# Explain why the enthalpy change of combustion per gram of hydrogen (H₂) is significantly greater than that of glucose (C₆H₁₂O₆), even though both compounds contain hydrogen atoms that are ultimately oxidised to water during combustion.

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

> The standard enthalpy of combustion of hydrogen is −286 kJ mol⁻¹ and that of glucose is −2803 kJ mol⁻¹.

## Mark scheme (4 marks)

1. Converting to a per-gram basis: hydrogen releases approximately 143 kJ g⁻¹ whereas glucose releases approximately 15.6 kJ g⁻¹, so hydrogen releases roughly 9 times more energy per gram.
2. Glucose has a very high molar mass (180 g mol⁻¹) compared to hydrogen (2 g mol⁻¹), so dividing the molar enthalpy by molar mass gives a much lower value per gram for glucose.
3. Glucose is already partially oxidised — it contains oxygen atoms bonded to carbon — so fewer bonds to oxygen need to be formed during combustion, releasing less energy per gram.
4. Hydrogen consists entirely of H–H bonds, which upon combustion are fully converted to H–O bonds in water; all bonding change contributes to energy release, so there is no 'wasted' mass from non-combustible atoms or pre-existing oxidised bonds.

## Key terms

- [enthalpy change of combustion](https://www.gradenine.co.uk/glossary/enthalpy-change-of-combustion)

## Related

- [Revision notes for IB DP Chemistry Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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