Natural gas is mostly methane, a hydrocarbon. Propane is another hydrocarbon used as a fuel in camping stoves. Explain what is meant by the term 'hydrocarbon' and describe the difference in molecular structure between methane and propane, including how each molecule is represented by its molecular formula.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Natural gas, used in home heating, is mostly methane. Propane is used as a portable fuel in camping stoves. Both are members of the alkane homologous series.
Model answer (5 marks)
A hydrocarbon is a compound that contains only carbon and hydrogen atoms.
Methane has the molecular formula CH₄ and consists of a single carbon atom bonded to four hydrogen atoms.
Propane has the molecular formula C₃H₈ and consists of a chain of three carbon atoms, each bonded to the appropriate number of hydrogen atoms, giving a total of eight hydrogens.
Thus methane contains one carbon atom, whereas propane contains three carbon atoms; both are alkanes with only single covalent bonds between C–H (and C–C in propane).
Methane has the molecular formula CH₄ and consists of a single carbon atom bonded to four hydrogen atoms.
Propane has the molecular formula C₃H₈ and consists of a chain of three carbon atoms, each bonded to the appropriate number of hydrogen atoms, giving a total of eight hydrogens.
Thus methane contains one carbon atom, whereas propane contains three carbon atoms; both are alkanes with only single covalent bonds between C–H (and C–C in propane).
Examiner tips
- Use the definition of a hydrocarbon first. Include the exact formulas CH₄ and C₃H₈. State the number of carbon atoms and note the single‑bonded alkane structure.
- common_mistakes
- :
- Writing the wrong formula for methane or propane. Forgetting to mention that both are alkanes with only single bonds. Using the wrong number of carbon atoms for either molecule.
Mark scheme (5 marks)
- A hydrocarbon contains hydrogen and carbon only / is a compound made of carbon and hydrogen atoms only
- Methane has the molecular formula CH₄
- Propane has the molecular formula C₃H₈
- Methane has one carbon atom whereas propane has three carbon atoms
- Both molecules contain only single covalent bonds between carbon and hydrogen (and carbon to carbon in propane) / both are alkanes with the general formula CₙH₂ₙ₊₂
Key terms in this question
hydrocarbon · molecular formula
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →