Zinc oxide reacts with carbon according to the balanced equation: 2ZnO + C → 2Zn + CO₂. A student reacts a small amount of carbon with excess zinc oxide. Explain what is meant by the terms 'limiting reactant' and 'conservation of mass', and describe how each applies to this reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The limiting reactant is the reactant that is completely used up during the reaction.
Carbon is the limiting reactant in this reaction because it is present in a small, non‑excess amount.
Conservation of mass states that no atoms are lost or made during a chemical reaction.
The mass of the products (Zn and CO₂) equals the mass of the reactants (ZnO and C) that actually reacted.
The equation is balanced (same number of each type of atom on both sides), which reflects conservation of mass.
Carbon is the limiting reactant in this reaction because it is present in a small, non‑excess amount.
Conservation of mass states that no atoms are lost or made during a chemical reaction.
The mass of the products (Zn and CO₂) equals the mass of the reactants (ZnO and C) that actually reacted.
The equation is balanced (same number of each type of atom on both sides), which reflects conservation of mass.
Examiner tips
- Define limiting reactant and conservation of mass explicitly. State which reactant is limiting and why. Show the mass balance and note the equation is balanced. Use correct terminology and UK spelling.
Common mistakes
- Confusing the roles of ZnO and C as limiting reactants. Failing to mention that ZnO is in excess. Omitting the statement that the equation is balanced.
Mark scheme (5 marks)
- The limiting reactant is the reactant that is completely used up during the reaction
- Carbon is the limiting reactant in this reaction (because it is present in a small / non-excess amount)
- Conservation of mass states that no atoms are lost or made during a chemical reaction
- The mass of the products (zinc and carbon dioxide) equals the mass of the reactants (zinc oxide and carbon) that actually reacted
- The equation is balanced (same number of each type of atom on both sides) which reflects conservation of mass
Key terms in this question
limiting reactant · conservation of mass · excess · balanced equation
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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