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Data about the hydrogenation of cyclohexene and of benzene are given - AQA - A-Level Chemistry - Question 6 - 2018 - Paper 2
Question 6
Data about the hydrogenation of cyclohexene and of benzene are given. $$\Delta H^ heta = -120 \text{ kJ mol}^{-1}$$ $$\Delta H^ heta = -208 \text{ kJ mol}^{-1}$$ ... show full transcript
Worked Solution & Example Answer:Data about the hydrogenation of cyclohexene and of benzene are given - AQA - A-Level Chemistry - Question 6 - 2018 - Paper 2
Step 1
Explain the bonding in and the shape of a benzene molecule:
Answer
Benzene consists of six carbon atoms arranged in a planar hexagonal structure, each bonded to one hydrogen atom. The bonding in benzene is characterized by
a) C-C bonds: Benzene has alternating single and double bonds, making the bonds equivalent due to resonance.
b) Resonance: The delocalization of electrons across the carbon atoms leads to bond angles of 120° and strengthened stability compared to localized double bonds in cyclohexa-1,3,5-triene, which cannot stabilize the structure effectively.
c) Shape: The symmetrical planar shape of benzene contributes to its stability, as all carbon-hydrogen bonds are equal, forming a stable resonance structure.
Step 2
Compare the stability of benzene with that of the hypothetical cyclohexa-1,3,5-triene molecule:
Answer
Benzene is more stable than cyclohexa-1,3,5-triene. The standard enthalpy of hydrogenation for benzene ( ) indicates that more energy is released when benzene is hydrogenated compared to cyclohexene and cyclohexa-1,3,5-triene. This suggests that benzene is more stable due to its resonance, which allows it to delocalize the electrons across the carbon atoms. In contrast, cyclohexa-1,3,5-triene does not benefit from such resonance stabilization and thus has higher energy and lower stability.
Step 3
Suggest a value for the enthalpy of hydrogenation of cyclohexa-1,3-diene and justify your value.
Answer
A reasonable value for the enthalpy of hydrogenation of cyclohexa-1,3-diene could be approximately -180 kJ mol^{-1}. This value is justified because cyclohexa-1,3-diene has two double bonds that are not fully delocalized like in benzene,. Thus, we expect the enthalpy of hydrogenation to be less exothermic than that of cyclohexene (-120 kJ mol^{-1}) but not double that value, supporting that the enthalpy is not exactly twice that of cyclohexene.