A level Chemistry · Edexcel 9CH0 · Topic 8C
Bond enthalpies
Key points
- Bond enthalpy: the energy needed to break one mole of a bond in gaseous molecules. Bond breaking is endothermic, bond making is exothermic.
- Mean bond enthalpy is averaged over many compounds, so calculations give approximate values.
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- Answers differ from Hess's law values because mean values are used and some substances are not gaseous.
- You can also work backwards from a known to find an unknown bond enthalpy.
- Stronger bonds are generally shorter; multiple bonds have higher bond enthalpies than single bonds.
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Bond enthalpies explain where the energy in a reaction comes from. Breaking bonds always takes energy in and making bonds always gives energy out, so the overall enthalpy change is the balance between the two.
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