Question

$\Delta H_f$ of $C_2{H}_4=12.5kcal$ Heat of atomisation of $C=171kcal$. Bond energy of $H_2=104.3kcal$. Bond energy $C-H=99.3kcal$. What is $C=C$ bond energy?

• A. 140.9 kcal
• B. 49 kcal
• C. 40 kcal
• D. 76 kcal

Answer 1

Answer: (A)

140.9 kcal

Reaction $2C+2H_2⟶C_2{H}_4$

Heat of formation = B.E of reagents - B.E. of products

$\Delta H_f=2\times$ Binding energy of carbon +$2\times$ Binding energy of $H_2+4\times$ bond energy of $C-H$ - Bond energy of $C=C$

$12.5=2\times 171+2\times 104.3+4\times 99.3-$Bond energy of $C=C$

Bond energy of $C=C=140.9Kcal$