Calculate the heat of formation of benzene from the following data, assuming no resonance. Bond energies :
`C-C=83 kcal, C=C=140 Kcal, C-H=99 kcal`
`{:("Heat of atomisation of ",C-170.9 kcal,,,),("Heat of atomisation of",H=6xx52.1 kcal,,,):}`

Explanation

We have to calculate of `Delta H` for the reaction
`6 C(s)+3H_(2)(g)rarrC_(6)H_(6)(g)`
For reactant:
Heat of atomisation of `6` moles of `C=6xx170.9 kcal`
heat of atomisation of `6` moles of `H=6xx52.1 kcal`
For products:
Heat of formation of `6` moles of `C-H "bonds"= -6xx99`
Heat of formation of `3` moles of `C-C= -3xx83`
Heat of formation of `3` moles of `C=C "bonds" = -3xx140`
on adding, we get heat of formation of `C_(6)H_(6)`,i.e.,
`Delta _(t)H=6xx170.9+6xx52.1-6xx99-3xx83-3xx140= -75 kcal`