Required Practical 10a (AQA A Level Chemistry)

Required Practical 10a: Preparation of an organic solid and a test of its purity

• This practical covers a number of key laboratory skills
  • use appropriate apparatus to record a range of measurements
  • use water bath or electric heater or sand bath for heating
  • use laboratory apparatus for a variety of experimental techniques
  • purify a solid product by recrystallisation
  • use melting point apparatus
  • safely and carefully handle solids and liquids

• There are a few choices of experiments that are commonly done in schools which cover these laboratory skills and three which can be done are
• The preparation of aspirin
• The purification of benzoic acid
• The nitration of methyl benzoate

• Recrystallisation is used to purify impure solids
• The principle is that a hot solvent is used to dissolve both the organic solid and the impurities and then as the solution cools the solid crystallises out and leaves behind the impurities in the solution
• The key is using the minimum amount of solvent to dissolve the solid and avoid loss of the product
• If any solid impurities remain in the solution, a hot filtration can be carried out
• Once the solution has cooled down to room temperature and crystallised then the product crystals can be recovered by filtration
• This is faster using Buchner apparatus in which filtration occurs under reduced pressure

Recrystallisation and Buchner filtration 

• After filtration the product is washed with fresh cold solvent and then allowed to dry on filter paper

• The melting point of a solid is indicative of its purity and identity
• A melting point can be matched to a known substance as a means of identification or confirmation of a desired product
• The proximity of a melting point to the actual data book value can express purity
  • Impurities tend to lower the melting point of a solid

• The melting point range also reveals the degree of purity
  • Pure substances have sharp well defined melting points
  • Impure substances have a broad melting point range, i.e. a large difference between when the substance first melts until it completely melts

• The skills needed in performing a melting point test are largely dependent on the specific melting point apparatus you are using
• However, there are some common key skills:
  • Correctly preparing the melting point tubes
  • Heating the tubes very slowly
  • Repeating to get a range of measurements (three would be normal)

• The sample solid must be totally dry and finely powdered - this can be achieved by crushing it with the back of a spatula onto some filter paper or the back of a white tile (this absorbs any moisture)
• Use the first tube to find the approximate melting point range and then repeat using a much slower heating rate

Melting point test using an oil bath

Melting point test using a Thiele tube

Melting point test using a melt station

Key steps in the procedure

• 6.0 g of salicylic acid is added to a conical flask along with 10 cm³ of ethanoic anhydride and 5 drops of concentrated sulfuric acid
• The mixture is swirled and held in a warm water bath around 60 ^oC for about 20 minutes
• The flask is then allowed to cool and the contents are added to 75 cm³ cold water in a beaker at which point the aspirin crystallises out
• The aspirin is recovered using Buchner filtration and left to dry

The preparation of aspirin using a hot water bath

Recrystallisation and melting point test

• The impure aspirin is recrystalised using ethanol, and when the solid has dissolved the solution is poured over 40 cm³ cold water and recovered using Buchner filtration as before
• The melting point of pure aspirin is 135 ^oC, so the purity of the product can be assessed

• Benzoic acid is a suitable substance for purification by recrystallisation as it
  • Readily available in most school labs
  • Dissolves in hot water, but not in cold water
  • Is relatively safe to handle in the laboratory

Key steps in the procedure

• 1.0g of impure benzoic acid is added to a small conical flask and the flask is place on a hot plate
• Using a supply of hot water, the minimum amount of hot water is added to the flask to dissolve the benzoic acid
  • It should be around 30-35 cm³ of water

• When is it dissolved a hot filtration is carried out to remove any insoluble impurities
• To do this a pre-heated glass funnel is rested on a hot conical flask and a piece of fluted filter paper is added
• The funnel should have a short stem to avoid the product solidifying prematurely and blocking the funnel
• The filtrate is allowed to cool and crystallise
  • Sometimes crystallisation can be difficult to achieve, so agitating the flask or scratch the inner walls with a glass rod can help initiate the process

• The solid product is then recovered by Buchner filtration, washed and dried

• The preparation of 3-nitromethyl benzoate is an experiment that utilises all the skills covered in Require Practical 10a
• The equation for the reaction is:

C₆H₅COOCH₃    +    HNO₃  → C₆H₄(NO₂)COOCH₃  + H₂O 

Key steps in the procedure

• 2.0 g of methyl benzoate is added to a small conical flask
• The flask is immersed into an ice-water bath and 4.0 cm³ of concentrated sulfuric acid is slowly added
• A mixture of 1.5 cm³ of concentrated nitric acid and 1.5 cm³ concentrated sulfuric acid is made in a separate test tube and immersed in the ice bath - this is the nitrating mixture
• A thermometer is placed in the conical flask: the temperature must be kept below 6 ^oC
• The nitrating mixture is very slowly added to the flask over about 15 mins ensuring the temperature remains low
• After the addition the mixture is allowed to remain at room temperature for around 15mins
• The contents of the flask are then poured over crushed ice where the product solidifies
• The ice is allowed to melt and the product is recovered by Buchner filtration

The nitration of methyl benzoate using an ice bath

Recrystallisation & melting point

• Recrystallisation is carried out in a water-ethanol mixture
• The crystals are recovered and dried
• The melting point of 3-nitromethylbenzoate is 78 ^oC

• The concentrated sulfuric and nitric acid are very corrosive and oxidising so safety glasses and gloves should be worn
• Methyl benzoate is moderately harmful
• Ethanol is flammable so recrystallisation should be done with a hot plate not a naked flame