Recristallisation de l'Acide Benzoïque

Purification d'un solide en le dissolvant dans un solvant chaud puis en le refroidissant

Crystallization Intermediate (High School) 45 min ~$10,00

Objectif

Purifier l'acide benzoïque impur par recristallisation dans l'eau chaude, démontrant la technique de purification standard des solides organiques.

Contexte

Recrystallization exploits the difference in solubility of a substance at different temperatures. Benzoic acid is very soluble in hot water but nearly insoluble in cold water. By dissolving impure benzoic acid in hot water and cooling slowly, pure crystals form while impurities remain in solution.

Avertissements de sécurité

  • Handle hot water and hot glassware with care
  • Benzoic acid dust can irritate airways
  • Use heat-resistant gloves when handling hot flasks

EPI requis

goggles lab_coat

Matériaux

  • Impure benzoic acid (3 g)
    Mixed with charcoal or sand
  • Distilled water (200 mL)
  • Activated charcoal (0.5 g)
    Optional, for decolorization

Équipement

250 mL Erlenmeyer flask Hot plate Buchner funnel Filter flask Vacuum pump Melting point apparatus Ice bath Watch glass

Procédure

1

Weigh 3 g of impure benzoic acid and record the mass. Note the appearance (color, particle size).

3 min
2

Add the impure sample to 100 mL of distilled water in the Erlenmeyer flask. Heat to boiling while stirring.

8 min Handle hot flask with care
3

If the solution is colored, add 0.5 g activated charcoal, swirl, and filter hot through fluted filter paper.

5 min
4

Allow the clear filtrate to cool slowly to room temperature. Do not disturb.

15 min
5

Cool further in an ice bath to maximize crystal yield.

5 min
6

Collect the crystals by vacuum filtration. Wash with a small amount of ice-cold water.

5 min
7

Dry the crystals on a watch glass. Weigh and calculate percent recovery.

3 min
8

Determine the melting point (pure benzoic acid: 122°C) and compare with the impure sample.

5 min

Résultats attendus

White, needle-like crystals with a sharp melting point at 121-123°C. Typical recovery is 60-80%. The purified crystals should be whiter and have a sharper melting point than the crude material.

Nettoyage

Dispose of filtrate down the drain (benzoic acid is food-grade). Rinse glassware. Dispose of charcoal in solid waste.

Détails

Catégorie
Crystallization
Difficulté
Intermediate (High School)
Durée
45 min
Coût estimé
$10,00
Étapes
8
Matériaux
3

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Frequently Asked Questions

What is the objective of Recristallisation de l'Acide Benzoïque?
Purifier l'acide benzoïque impur par recristallisation dans l'eau chaude, démontrant la technique de purification standard des solides organiques.
How difficult is Recristallisation de l'Acide Benzoïque?
This experiment is rated as Intermediate (High School). It takes approximately 45 minutes to complete.
What safety precautions are needed for Recristallisation de l'Acide Benzoïque?
Key safety precautions include: Handle hot water and hot glassware with care; Benzoic acid dust can irritate airways; Use heat-resistant gloves when handling hot flasks.
What materials are needed for Recristallisation de l'Acide Benzoïque?
The main materials required are: Impure benzoic acid, Distilled water, Activated charcoal.
What results should I expect from Recristallisation de l'Acide Benzoïque?
White, needle-like crystals with a sharp melting point at 121-123°C. Typical recovery is 60-80%. The purified crystals should be whiter and have a sharper melting point than the crude material.