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

Options

A. `- 170 kJ mol^(-1)`
B. `- 260 kJ mol^(-1)`
C. `- 400 kJ mol^(-1)`
D. `- 450 kJ mol^(-1)`

Answer

A

Explanation

`CH_(2) = CH_(2) + H_(2) rarr CH_(3)CH_(3)`
or `underset(underset(H)(|))overset(overset(H)(|))(C) = underset(underset(H)(|))overset(overset(H)(|))(C) + H - H rarr H - underset(underset(H)(|))overset(overset(H)(|))(C) - underset(underset(H)(|))overset(overset(H)(|))(C) - H`
`Delta_(r) H = Sum B. E ("Reactants) - Sum B. E ("Products")`
`= B. E. 4(C - H) + B. E. (C = C) + B. E. (H - H) - B. E. 6 (C - H) - B. E. (C - C)`
`= E. (C = C) + B. E. (H - H) - 2B. E. (C - H) - B. E. (C - C)`
`= 600 + 400 - 2 xx 410 - 350`
`= - 170 kJ`