Explain why the energy density of ethanol (C₂H₅OH) is lower than that of octane (C₈H₁₈) when both values are expressed per gram of fuel.

IB DP Chemistry Higher Level (2023 syllabus) — R1.3 Energy from fuels · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Biofuels such as ethanol are increasingly blended with petrol to reduce dependence on fossil fuels. The energy densities of ethanol and octane, expressed per gram, differ significantly.

Model answer (4 marks)

Octane contains a greater proportion of C–H bonds by mass than ethanol, and C–H bonds release more energy on combustion than C–O bonds.

C–H bonds are more energetically favourable to oxidise than C–O bonds; the C–O bond in ethanol is already partially oxidised, so less energy is released when it is further oxidised to CO₂.

Ethanol has a lower molar mass relative to the number of C–H bonds it contains compared to octane, meaning less chemical energy is stored per gram.

Therefore, per gram, octane releases more energy on complete combustion, giving it a higher energy density than ethanol.

Examiner tips

Use the bond‑energy argument and mention mass proportion of C–H bonds; link to partial oxidation of C–O in ethanol; keep answer concise and to the point.
Show the logical flow: higher C–H proportion → more energy per gram; partial oxidation of C–O → less energy; lower molar mass of ethanol → less energy per gram.
Use the exact terminology: ‘energy density’, ‘C–H bonds’, ‘C–O bonds’, ‘partial oxidation’.
Ensure you answer the ‘why’ – focus on bond types and mass proportion, not just give numbers.

Common mistakes

Confusing energy per mole with energy per gram; students may give molar energy values. Failing to mention the C–O bond in ethanol is already partially oxidised. Using vague terms like ‘more carbon’ or ‘less carbon’ without linking to bond types and mass proportion.

Mark scheme (4 marks)

Octane contains a greater proportion of C–H bonds (by mass) than ethanol, and C–H bonds release more energy per bond on combustion than C–O bonds.
C–H bonds are more energetically favourable to combust than C–O bonds; the C–O bond in ethanol is already partially oxidised, so less energy is released when it is further oxidised to CO₂.
Ethanol has a lower molar mass relative to the number of C–H bonds it contains compared to octane, meaning less chemical energy is stored per gram.
Therefore, per gram, octane releases more energy on complete combustion, giving it a higher energy density than ethanol.

Key terms in this question

energy density