Heat of Combustion of Magnesium

Determining ΔH using Hess's law and magnesium oxide

Thermochemistry Advanced (University) 50 min ~$12.00

Objective

Determine the enthalpy of combustion of magnesium indirectly using Hess's law, since direct measurement is impractical.

Background

The combustion of Mg in O₂ is too vigorous for direct calorimetry. Instead, we measure: (1) Mg + HCl → MgCl₂ + H₂, and (2) MgO + HCl → MgCl₂ + H₂O. Combined with the known ΔH for H₂ + 0.5O₂ → H₂O, Hess's law gives ΔH(combustion) of Mg.

Safety Warnings

  • HCl is corrosive
  • Mg + HCl produces flammable H₂ gas
  • Do not perform near open flames
  • Wear goggles and gloves

Required PPE

goggles gloves lab_coat

Materials

  • Magnesium ribbon (0.5 g)
  • Magnesium oxide (MgO) (1 g)
  • Hydrochloric acid (HCl) (200 mL)
    1M

Equipment

Polystyrene cup calorimeter Thermometer Analytical balance Graduated cylinder Lid

Procedure

1

Reaction 1: Add 50 mL 1M HCl to the calorimeter. Record temperature. Add 0.25 g Mg ribbon. Record max temperature.

8 min H₂ produced — no flames
2

Calculate ΔH1 for Mg + 2HCl → MgCl₂ + H₂.

5 min
3

Clean the calorimeter. Reaction 2: Add 50 mL 1M HCl. Record temperature. Add 0.5 g MgO. Record max temperature.

8 min
4

Calculate ΔH2 for MgO + 2HCl → MgCl₂ + H₂O.

5 min
5

Use Hess's law: ΔH(combustion) = ΔH1 - ΔH2 + ΔH(H₂ combustion). ΔH(H₂ + 0.5O₂ → H₂O) = -285.8 kJ/mol.

5 min
6

Compare calculated ΔH with accepted value (-601.6 kJ/mol).

5 min

Expected Results

ΔH1 ≈ -462 kJ/mol, ΔH2 ≈ -146 kJ/mol. ΔH(comb) = -462 - (-146) + (-286) = -602 kJ/mol, close to the accepted -601.6 kJ/mol.

Cleanup

Neutralize HCl waste. Dispose of MgCl₂ solution down the drain. Rinse calorimeter.

Details

Category
Thermochemistry
Difficulty
Advanced (University)
Duration
50 min
Est. Cost
$12.00
Steps
6
Materials
3

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Frequently Asked Questions

What is the objective of Heat of Combustion of Magnesium?
Determine the enthalpy of combustion of magnesium indirectly using Hess's law, since direct measurement is impractical.
How difficult is Heat of Combustion of Magnesium?
This experiment is rated as Advanced (University). It takes approximately 50 minutes to complete.
What safety precautions are needed for Heat of Combustion of Magnesium?
Key safety precautions include: HCl is corrosive; Mg + HCl produces flammable H₂ gas; Do not perform near open flames.
What materials are needed for Heat of Combustion of Magnesium?
The main materials required are: Magnesium ribbon, Magnesium oxide (MgO), Hydrochloric acid (HCl).
What results should I expect from Heat of Combustion of Magnesium?
ΔH1 ≈ -462 kJ/mol, ΔH2 ≈ -146 kJ/mol. ΔH(comb) = -462 - (-146) + (-286) = -602 kJ/mol, close to the accepted -601.6 kJ/mol.