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Organic preparations at A2 and recrystallisation (Core Practical 16)

Edexcel IAL Chemistry · WCH16 Practical Skills in Chemistry II

A2 preparations make solids such as aspirin and azo dyes, so purification is by recrystallisation rather than distillation. Core Practical 16 (aspirin) is taught in the Unit 5 practical techniques lesson; here you practise the exam-style reasoning.

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Controlling conditions

Temperature: diazotisation of phenylamine is done at 0 to 5 °C. Below 0 °C the reaction is too slow; above about 5 °C the diazonium ion decomposes, giving nitrogen and phenol as a side product. It is the diazonium salt, not phenylamine, that decomposes (October 2024 report).
Excess reagent: one reactant (such as ethanoic anhydride for aspirin) is used in excess so the more valuable one reacts fully.
Catalyst: a few drops of concentrated acid speed up the acylation.

Recrystallisation

1. Choose a solvent in which the product is very soluble when hot and only slightly soluble when cold, and impurities are either very soluble or insoluble.

2. Dissolve in the minimum volume of hot solvent (heat flammable solvents in a water bath).

3. Filter hot (gravity, with a warmed funnel) to remove insoluble impurities.

4. Cool slowly, then in ice, so crystals form; soluble impurities stay in solution.

5. Filter under reduced pressure, wash with a little cold solvent, dry.

You will be asked to draw step 5: Buchner funnel with perforated plate, filter paper, side-arm flask, vacuum.

Yield and losses

% yield = actual / theoretical x 100. Yields are lower because of incomplete reaction, side reactions, product remaining dissolved in the recrystallisation solvent, and losses on filter paper and glassware. A liquid reagent's volume is found from mass / density.

Worked example

3.50 g of 2-hydroxybenzoic acid (M = 138.0) reacts with excess ethanoic anhydride, giving 3.20 g of aspirin (M = 180.0). Calculate the % yield.

n = 3.50 / 138.0 = 0.02536 mol
Theoretical mass = 0.02536 x 180.0 = 4.57 g
% yield = 3.20 / 4.57 x 100

Answer: 70.1%

Key idea: Diazotise at 0 to 5 °C (diazonium decomposes to phenol above). Recrystallise: minimum hot solvent, hot filter, cool, filter under reduced pressure, cold wash, dry. Losses lower the yield.

Check yourself

2.80 g of 2-hydroxybenzoic acid (M = 138.0) gives 2.65 g of aspirin (M = 180.0). Calculate the percentage yield.
Show answer
72.6 %. Theoretical = 2.80 / 138.0 x 180.0 = 3.65 g; yield = 72.6%.
What volume of ethanoic anhydride (M = 102.0, density 1.08 g cm^-3) reacts exactly with 2.00 g of 2-hydroxybenzoic acid (M = 138.0)?
Show answer
1.37 cm³. n = 2.00 / 138.0 = 0.01449 mol; mass = 1.478 g; volume = mass / 1.08 = 1.37 cm³.
Why is the diazotisation of phenylamine carried out at 0 to 5 °C?
- Phenylamine boils at room temperature
- Nitrous acid freezes at higher temperatures
- At higher temperatures the diazonium ion decomposes to phenol and nitrogen
- The azo dye is only coloured when cold
Show answer
At higher temperatures the diazonium ion decomposes to phenol and nitrogen. Below 0 °C it is too slow; warmer, the diazonium salt breaks down.
Which property makes a solvent suitable for recrystallising a product?
- The product is insoluble in it at all temperatures
- The product is very soluble in it when hot and only slightly soluble when cold
- It reacts with the impurities
- It has a higher boiling point than the product's melting point
Show answer
The product is very soluble in it when hot and only slightly soluble when cold. Then most of the product crystallises on cooling.
Why is the minimum volume of hot solvent used?
- So that the solvent boils faster
- To dissolve insoluble impurities
- To lower the melting temperature
- So that as much product as possible crystallises on cooling
Show answer
So that as much product as possible crystallises on cooling. Excess solvent keeps more product in solution.
Which is a reason the yield of a recrystallised product is below 100%?
- The product is weighed when wet
- Some product stays dissolved in the cold solvent
- The catalyst adds mass
- The solvent is too cold
Show answer
Some product stays dissolved in the cold solvent. No product is completely insoluble in the cold solvent.
Why is the hot solution filtered before cooling?
- To remove soluble impurities
- To speed up crystallisation
- To recover the solvent
- To remove insoluble impurities
Show answer
To remove insoluble impurities. Insoluble solids are removed while the product is dissolved.

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Last updated 8 November 2026. Spotted a mistake? Tell us.