Butane (C₄H₁₀) is a hydrocarbon used in camping gas. A student says that when butane burns completely in oxygen, the total mass of the products must equal the total mass of the reactants. Explain why the student is correct, and describe what products are formed during complete combustion of butane.

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).

Butane (C₄H₁₀) is a hydrocarbon used as a fuel in camping gas canisters. When ignited, butane reacts with oxygen in the air.

Model answer (5 marks)

The student is correct because of the law of conservation of mass – no atoms are lost or created during a chemical reaction. The atoms are simply rearranged to form new substances, so the same atoms appear on both sides of the equation.

During complete combustion of butane the products are carbon dioxide (CO₂) and water (H₂O).

Examiner tips

  • Use the phrase "law of conservation of mass" to earn the first mark. Show the rearrangement idea to get the second and third marks. State CO₂ and H₂O explicitly for the final two marks.

Common mistakes

  • Writing incomplete combustion products such as CO or soot. Forgetting to mention the conservation of mass. Using vague terms like "new substances" without naming CO₂ and H₂O.

Mark scheme (5 marks)

  1. The student is correct because of the law of conservation of mass
  2. No atoms are lost or made during a chemical reaction
  3. The atoms are simply rearranged to form new substances / the same atoms appear on both sides of the equation
  4. Carbon dioxide (CO₂) is produced during complete combustion
  5. Water (H₂O) is produced during complete combustion

Key terms in this question

complete combustion · hydrocarbon

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