# The enthalpy change of hydrogenation of cyclohexene is −120 kJ mol⁻¹. If benzene behaved like a molecule with three isolated carbon-carbon double bonds, its enthalpy change of hydrogenation would be expected to be −360 kJ mol⁻¹. The experimentally measured value for benzene is only −208 kJ mol⁻¹. Explain what this difference in values tells us about the stability and bonding of benzene.

> Eduqas A-Level Chemistry — 4.2 Aromaticity · Explain · 5 marks

> The enthalpy change of hydrogenation of cyclohexene is −120 kJ mol⁻¹. A hypothetical molecule with three isolated double bonds would be expected to release −360 kJ mol⁻¹ on complete hydrogenation. The experimentally measured enthalpy change of hydrogenation of benzene is −208 kJ mol⁻¹.

## Mark scheme (5 marks)

1. The difference between the expected value (−360 kJ mol⁻¹) and the actual value (−208 kJ mol⁻¹) is approximately 152 kJ mol⁻¹ less energy released than expected
2. This means benzene is more stable than a hypothetical molecule with three isolated/localised double bonds
3. The extra stability arises because the six pi electrons are delocalised over all six carbon atoms in the ring (rather than being in three fixed double bonds)
4. Because the electrons are delocalised, all carbon-carbon bonds in benzene are equal / intermediate in length and strength between a single and a double bond
5. The energy difference (approximately 152 kJ mol⁻¹) is called the delocalisation / stabilisation / resonance energy, representing the extra stability gained through delocalisation

## Key terms

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

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Source: [GradeNine](https://www.gradenine.co.uk/q/the-enthalpy-change-of-hydrogenation-of-47fc8246) · Published by Druglandscape Ltd.