Methane (CH₄) is a hydrocarbon that burns completely in oxygen. The balanced equation for this reaction is: CH₄ + 2O₂ → CO₂ + 2H₂O. Explain what the term 'hydrocarbon' means, state the law that explains why no mass is lost during this reaction, and explain why the equation must have the same number of atoms of each element on both sides.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Methane is the main component of natural gas and is used as a fuel in homes and industry.
Model answer (5 marks)
A hydrocarbon is a compound that contains only carbon and hydrogen atoms.
The law of conservation of mass states that no atoms are lost or made during a chemical reaction.
Because no atoms are lost or made, the total mass of the products equals the total mass of the reactants.
Atoms are rearranged during a chemical reaction but are not lost or created, so the same atoms must appear on both sides of the equation.
Therefore the equation must be balanced – the number of atoms of each element is the same on both sides.
The law of conservation of mass states that no atoms are lost or made during a chemical reaction.
Because no atoms are lost or made, the total mass of the products equals the total mass of the reactants.
Atoms are rearranged during a chemical reaction but are not lost or created, so the same atoms must appear on both sides of the equation.
Therefore the equation must be balanced – the number of atoms of each element is the same on both sides.
Examiner tips
- Use the exact definition of ‘hydrocarbon’ – carbon and hydrogen only.
- Mention the law of conservation of mass and link it to mass balance.
- Explain that atoms are rearranged, not lost, so counts must match.
- Show the logical flow from conservation to balancing the equation.
Common mistakes
- Confusing ‘hydrocarbon’ with any organic compound, including heteroatoms.
- Forgetting to state the law of conservation of mass explicitly.
- Claiming that atoms are created or destroyed instead of rearranged.
Mark scheme (5 marks)
- A hydrocarbon contains hydrogen and carbon only
- The law of conservation of mass states that no atoms are lost or made during a chemical reaction
- Because no atoms are lost or made, the mass of products equals the mass of reactants
- Atoms are rearranged during a chemical reaction (but not lost or made), so the same atoms must appear on both sides of the equation
- Therefore the equation must be balanced — the number of atoms of each element is the same on both sides
Key terms in this question
hydrocarbon · balanced equation
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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