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 · View as Markdown

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

Model answer (4 marks)

Bond enthalpy values are average values taken from many different compounds, so they are not specific to the exact bonds in the hydrocarbon being combusted.
The calculations assume all species are gaseous, whereas real combustion involves liquid fuel and often liquid water.
Standard enthalpies of formation are measured experimentally under defined standard conditions (298 K, 100 kPa, standard states), so they represent the actual physical states and structures of the reactants and products.
Using Hess’s law with these formation enthalpies therefore gives a value that correctly accounts for the real states of all species, making it a more reliable estimate of the combustion enthalpy.

Examiner tips

  • Mention the average nature of bond enthalpies and the gaseous assumption; contrast with measured formation enthalpies under standard conditions; explain why this gives a more accurate result.
  • Use the exact terminology: bond enthalpy, standard enthalpy of formation, Hess’s law, standard states, gaseous vs liquid states.

Common mistakes

  • Assuming bond enthalpies are exact for the specific molecule; ignoring the state of water or fuel; not citing the standard conditions for formation enthalpies.

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 in this question

bond enthalpy · Hess's law

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