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

Answer 1

Answer: (C)

Aim: $6C\left(s\right)+3H_2\left(g\right)\to C_6{H}_6\left(g\right)$


$\Delta H=?$
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