Q. The standard molar heat of formation of ethane, $CO_{2}$ and water $(l)$ are respectively $- 21.1 - 94.1$ and $- 68.3$ kcal. The standard molar heat of combustion of ethane will be
A) $- 372$ kcal
B) 162 kcal
C) $- 240$ kcal
D) 183.5 kcal
Explanation
(a)
Given, $2C + 3H_{2} \rightarrow C_{2}H_{6};\ \mathrm{\Delta}H = - 21.1$
$C + O_{2} \rightarrow CO_{2};\ \mathrm{\Delta}H = - 94.1$
$H_{2} + \frac{1}{2}O_{2} \rightarrow H_{2}O;\ \mathrm{\Delta}H = - 68.3$
Eqs .2(ii) + 3(iii)$-$(i)
$C_{2}H_{6} + \frac{3}{2}O_{2} \rightarrow 2CO_{2} + 3H_{2}O$
$\mathrm{\Delta}x = 2( - 94.1) + 3( - 68.3) - ( - 21.1)$
$= - 372\ kcal$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.