Explain why the standard enthalpy of combustion of hydrogen is considered a more environmentally favourable fuel than petrol, and identify one limitation of using hydrogen as a fuel despite its high energy output.

IB DP Chemistry Standard 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).

Hydrogen gas is increasingly discussed as an alternative to fossil fuels. Its standard enthalpy of combustion is approximately −286 kJ mol⁻¹, and its only combustion product is water.

Model answer (4 marks)

1. Combustion of hydrogen gives only water, so no CO₂ or other greenhouse gases are released, avoiding the enhanced greenhouse effect.
2. Petrol combustion releases CO₂, CO and NOₓ, which are greenhouse gases and air‑polluting.
3. Hydrogen has a very high specific energy (energy per unit mass), making it an efficient fuel.
4. A limitation is that hydrogen is difficult to store and transport safely because it is a low‑density gas at room temperature, requiring high‑pressure tanks or cryogenic conditions, or that most hydrogen is produced from fossil fuels, so it is not truly carbon‑neutral.

Examiner tips

  • Use the word ‘only’ to emphasise water as the sole product; mention CO₂, CO, NOₓ for petrol; state specific energy; give a clear limitation such as storage or production source.
  • Structure answer in four clear points matching the mark scheme. Use UK spelling (e.g., ‘favourable’).

Common mistakes

  • Confusing energy per unit volume with energy per unit mass; writing ‘high energy output’ without specifying specific energy.
  • Failing to mention that hydrogen combustion produces no CO₂ or that petrol releases CO₂/NOₓ; or giving a limitation unrelated to storage/production.

Mark scheme (4 marks)

  1. Combustion of hydrogen produces only water (no carbon dioxide or other greenhouse gases / pollutants), so it does not contribute to the enhanced greenhouse effect / climate change.
  2. Petrol combustion produces carbon dioxide (and other pollutants such as CO or NOₓ), which are greenhouse gases / contribute to air pollution / climate change.
  3. Hydrogen has a very high specific energy / energy per unit mass (energy density by mass), making it an efficient fuel.
  4. One limitation: hydrogen is difficult / expensive to store and transport safely because it is a gas at room temperature and has a very low density / requires high-pressure tanks or cryogenic conditions; OR most hydrogen is currently produced from fossil fuels (steam reforming), so its production is not truly carbon-neutral.

Key terms in this question

standard enthalpy of combustion

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