Question

The heat of combustion of ethene gas is $330\, kcal\, mol ^{-1}$ calculate bond energy (in kcal) assuming that bond energy of $C - H$ bond is $93.6 \,kcal\, mol ^{-1}$
Given: $\Delta H _{ f }^{\circ}$ for $CO _{2}( g )$ and $H _{2} O ( l )$ are $-94.2$ and $-61\,kcal\, mol ^{-1}$ respectively. Heat of atomisation of carbon and hydrogen are $150$ and $51.5 \,kcal\, mol ^{-1}$ respectively.

Answer 1

$ CH _{2}= CH _{2}( g )+3 O _{2}( g ) \longrightarrow 2 CO _{2}( g )+2 H _{2} O ( l )$
$\therefore -330=[2 \times(-94.2)+2(-61)]-\left[\Delta H _{ f }^{\circ} C _{2} H _{4}\right] $
$\therefore \Delta H _{ f }^{\circ}\left( C _{2} H _{4}\right)=19.6\, Kcal\, mol ^{-1} $
$\therefore 2 C ( s )+2 H _{2}( g ) \longrightarrow CH _{2}= CH _{2}( g ) ; \Delta H _{ f }=19.6$
$\therefore \Delta H \text { Reaction }=\sum \text { B.E }_{\text {(Readants) }}-\sum \text { B.E }_{\text {(Poducts) }}$
on substituting the value in above relation we get
B.E $_{( C = C )}=122\, kcal / mol$