Hydrogen and its compoundMedium
Question
The enthalpy of combustion at 25oC of H2, cyclohexane and cyclohexene are - 241, - 3920 and - 3800 kJ/mol respectively. The heat of hydrogenation of cyclohexene is?
Options
A.-121 kJ/mol
B.+ 121 kJ/mol
C.-242 kJ/mol
D.+ 242 kJ/mol
Solution
(i) H(g) + 
O2(g) → H2O(l) ᐃH = - 241 kJ
(ii) C6H10 +
O2(g) → 6CO2(g) + 5H2O(l) ᐃH =- 3800 kJ
(iii) C6H12 + 9O2(g) → 6CO2(g) + 6H2O(l) ᐃH = - 3920 kJ
(i) + (ii) - (iii) give
ᐃH = - 241 - 3800 (- 3920) = - 121 kJ
O2(g) → H2O(l) ᐃH = - 241 kJ(ii) C6H10 +
O2(g) → 6CO2(g) + 5H2O(l) ᐃH =- 3800 kJ(iii) C6H12 + 9O2(g) → 6CO2(g) + 6H2O(l) ᐃH = - 3920 kJ
(i) + (ii) - (iii) give
ᐃH = - 241 - 3800 (- 3920) = - 121 kJ
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