Question

If Bond energies of $C-C, \, C=C, \, C-H$ bonds are $83$ Kcal, $140$ Kcal & $99$ Kcal resp. The calculate the heat of formation of benzene.

• A. $-65 \, Kcal$
• B. $-70 \, Kcal$
• C. $-75 \, Kcal$
• D. $-80 \, Kcal$

Answer 1

Answer: (C)

Aim: $6C \, \left(s\right)+3H_{2}\left(g\right) \rightarrow C_{6}H_{6}\left(g\right)$


$\Delta H =?$
For reactant
Heat of atomization of 6 moles of $C=6 \times 170.9$
Hear of atomization of 6 moles of $H=6 \times 52.1$
For product
Heat of formation of 6 moles of $C-H$ bonds $=-6 \times 99$
Heat of formation of 3 moles of $C-C$ bonds $=-3 \times 83$
Heat of formation of 3 moles of $C=C$ bonds $=-3 \times 140$
On adding :
$\Delta H =6 \times 170.9+6 \times 52.1+6 \times(-99)+3 \times(-83)+3 \times(-140)=-75 $ Kcal