The standard enthalpy of combustion at 25°C of hydrogen, cyclohexene (C6H10) and cyclohexane (C6H12) are -241, -3800 and -3920 kJ/mole respectively. Calculate the heat of hydrogenation of cyclohexene.
(a) -111 kJ/mol
(b) -121 kJ/mol
(c) -118 kJ/mol
(d) -128 kJ/mol
(b) -121 kJ/mol
Explanation:
The required reaction is
C6H10+ H2 → C6H12, ΔH1 = ? ....(1)
Let us write the given facts
H2 + 1/2O2 → H2O;
ΔH2 = -241 kJ/mol .....(2)
C6H10+ 17/2O2 → 6CO2 + 5H2O
ΔH3 = -3800 kJ/mol .....(3)
C6H12 + 9O2 → 6CO2 + 6H2O
ΔH4 = -3920 kJ/mol .....(4)
The required reaction (1) can be obtained by adding equations (2) and (3), and subtracting (4) from the sum of (2) and (3).
C6H10+ H2 → C6H12
ΔH1 = (ΔH2 + ΔH3) - ΔH4
= [-241 + (-3800)] - (-3920)
= (-241 – 3800) - (-3920)
= - 4041 + 3920 = -121 kJ/mol