Combustion & Reduction
This practical lesson covers:
- How to determine the formula of a metal oxide by combustion or by reduction
Investigating combustion and reduction
Aim
To determine the formula of a metal oxide by combustion or by reduction.
Background information
Combustion and reduction are two chemical processes used to analyse the composition of compounds, particularly metal oxides. In this practical, you will use combustion to react magnesium with oxygen, forming magnesium oxide. By measuring the mass of magnesium before and after combustion, and comparing it to the mass of the final product (magnesium oxide), you will be able to calculate the formula of magnesium oxide.
Equipment
- Bunsen burner
- Crucible with lid
- Tripod and Gauze
- Digital mass balance
- Tongs
- Magnesium oxide
Method
- Place the balance on a level surface and turn on. Ensure the balance reads zero. If it doesn't read zero, the balance will need zeroing by pressing the reset.
- Record the mass of the crucible and lid using the balance.
- Add a sample of magnesium to the crucible and record the new mass.
- Calculate the mass of magnesium by finding the difference between the two mass readings.
- Place the crucible on a tripod with gauze and heat using a Bunsen burner.
- Periodically lift the lid of the crucible using tongs to let air in. This allows the magnesium to fully oxidise.
- After several minutes have elapsed, measure the mass of the crucible, contents and lid.
- Heat for a further minute. Record the mass. If the mass is still changing, continue to heat the crucible. If the mass has stabilised, stop heating and record the final mass.
- Calculate the mass of magnesium oxide by subtracting the mass of the empty crucible and lid from the mass recorded in step 8.
Safety
- Take care when using the Bunsen burner. Tie long hair back.
- The crucible, lid and contents will be extremely hot. Take care when moving the crucible with tongs. Run any skin burns under cold running water for up to 10 minutes.
What is the purpose of periodically lifting the lid of the crucible using tongs during the heating process?
to let the heat out
to let air in and allow the magnesium to fully oxidise
to prevent the magnesium from burning
to check if the magnesium has turned into magnesium oxide
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What is the purpose of using a digital mass balance in this experiment?
to measure the mass of the magnesium before and after combustion
all of the above
to measure the mass of the crucible and lid
to measure the mass of the magnesium oxide
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What should you do if the balance does not read zero before starting the experiment?
continue with the experiment
measure the mass of the crucible and lid
add more magnesium to the crucible
the balance will need zeroing by pressing the reset
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