Vérification de la Loi de Hess

Confirmation que l'enthalpie est une fonction d'état à l'aide de trois réactions

Thermochemistry Advanced (University) 60 min ~$15,00

Objectif

Vérifier la loi de Hess en mesurant les variations d'enthalpie de trois réactions liées et en montrant que ΔH₁ + ΔH₂ = ΔH₃.

Contexte

Hess's law states that the total enthalpy change for a reaction is independent of the pathway. This is verified by measuring: (1) NaOH(s) → NaOH(aq), (2) NaOH(aq) + HCl(aq) → NaCl(aq), and (3) NaOH(s) + HCl(aq) → NaCl(aq). The sum ΔH1 + ΔH2 should equal ΔH3.

Avertissements de sécurité

  • NaOH pellets are extremely corrosive and generate heat when dissolved
  • HCl is corrosive
  • Wear goggles and gloves throughout

EPI requis

goggles gloves lab_coat

Matériaux

  • Sodium hydroxide pellets (8 g)
    Anhydrous
  • Hydrochloric acid (HCl) (150 mL)
    1M
  • Sodium hydroxide solution (50 mL)
    1M
  • Distilled water (200 mL)

Équipement

Polystyrene cup calorimeter (2 cups nested) Thermometer Analytical balance Graduated cylinder Stopwatch Lid with hole

Procédure

1

Reaction 1: Add 50 mL distilled water to the calorimeter. Record temperature. Add 2 g NaOH pellets. Stir and record max temperature.

8 min Exothermic — NaOH dissolving
2

Calculate ΔH1 for dissolution. Repeat for accuracy.

5 min
3

Reaction 2: Add 50 mL 1M HCl to the calorimeter. Record temperature. Add 50 mL 1M NaOH. Record max temperature.

8 min
4

Calculate ΔH2 for neutralization.

5 min
5

Reaction 3: Add 50 mL 1M HCl to the calorimeter. Record temperature. Add 2 g NaOH pellets directly. Record max temperature.

8 min Very exothermic
6

Calculate ΔH3 for combined dissolution + neutralization.

5 min
7

Compare ΔH1 + ΔH2 with ΔH3. They should be approximately equal, verifying Hess's law.

8 min

Résultats attendus

ΔH1 (dissolution) ≈ -44 kJ/mol, ΔH2 (neutralization) ≈ -57 kJ/mol, ΔH3 (combined) ≈ -101 kJ/mol. ΔH1 + ΔH2 should equal ΔH3 within experimental error (±10%).

Nettoyage

Neutralize all solutions before disposal. Rinse calorimeter cups. Dry and store equipment.

Détails

Catégorie
Thermochemistry
Difficulté
Advanced (University)
Durée
60 min
Coût estimé
$15,00
Étapes
7
Matériaux
4

Plus d'expériences

Calorimétrie en Gobelet de Café 40 min Capacité Calorifique Spécifique des Métaux 40 min Chaleur de Combustion du Magnésium 50 min Chimie des Poches de Froid 20 min Cristallisation Exothermique de l'Acétate de Sodium 30 min Démonstration de Réaction Endothermique 20 min Enthalpie de Fusion de la Glace 25 min La Dissolution n'Est Pas Toujours Exothermique 25 min Simulation de Calorimétrie en Bombe 45 min

Frequently Asked Questions

What is the objective of Vérification de la Loi de Hess?
Vérifier la loi de Hess en mesurant les variations d'enthalpie de trois réactions liées et en montrant que ΔH₁ + ΔH₂ = ΔH₃.
How difficult is Vérification de la Loi de Hess?
This experiment is rated as Advanced (University). It takes approximately 60 minutes to complete.
What safety precautions are needed for Vérification de la Loi de Hess?
Key safety precautions include: NaOH pellets are extremely corrosive and generate heat when dissolved; HCl is corrosive; Wear goggles and gloves throughout.
What materials are needed for Vérification de la Loi de Hess?
The main materials required are: Sodium hydroxide pellets, Hydrochloric acid (HCl), Sodium hydroxide solution, Distilled water.
What results should I expect from Vérification de la Loi de Hess?
ΔH1 (dissolution) ≈ -44 kJ/mol, ΔH2 (neutralization) ≈ -57 kJ/mol, ΔH3 (combined) ≈ -101 kJ/mol. ΔH1 + ΔH2 should equal ΔH3 within experimental error (±10%).