Question

The enthalpy of combustion of H2, cyclohexene (C6H10) and cyclohexene (C6H12) are -241, -3800 and -3920 kJ per mol respectively. Heat of hydrogenation of cyclohexene is:

• A. -121 kJ per mol
• B. +121 kJ per mol
• C. +484 kJ per mol
• D. -484 kJ per mol

Answer 1

Answer: (A)

-121 kJ per mol

Answer 2

Given the equations:
H₂ + $\frac 12$O₂ $→$ H₂O, ΔH = -241 kJ/mol ……..(1)
C₆H₁₀ + $\frac {17}{2}$O₂ $→$ 6CO₂ + 5H₂O, ΔH = -3800 kJ/mol ……..(2)
C₆H₁₂ + 9O₂ $→$ 6CO₂ + 6H₂O, ΔH = -3920 kJ/mol ……..(3)
Equation (1) and equation (2) are added, and then equation (3) is subtracted to yield the following result:
C₆H₁₂ + H₂ $→$ C₆H₁₂
Calculation of Enthalpy Change for Hydrogenation:
The following formula is used to calculate the enthalpy change for hydrogenation, abbreviated as H Hydrogen:
ΔH Hydrogen = (-241 kJ/mol) + (-3800 kJ/mol) - (-3920 kJ/mol) = -121 kJ/mol

So, the correct option is (A): -121 kJ per mol