5.6.5Unit 5. Transition Metals and Organic Nitrogen Chemistry
Organic Techniques – Purification — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 42 of 43 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4Purifying a Liquid Product
1
Transfer the cooled distillate to a separating funnel and add water or a wash solution so two layers form.
2
Run off the lower layer, working out which layer contains the organic product from its density relative to water.
3
Wash the organic layer with sodium hydrogencarbonate solution to remove acidic impurities; carbon dioxide forms, so open the tap often to release pressure.
4
Dry the organic layer with an anhydrous drying agent such as anhydrous magnesium sulfate or calcium chloride until it is no longer cloudy.
5
Filter off the drying agent and redistil the liquid, collecting only the fraction at the correct boiling point.
Part 2 of 4Separating Funnel and Drying
Water and acid are the commonest impurities in a liquid product.
Immiscible layers
- Immiscible layers The organic layer and the aqueous layer do not mix, so they can be separated by running off one layer.
Why wash
- Why wash Sodium hydrogencarbonate reacts with acid impurity: RCOOH + NaHCO₃ → RCOONa + H₂O + CO₂.
Drying agent
- Drying agent Anhydrous salts take up traces of dissolved water by forming hydrates and are easily removed by filtration.
Sign of dryness
- Sign of dryness The liquid turns from cloudy to clear and some of the solid drying agent stays free-flowing.
Part 3 of 4Purifying a Solid by Recrystallisation
Purifying a Solid by Recrystallisation
Choose solvent
- Choose solvent The solid should dissolve well in the hot solvent but poorly in the cold solvent.
Dissolve
- Dissolve Add the minimum volume of hot solvent so that the solution is saturated and the maximum yield is recovered.
Hot filtration
- Hot filtration Filtering while hot removes insoluble impurities before the product starts to crystallise.
Cool and collect
- Cool and collect Slow cooling gives larger purer crystals, which are filtered under reduced pressure and washed with a little cold solvent.
| Method | Best for | Removes |
|---|---|---|
| Redistillation | Liquids | Impurities of different boiling point |
| Recrystallisation | Solids | Soluble and insoluble impurities |
| Solvent extraction | Mixtures in two solvents | Water-soluble or acid impurities |
Part 4 of 4Exam Checklist & Tips
5.6.5 – Key Points for the Exam
- Immiscible Liquids that do not mix and form two separate layers.
- Drying agent An anhydrous solid added to remove traces of water from a liquid.
- Recrystallisation Purification of a solid by dissolving in hot solvent and cooling to form crystals.
- Filtration under reduced pressure Faster filtration using a vacuum with a Buchner funnel.
Common Mistakes
- Using too much hot solvent in recrystallisation, which lowers the yield.
- Failing to vent the separating funnel when washing with carbonate, so pressure builds up.
- Using a wet or hydrated drying agent that cannot remove more water.
- Washing crystals with warm solvent, which dissolves some product.
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