A student burns magnesium in excess oxygen. The reaction produces magnesium oxide. The student notices that the mass of the product is greater than the mass of magnesium used at the start. Explain why the mass of magnesium oxide formed is greater than the mass of magnesium used, and explain what is meant by the term 'limiting reactant' in this reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When magnesium burns in excess oxygen, the balanced equation for the reaction is: 2Mg + O₂ → 2MgO
Model answer (5 marks)
The mass of the product is greater because no atoms are lost or created during the reaction (conservation of mass). The magnesium atoms combine with oxygen atoms from the air/oxygen gas. Therefore the mass of magnesium oxide equals the mass of magnesium plus the mass of oxygen that has reacted.
The limiting reactant is the reactant that is completely used up in a reaction. In this reaction magnesium is the limiting reactant because it limits the amount of magnesium oxide that can be formed – oxygen is present in excess.
The limiting reactant is the reactant that is completely used up in a reaction. In this reaction magnesium is the limiting reactant because it limits the amount of magnesium oxide that can be formed – oxygen is present in excess.
Examiner tips
- Use the phrase ‘conservation of mass’ to justify the increase in mass.
- State that magnesium combines with oxygen to give MgO.
- Define limiting reactant as the reactant that is completely consumed.
- Mention that magnesium is limiting because oxygen is in excess.
Common mistakes
- Saying the mass increases because of ‘extra energy’ or ‘heat’, not conservation of mass.
- Confusing the limiting reactant with the excess reactant.
- Failing to mention that oxygen comes from the air/oxygen gas.
Mark scheme (5 marks)
- No atoms are lost or made during a chemical reaction (conservation of mass)
- The magnesium atoms combine with oxygen atoms from the air/oxygen gas
- The mass of magnesium oxide equals the mass of magnesium plus the mass of oxygen that reacted
- The limiting reactant is the reactant that is completely used up
- In this reaction, magnesium is the limiting reactant because it limits the amount of magnesium oxide (product) formed / oxygen is in excess
Key terms in this question
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