Methane is a hydrocarbon that is the main component of natural gas. When methane burns completely in oxygen, carbon dioxide and water are produced. Explain why the total mass of carbon dioxide and water produced is always equal to the total mass of methane and oxygen that reacted.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Methane has the molecular formula CH₄. The balanced equation for the complete combustion of methane is: CH₄ + 2O₂ → CO₂ + 2H₂O
Model answer (5 marks)
1. In a chemical reaction no atoms are lost or created – they are only rearranged.
2. This is the law of conservation of mass.
3. The balanced equation shows that the same atoms are present on both sides of the equation.
4. Because the number of each type of atom is the same on both sides, the total mass of the products equals the total mass of the reactants.
5. Therefore the mass of CO₂ and H₂O produced is always equal to the mass of CH₄ and O₂ that reacted.
2. This is the law of conservation of mass.
3. The balanced equation shows that the same atoms are present on both sides of the equation.
4. Because the number of each type of atom is the same on both sides, the total mass of the products equals the total mass of the reactants.
5. Therefore the mass of CO₂ and H₂O produced is always equal to the mass of CH₄ and O₂ that reacted.
Examiner tips
- Use the phrase "law of conservation of mass". Show the balanced equation to prove atoms are unchanged. Explain that equal numbers of atoms give equal mass. Keep answer concise and directly linked to the marks.
Common mistakes
- Confusing the law of conservation of energy with mass. Failing to mention that atoms are rearranged rather than lost or made. Giving a general statement without citing the balanced equation.
Mark scheme (5 marks)
- No atoms are lost or made during a chemical reaction
- This is the law of conservation of mass
- The same atoms are present on both sides of the equation / the atoms are rearranged to form new substances
- Because the number of each type of atom is the same on both sides of the balanced equation
- Therefore the mass of the products equals the mass of the reactants
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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