Chemistry practical 10 - recrystallisation

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What features must an appropriate solvent have to be used in recrystallisation? (2)
1) One that will dissolve both compound and impurities when hot 2) One in which the organic compound itself doesn't dissolve well when cold.
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Why is the minimum volume of solvent used?
To obtain a saturated solution and enable crystallisation on cooling.
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Why is the solvent hot?
Molecules have more KE, so intermolecular forces disrupted so solutes dissolve as their molecules get in between solvent molecules.
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Why is the solution filtered through filter paper?
To remove any insoluble impurities.
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Why is the solution filtered hot?
To prevent crystals re - forming during filtration.
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Why is the filtered solution cooled?
To increase the yield of crystals.
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Why will soluble impurities remain in solution?
Because they are present in small quantities so the solution is not saturated.
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Why are the crystals filtered using a buchner flask?
Water pump connected to the buchner flask reduces pressure and speeds up the filtration.
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Why are the crystals washed with distilled water?
To remove soluble impurities.
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What is the last step before weighing and why must this be done thoroughly?
Dry the crystals. If not dried properly the mass will be larger than expected which can lead to a false % yield calculation (may be >100%)
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Give 3 reasons why the actual yield may be lower than the expected yield.
1) Crystals lost when filtering or washing 2) Some product may stay in solution after recrystallisation 3) Other side reactions may occur (with the organic product and other solutions).
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Other cards in this set

Card 2

Front

Why is the minimum volume of solvent used?

Back

To obtain a saturated solution and enable crystallisation on cooling.

Card 3

Front

Why is the solvent hot?

Back

Preview of the front of card 3

Card 4

Front

Why is the solution filtered through filter paper?

Back

Preview of the front of card 4

Card 5

Front

Why is the solution filtered hot?

Back

Preview of the front of card 5
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