Question

The heat of combustion of ethene gas is $330kcalmo{l}^{-1}$ calculate bond energy (in kcal) assuming that bond energy of $C-H$ bond is $93.6kcalmo{l}^{-1}$
Given: $\Delta H_f^{\circ }$ for $C{O}_2(g)$ and $H_{2}O(l)$ are $-94.2$ and $-61kcalmo{l}^{-1}$ respectively. Heat of atomisation of carbon and hydrogen are $150$ and $51.5kcalmo{l}^{-1}$ respectively.

Answer 1

Answer: 122

$C{H}_2=C{H}_2(g)+3O_2(g)⟶2C{O}_2(g)+2H_{2}O(l)$
$\therefore -330=[2\times (-94.2)+2(-61)]-\left[\Delta H_f^{\circ }{C}_2{H}_4\right]$
$\therefore \Delta H_f^{\circ }\left(C_2{H}_4\right)=19.6Kcalmo{l}^{-1}$
$\therefore 2C(s)+2H_2(g)⟶C{H}_2=C{H}_2(g);\Delta H_f=19.6$
$\therefore \Delta H\text{ Reaction }=\sum {\text{ B.E }}_{\text{(Readants) }}-\sum {\text{ B.E }}_{\text{(Poducts) }}$
on substituting the value in above relation we get
B.E ${}_{(C=C)}=122kcal/mol$