Required Practical 10a (AQA A Level Chemistry): Revision Note

Introduction to required practical 10a

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

• This practical covers several key laboratory skills:

  • use appropriate apparatus to record a range of measurements

  • use water bath or electric heater, or a 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 of which can be done are

• The preparation of aspirin

• The purification of benzoic acid

• The nitration of methyl benzoate

Purification by 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

  • product is very soluble in hot / nearly insoluble cold water

  • impurities soluble at all temperatures

  • solvent is volatile (low bp)

  • solvent does not react with the product

  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, the product crystals can be recovered by filtration

• This is faster using a Buchner apparatus, in which filtration occurs under reduced pressure

• 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 AND broaden the melting range of a solid

  • e.g. impure 127 – 132 °C (5° range) vs pure 135 – 136 °C (1° range).

• 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 and when 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

Preparation of Aspirin

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 at around 60 °C for about 20 minutes

• The flask is then allowed to cool, and the contents are added to 75 cm³ of cold water in a beaker, at which point the aspirin crystallises out

• The aspirin is recovered using Buchner filtration and left to dry

Recrystallisation and melting point test

• The impure aspirin is recrystallised using ethanol, and when the solid has dissolved, the solution is poured over 40 cm³ of cold water and recovered using Buchner filtration as before

• The melting point of pure aspirin is 135 °C, so the purity of the product can be assessed

Purification of Benzoic Acid

• 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

• 1.0 g of impure benzoic acid is added to a small conical flask, and the flask is placed 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 it is 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 scratching the inner walls with a glass rod can help initiate the process

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

Nitration of Methyl Benzoate

• The preparation of methyl 3-nitrobenzoate is an experiment that utilises all the skills covered in Required Practical 10a

• The equation for the reaction is:

${\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{COOCH}}_3+{\mathrm{HNO}}_3\to \stackrel{\mathrm{conc}.{\mathrm{H}}_2{\mathrm{SO}}_4}{\to }{\mathrm{C}}_6{\mathrm{H}}_4\left({\mathrm{NO}}_2\right){\mathrm{COOCH}}_3+{\mathrm{H}}_2\mathrm{O}$

Key steps in the procedure

• 2.0 g of methyl benzoate is added to a small conical flask

• The flask is immersed in 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 of 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 °C

• 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 15 mins

• 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

Recrystallisation & melting point

• Recrystallisation is carried out in a water-ethanol mixture

• The crystals are recovered and dried

• The melting point of methyl 3-nitrobenzoate is 78 °C

Key hazards

• The concentrated sulfuric and nitric acids are very corrosive and oxidising → safety glasses and gloves should be worn

• Methyl benzoate is moderately harmful → safety glasses and gloves should be worn

• Ethanol is flammable →  heat in a water bath / no naked flame

Links to related notes

• The identity of methyl 3-nitrobenzoate can be confirmed using thin-layer chromatography

• Organic preparations can often include percentage yield calculations