Find the enthalpy of gas-phase trimerisation of ethyne to benzene (Kekule structure with three C=C and three C-C bonds), $3 HC \equiv CH (g) \to C_{6} H_{6} (g)$ , in kJ per mole of benzene formed.
Bond energies (kJ/mol): $ε_{C \equiv C} = 835$ , $ε_{C = C} = 610$ , $ε_{C - C} = 345$ , $ε_{C - H} = 410$ .

Question

Find the enthalpy of gas-phase trimerisation of ethyne to benzene (Kekule structure with three C=C and three C-C bonds), $3 HC \equiv CH (g) \to C_{6} H_{6} (g)$ , in kJ per mole of benzene formed.

Bond energies (kJ/mol): $ε_{C \equiv C} = 835$ , $ε_{C = C} = 610$ , $ε_{C - C} = 345$ , $ε_{C - H} = 410$ .

TestHub · Accepted Answer

Break 3 C ≡ C bonds: + 3 × 835 = + 2505 kJ . Form 3 C = C and 3 C − C bonds: − ( 3 × 610 + 3 × 345 ) = − ( 1830 + 1035 ) = − 2865 kJ . C − H bonds are unchanged (6 before, 6 after). Δ H = 2505 − 2865 = − 360 kJ per mole of benzene ( − 120 per mole of ethyne). Answer: (A).

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