If at 298 K the bond energies of $\mathrm{C}-\mathrm{H}, \mathrm{C}-\mathrm{C}$, $\mathrm{C}=\mathrm

Question

If at 298 K the bond energies of $\mathrm{C}-\mathrm{H}, \mathrm{C}-\mathrm{C}$, $\mathrm{C}=\mathrm{C}$ and $\mathrm{H}-\mathrm{H}$ bonds are respectively 414, 347, 615 and $435 \mathrm{~kJ} \mathrm{~mol}^{-1}$, the value of enthalpy change for the reaction,\ $\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}_{2(g)}+\mathrm{H}_{2(g)} \longrightarrow \mathrm{H}_{3} \mathrm{C}-\mathrm{CH}_{3(g)}$ at 298 K will be\

Professor PC Thomas Classes · Answer

(d) : $\mathrm{CH}_2=\mathrm{CH}_2+\mathrm{H}_2 \longrightarrow \mathrm{CH}_3-\mathrm{CH}_3$
$
\begin{aligned}
& \Delta H=(B E)_{	ext {reactants }}-(B E)_{	ext {products }} \
& =4(B E)_{\mathrm{C}-\mathrm{H}}+(B E)_{\mathrm{C}=\mathrm{C}}+(B E)_{\mathrm{H}-\mathrm{H}}-\left[6(B E)_{\mathrm{C}-\mathrm{H}}+(B E)_{\mathrm{C}-\mathrm{C}}ight] \
& =[4 	imes 414+615+435]-[6 	imes 414+347]=-125 \mathrm{~kJ} \mathrm{~mol}^{-1}
\end{aligned}
$