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/mol
• B. $-400$ kJ/mol
• C. $-260$ kJ/mol
• D. $-450$ kJ/mol

Answer 1

The correct option is A $-170$ kJ/mol

$\underset{1C=C+4C-H}{\underset{}{C_2{H}_2}}+\underset{1H-H}{\underset{}{H_2}}⟶\underset{1C-C+6C-H}{\underset{}{C_2{H}_6}}$

$\Delta H_{reaction}=\Delta H_{C-C}^{BE}+6\Delta H_{C-H}^{BE}-\Delta H_{H-H}^{BE}-\Delta H_{C=C}^{BE}-4\Delta H_{C-H}^{BE}$

$⟹\Delta H_{reaction}=350+2\times 410-400-600$

$⟹\Delta H_{reaction}=170$ $kJ/mol$

$\Delta H_{hydrogenation}^{-2H_6}=-\Delta H_{reaction}=-170$ $kJ/mol$