Question

The bond energies of $ C-C $ , $ C=C $ , $ H-H $ and $ C-H $ linkages are 350, 600, 400 and 410 kJ/mol respectively. The heat of hydrogenation of ethylene is

• A. $ -170\,kJ\,mo{{l}^{-1}} $
• B. $ -260\,kJ\,mo{{l}^{-1}} $
• C. $ -400\,kJ\,mo{{l}^{-1}} $
• D. $ -450\,kJ\,mo{{l}^{-1}} $

Answer 1

Answer: (A)

$ H-\overset{H}{\mathop{\overset{|}{\mathop{C}}\,}}\,=\overset{H}{\mathop{\overset{|}{\mathop{C}}\,}}\,-H+H-H\to H-\underset{H}{\mathop{\underset{|}{\mathop{\overset{H}{\mathop{\overset{|}{\mathop{C}}\,}}\,}}\,}}\,-\underset{H}{\mathop{\underset{|}{\mathop{\overset{H}{\mathop{\overset{|}{\mathop{C}}\,}}\,}}\,}}\,-H $ $ \Delta {{H}_{reaction}}=\Sigma BE\text{(Reactants)-}\Sigma \text{BE(Products)} $ $ =[BE(4C-H)+BE(C=C)+BE(H-H)] $ $ -[BE\,(6C-H)+BE(C-C)] $ $ =BE(C=C)+BE(H-H)-BE(2C-H) $ $ -BE(C-C) $ $ =600+400-2\times 410-350 $ $ =1000-1170=-170kJ\,mo{{l}^{-1}} $