The enthalpy of formation of ethane from ethylene by addition of hydrogen where the bond energies of C – H, C – C, C = C, H – H are 414 kJ, 347 kJ, 615 kJ and 435 kJ respectively is - ________ kJ.

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Solution

Q.
The enthalpy of formation of ethane ( $C_{2} H_{6}$ ) from ethylene by addition of hydrogen where the bond energies of C – H, C – C, C = C, H – H are 414 kJ, 347 kJ, 615 kJ and 435 kJ respectively is - ________ kJ. [2024]

Ans.
(125)

$H_{2} C = {CH}_{2} + H_{2} \to H_{3} C - {CH}_{3}$

${ΔH}_{r} = {ΔH}_{B . E .} (reactants) - {ΔH}_{B . E .} (products)$

${ΔH}_{r} = [{ΔH}_{B . E .} (H_{2}) + {ΔH}_{B . E .} (C = C) + 4 \times {ΔH}_{B . E .} (C - H)] - [{ΔH}_{B . E .} (C - C) + 6 \times {ΔH}_{B . E .} (C - H)]$

$= [435 + 615 + 4 \times 414] kJ - [347 + 6 \times 414] k J$

$= - 125 kJ$

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