# Explain why bond enthalpy data gives only an approximate value for the standard enthalpy of combustion of a hydrocarbon fuel, and suggest why the value obtained using Hess's law with standard enthalpies of formation is considered more reliable.

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

## Mark scheme (4 marks)

1. Bond enthalpy values are average values taken from many different compounds, not specific to the exact bonds in the molecule under consideration.
2. Bond enthalpy calculations assume all reactants and products are in the gaseous state, whereas real combustion reactions involve liquids and/or solids (e.g. liquid fuel, liquid water product).
3. Standard enthalpies of formation are measured experimentally under precisely defined standard conditions (298 K, 100 kPa, standard states), so they reflect the actual states and structures of the compounds involved.
4. Applying Hess's law with standard enthalpies of formation therefore gives a value that accounts for the correct physical states of all reactants and products, making it specific to the reaction as it actually occurs.

## Key terms

- [bond enthalpy](https://www.gradenine.co.uk/glossary/bond-enthalpy)
- [Hess's law](https://www.gradenine.co.uk/glossary/hess-s-law)

## Related

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- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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