Question

The standard heat of combustion of ethene gas is $-330Kcalmo{l}^{-1}$. Calculate $C=C$ bond energy (in kcal/mol) assuming that bond energy of $C-H$ bond is $93.6Kcalmo{l}^{-1}$

Given : $\Delta H_f^0$ for $C{O}_2\left(g\right)$ and $H_{2}O\left(l\right)$ are $-94.2$ and $-61Kcalmo{l}^{-1}$ respectively. Heat of atomisation of carbon and hydrogen are $150$ and $51.5Kcalmo{l}^{-1}$ respectively.

• A. 82 Kcal/mol
• B. 92 Kcal/mol
• C. 102 Kcal/mol
• D. 112 Kcal/mol

Answer 1

Answer: (D)

112 Kcal/mol

$C{H}_2=C{H}_2\left(g\right)+3O_2\left(g\right)\to 2C{O}_2\left(g\right)$ + $2H_{2}O\left(g\right)$

$-330=[2\times (-94.2)+2\times (-61\left)\right]$ - [$\Delta H_f^0{C}_2{H}_4$ ]

$\Delta H_f^0\left(C_2{H}_4\right)=19.6Kcalmo{l}^{-1}$

Again,

$2C\left(s\right)+2H_2\left(g\right)\to C{H}_2=C{H}_2\left(g\right);\Delta H_f=19.6$

$\Delta H_{Reaction}=2\Delta H_{atm\left(C\right)}+4\Delta H_{atm\left(H\right)}-4B.E{.}_{(C-H)}-B.E{.}_{(C=C)}$

$19.6=2\times 150+4\times 51.5-4\times 93.6$ $-B.E{.}_{(C=C)}$

$B.E{.}_{(C=C)}=$ 112 Kcal / mol