Techniques (OCR A Level Chemistry)

• This preparation and purification 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 preparation of benzoic acid
  • The nitration of methyl benzoate
    • These are covered in more detail in Unit 1. Development of practical skills in chemistry

Recrystallisation

• 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

Determination of Melting point

• 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