Required Practical 10: Aspirin Flashcards
Draw a diagram to show the apparatus you would use to filter the crude product under reduced pressure (2 marks)
- Side-arm flask
- Flat-bottomed filter funnel + filter paper
Describe briefly how you would determine an accurate value for the melting point of aspirin (2 marks)
- Heat the melting point tube in an oil bath
- Slowly near the melting point
State two observations, during the melting point determination of aspirin, that would indicate that the sample of aspirin is not pure (2 marks)
- Melting point range would be wide
- And below the true melting point
Suggest why a pure sample of aspirin may sometimes appear to melt at a temperature different from 135 °C (1 mark)
The temperature on the thermometer may not actually be the same as the sample
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)
- 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
Explain why anti-bumping granules are used during reflux (2 marks)
- Provide a larger surface area
- Allowing smaller bubbles to form
Explain why reflux is beneficial (2 marks)
- Allows strong heating without losing volatile reactants and products
- They evaporate and condense and fall back into the flask
Describe what happens when a reaction mixture is refluxed (2 marks)
- 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
State the catalyst used in the production of aspirin (1 mark)
Concentrated sulfuric acid
Outline the method of recrystallisation (4 marks)
- 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
Suggest 3 reasons why there could be a loss of yield during the recrystallisation of aspirin [3 marks]
- Crystals lost when filtering or washing
- Some products stay in the solution after recrystallisation
- Other side reactions could have occured
Suggest why there could be a larger than 100% percentage yield of aspirin [1 mark]
- The crystals may not have been dried properly
Describe how you would prepare aspirin [5 marks]
- 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
Explain why crystals are formed when the aspirin mixture is added to water [1 mark]
- Aspirin is insoluble so it forms crystals
Suggest why phosphoric acid is added during the preparation of aspirin [1 mark]
- It acts as a catalyst so speeds up the rate of the reaction
Describe and explain how you would purify aspirin via recrystallisation [5 marks]
- 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
Suggest why aspirin is filtered off using a Buchner flask [2 marks]
- Water pump connected to the flask reduces the pressure
- Speeding up the filtration
Suggest three reasons why ethanoic anhydride is used as opposed to acid chlorides during the formation of aspirin [3 marks]
- Cheaper
- Less corrosive
- Less vulnerable to hydrolysis
- Less dangerous to use
If a sample of aspirin is impure, how will this affect the melting point [1 mark]
- Melting point would be much lower
Suggest 3 ways in which impure aspirin would differ in appearance to pure aspirin [3 marks]
- Larger
- Needle like
- Lighter in colour
Describe how you would purify an organic liquid [6 marks]
- 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
Suggest why the organic layer is usually formed on the top in a separating funnel [1 mark]
- It has a lower density than the aqueous layer
State two properties of the drying agent [2 marks]
- Insoluble in the organic liquid
- Should not react with the organic liquid
