The change in mass when magnesium burns | Experiment

The change in mass when magnesium burns | Experiment

When magnesium burns, it reacts with oxygen from the air to form magnesium oxide, a reaction that significantly changes the compound's mass.

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This practical activity typically lasts around 30-45 minutes, depending on the class's proficiency. Students should all stand and wear appropriate eye protection. Those with long hair should tie it back for safety.

Preparation and Procedure

Preparation

It is advisable for students to practice lifting the lid on and off the crucible and removing the crucible from the pipe clay triangle before starting the experiment. This also ensures that the tongs are functioning correctly.

To light their Bunsen burners, students will need matches or lighters. Alternatively, instructors can light one or two Bunsen burners around the room, and students can light their own using a splint.

One major hazard in this experiment is the hot apparatus, which can take some time to cool down. Students should be cautioned about this.

For classes with shorter attention spans, the final step of heating to a constant mass can be omitted.

Equipment

Apparatus

• Eye protection
• Access to a balance (2 decimal places)
• Per pair or group of students:
  • Crucible with lid
  • Tongs
  • Pipe clay triangle
  • Bunsen burner
  • Tripod
  • Heat-resistant mat
  • Emery paper (optional)

Chemicals

• Magnesium ribbon, about 10-15 cm

Health, Safety, and Technical Notes

• Refer to standard health and safety guidelines
• Wear eye protection
• Magnesium ribbon, Mg(s) – see CLEAPSS Hazcard HC059a. Fresh, clean magnesium is best for this experiment. Tarnished magnesium can be cleaned with emery or sandpaper.

Experimental Procedure

1. Cut a piece of magnesium 10–15 cm long. Clean it with emery paper if it looks tarnished. Twist it into a loose coil.
2. Weigh the crucible with the lid (mass 1) and then with the magnesium inside (mass 2).
3. Set up the Bunsen burner on a heat-resistant mat with the tripod. Place the pipe clay triangle securely on the tripod. Place the crucible with the magnesium in the triangle and cover it with the lid.
4. Light the Bunsen burner and begin heating the crucible. Start with a gentle blue flame, then shift to a roaring flame to initiate the reaction.
5. Once the crucible is hot, carefully lift the lid slightly to allow oxygen in. You may see magnesium flare up. Avoid leaving the lid off for too long to prevent magnesium oxide from escaping.
6. Continue heating and lifting the lid until the reaction ceases. Then, remove the lid and heat for an additional couple of minutes. Replace the lid if product loss is observed.
7. Turn off the Bunsen burner and allow the apparatus to cool.
8. Re-weigh the crucible and lid containing the final product (mass 3).
9. Reheat the crucible for a few more minutes and allow it to cool again. Repeat until mass readings remain consistent (heating to constant mass).

Teaching Notes

Students should record the following masses:

• mass 1 = crucible + lid
• mass 2 = crucible + lid + magnesium
• mass 3 = crucible + lid + product

These values allow calculation of the mass of magnesium (mass 2 - mass 1) and the mass of the product (mass 3 - mass 1). The increase in mass (mass 3 - mass 2) corresponds to the mass of oxygen.

The reaction equation is:

• Magnesium + oxygen → magnesium oxide
• 2Mg + O2 → 2MgO

Occasionally, students may get unconvincing results due to:

• Magnesium oxide product escaping as they lift the lid
• Incomplete reaction, indicated by a gray rather than white product
• Product loss due to prodding with splints
• Incorrect balance taring
• Magnesium coiled too tightly, preventing full reaction

Finding the Formula of Magnesium Oxide

Method One

• To find the formula of magnesium oxide, students need the masses of magnesium and oxygen, along with their relative atomic masses (Magnesium = 24, Oxygen = 16)
• Divide each mass by its atomic mass to find the number of moles
• Determine the ratio by dividing both mole values by the smaller number of moles
• The ratio should approximate 1:1, indicating the formula MgO
• Example:
  • Mass of magnesium = 2.39 g
  • Mass of magnesium oxide = 3.78 g
  • Mass of oxygen = 1.39 g
  • Number of moles Mg = 2.39/24 = 0.0995
  • Number of moles O = 1.39/16 = 0.0868
  • Ratio approximately = 1 Mg : 1 O
  • Formula = MgO

Method Two

• Students need the masses of magnesium and combined oxygen. Provide a graph for the class.
• Students plot their magnesium and oxygen masses onto the graph. Most data points should align with the line representing the formula MgO (1:1 ratio), highlighting any anomalies and reinforcing the correct formula MgO.

Magnesium Oxide Relationship with Magnesium Hydroxide

In wastewater treatment, "MagOx" often refers to either Magnesium Hydroxide or Magnesium Oxide. Despite being chemically related, they have distinct properties, which are crucial for application selection.

Magnesium Oxide (MgO) and Magnesium Hydroxide (Mg(OH)2) both derive from the Magnesium di-cation (Mg2+), an essential macronutrient and core element in chlorophyll. Differences in their production processes result in varied physical and chemical properties.

MgO is obtained by calcining Magnesite ore (MgCO3), controlling temperature and speed to influence structure, porosity, and reactivity. Such control develops diverse MgO products for various industrial applications.

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