Explain why the standard enthalpy of combustion of methane is more exothermic per mole than that of methanol, despite both molecules containing one carbon atom.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Methane (CH₄) has a standard enthalpy of combustion of −890 kJ mol⁻¹, whilst methanol (CH₃OH) has a standard enthalpy of combustion of −726 kJ mol⁻¹.
Model answer (4 marks)
Methanol already contains an oxygen atom (C–O and O–H bonds), so it is partially oxidised before combustion.
Because combustion is an oxidation reaction, the more oxidised the reactant, the less energy is released when it is fully oxidised to CO₂ and H₂O.
Methane has four C–H bonds, whereas methanol has only three; C–H bonds give a larger energy release on oxidation than C–O or O–H bonds.
Thus the net enthalpy change for methane is more negative: the difference between the energy of bonds broken and bonds formed is larger because the reactant bonds are at a lower oxidation state.
Because combustion is an oxidation reaction, the more oxidised the reactant, the less energy is released when it is fully oxidised to CO₂ and H₂O.
Methane has four C–H bonds, whereas methanol has only three; C–H bonds give a larger energy release on oxidation than C–O or O–H bonds.
Thus the net enthalpy change for methane is more negative: the difference between the energy of bonds broken and bonds formed is larger because the reactant bonds are at a lower oxidation state.
Examiner tips
- Use the word ‘oxidised’ and mention C–O/O–H bonds in methanol; link to bond energies; keep answer to 4 points
Common mistakes
- Confusing ‘more exothermic’ with ‘more endothermic’; writing that methanol has more bonds; ignoring the role of oxygen in methanol
Mark scheme (4 marks)
- Methanol already contains an oxygen atom (or a C–O bond / O–H bond), meaning it is partially oxidised compared to methane.
- Combustion is an oxidation process; since methanol is already partially oxidised, less energy is released when it is fully oxidised to CO₂ and H₂O.
- Methane has more C–H bonds (4) than methanol (3), and C–H bonds release more energy per bond when oxidised than C–O or O–H bonds that replace them in methanol.
- The overall enthalpy of combustion depends on the difference in bond energies broken and formed; for methane, the net energy released is greater because the reactant bonds are all at a lower oxidation state, giving a larger negative ΔH.
Key terms in this question
standard enthalpy of combustion · exothermic
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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