Hess's Law Verification

Confirming that enthalpy is a state function using three reactions

Thermochemistry Advanced (University) 60 min ~$15.00

Objective

Verify Hess's law by measuring the enthalpy changes of three related reactions and showing that ΔH1 + ΔH2 = ΔH3.

Background

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.

Safety Warnings

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

Required PPE

goggles gloves lab_coat

Materials

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

Equipment

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

Procedure

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

Expected Results

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

Cleanup

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

Details

Category
Thermochemistry
Difficulty
Advanced (University)
Duration
60 min
Est. Cost
$15.00
Steps
7
Materials
4

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Frequently Asked Questions

What is the objective of Hess's Law Verification?
Verify Hess's law by measuring the enthalpy changes of three related reactions and showing that ΔH1 + ΔH2 = ΔH3.
How difficult is Hess's Law Verification?
This experiment is rated as Advanced (University). It takes approximately 60 minutes to complete.
What safety precautions are needed for Hess's Law Verification?
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 Hess's Law Verification?
The main materials required are: Sodium hydroxide pellets, Hydrochloric acid (HCl), Sodium hydroxide solution, Distilled water.
What results should I expect from Hess's Law Verification?
ΔH1 (dissolution) ≈ -44 kJ/mol, ΔH2 (neutralization) ≈ -57 kJ/mol, ΔH3 (combined) ≈ -101 kJ/mol. ΔH1 + ΔH2 should equal ΔH3 within experimental error (±10%).