and stir to induce crystallisation
• Then reduce under pressure

Question 14

Explain why crystals are formed when the aspirin mixture is added to water [1 mark]

Answer 14

• Aspirin is insoluble so it forms crystals

Question 15

Suggest why phosphoric acid is added during the preparation of aspirin [1 mark]

Answer 15

• It acts as a catalyst so speeds up the rate of the reaction

Question 16

Describe and explain how you would purify aspirin via recrystallisation [5 marks]

Answer 16

• Dissolve the filtered crystals in a minimum volume of hot solvent to obtain a saturated solution
• Stir the contents until all the aspirin has dissolved
• Pour the hot solution through a warm filter funnel to remove insoluble impurities and to prevent crystals from forming during the filtration
• Then pour the filtrate (solution remaining) into a beaker of water and allow crystals to form by placing in an ice bath
• Filter off the purified solid under reduced pressure and allow it to dry on filter paper

Question 17

Suggest why aspirin is filtered off using a Buchner flask [2 marks]

Answer 17

• Water pump connected to the flask reduces the pressure

- Speeding up the filtration

Question 18

Suggest three reasons why ethanoic anhydride is used as opposed to acid chlorides during the formation of aspirin [3 marks]

Answer 18

• Cheaper
• Less corrosive
• Less vulnerable to hydrolysis
• Less dangerous to use

Question 19

If a sample of aspirin is impure, how will this affect the melting point [1 mark]

Answer 19

• Melting point would be much lower

Question 20

Suggest 3 ways in which impure aspirin would differ in appearance to pure aspirin [3 marks]

Answer 20

• Larger
• Needle like
• Lighter in colour

Question 21

Describe how you would purify an organic liquid [6 marks]

Answer 21

• Add the impure product to a separating funnel
• Add NaHCO₃, shake and release the pressure from the CO₂ produced
• Add saturated NaCl solution to allow the layers to separate
• Then run and discard the aqueous layer (bottom layer)
• Run the organic layer into a conical flask, add some drying agent to dry the organic liquid
• Carefully decant the liquid into a distillation flask and distil to collect the pure product

Question 22

Suggest why the organic layer is usually formed on the top in a separating funnel [1 mark]

Answer 22

• It has a lower density than the aqueous layer

Question 23

State two properties of the drying agent [2 marks]

Answer 23

• Insoluble in the organic liquid

- Should not react with the organic liquid