Propane (C₃H₈) is a hydrocarbon used as a fuel. When propane burns completely in oxygen, carbon dioxide and water are produced. The balanced equation for this reaction is: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. Explain why the total mass of carbon dioxide and water produced is always equal to the total mass of propane and oxygen used up.

Eduqas A-Level Chemistry — 3.2 Hydrocarbons · Explain · 5 marks · View as Markdown

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

Propane (C₃H₈) is a hydrocarbon used as a fuel in gas-powered appliances. When propane burns completely in oxygen, the balanced equation is: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.

Model answer (5 marks)

The law of conservation of mass states that mass cannot be created or destroyed in a chemical reaction. In the combustion of propane no atoms are lost or made; the carbon, hydrogen and oxygen atoms are simply rearranged. The same atoms that are present in the reactants (C₃H₈ and 5O₂) are found in the products (3CO₂ and 4H₂O). Because the number of atoms of each element is identical on both sides of the equation, the total mass of the products equals the total mass of the reactants.

Examiner tips

  • Use the phrase "law of conservation of mass" first. Show that atoms are rearranged, not created or destroyed. Mention that the same atoms appear on both sides. Conclude that mass is unchanged.

Common mistakes

  • Forgetting to state the law of conservation of mass. Saying atoms are 'made' or 'destroyed' instead of 'rearranged'. Failing to mention that the same atoms appear on both sides of the equation.

Mark scheme (5 marks)

  1. States the law of conservation of mass
  2. No atoms are lost or made / created or destroyed during the reaction
  3. The atoms are rearranged / the same atoms are present in the reactants and the products
  4. Because the number of atoms of each element is the same on both sides of the equation
  5. Therefore the mass of the products equals the mass of the reactants

Key terms in this question

balanced equation

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