Magnesium burns in oxygen according to the balanced equation: 2Mg + O₂ → 2MgO. A student burns magnesium in a limited supply of oxygen and finds that the reaction stops before all the magnesium is used up. Explain why the reaction stops, identify which reactant is the limiting reactant, and explain what the law of conservation of mass means for this reaction.

WJEC A-Level Chemistry (Wales) — 3.1 Introduction to organic chemistry · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Magnesium is a reactive metal that burns brightly in oxygen to form magnesium oxide. The balanced equation for this reaction is: 2Mg + O₂ → 2MgO.

Model answer (5 marks)

The reaction stops because the oxygen is completely used up.

Oxygen is the limiting reactant because it is the reactant that is completely consumed and therefore limits the amount of product that can form.

The magnesium is in excess – some of it does not react.

The law of conservation of mass states that no atoms are lost or made during a chemical reaction.

The mass of the products (magnesium oxide) equals the mass of the reactants that actually react – the magnesium and oxygen that are used up.

Examiner tips

  • Use the word ‘limiting reactant’ explicitly. Show that oxygen is the limiting reactant by noting it is completely consumed. Mention that magnesium is in excess. State the conservation of mass in terms of atoms not being lost or created. Relate the mass of MgO to the mass of the reactants that reacted.

Common mistakes

  • Confusing the limiting reactant with the excess reactant. Forgetting to state that the mass of MgO equals the mass of the reactants that reacted. Using the phrase ‘mass is conserved’ without explaining that atoms are neither lost nor created.

Mark scheme (5 marks)

  1. The reaction stops because the oxygen is completely used up / runs out
  2. Oxygen is the limiting reactant because it is completely used up and limits the amount of product formed
  3. The magnesium is in excess / not all the magnesium reacts
  4. The law of conservation of mass states that no atoms are lost or made during a chemical reaction
  5. The mass of the products (magnesium oxide) equals the mass of the reactants that actually react (the magnesium and oxygen that are used up)

Key terms in this question

limiting reactant · conservation of mass

Related

More Introduction to organic chemistry questions

▶ Try answering this question with AI marking (free) →