Q10 / 120 · VITEEE 2014 · Chemistry
The standard molar heat of formation of ethane, $\mathrm{CO}_{2}$ and water (1) are respectively $-21.1,-94.1$ and $-68.3 \mathrm{kcal}$. The standard molar heat of combustion of ethane will be
- Correct answerA$-372 \mathrm{kcal}$
- B$162 \mathrm{kcal}$
- C$-240 \mathrm{kcal}$
- D$183.5 \mathrm{kcal}$
Solution
Given,
(a) $2 \mathrm{C}+3 \mathrm{H}_{2} \longrightarrow \mathrm{C}_{2} \mathrm{H}_{6} ; \Delta \mathrm{H}=-21.1 \mathrm{kcal}$
(b) $\mathrm{C}+\mathrm{O}_{2} \longrightarrow \mathrm{CO}_{2} ; \Delta \mathrm{H}=-94.1 \mathrm{kcal}$
(c) $\mathrm{H}_{2}+\frac{1}{2} \mathrm{O}_{2} \longrightarrow \mathrm{H}_{2} \mathrm{O} ; \Delta \mathrm{H}=-68.3 \mathrm{kcal}$
Now, eqs. $2 \times(\mathrm{b})+3 \times(\mathrm{c})-(\mathrm{a})$
$\mathrm{C}_{2} \mathrm{H}_{6}+\frac{3}{2} \mathrm{O}_{2} \longrightarrow 2 \mathrm{CO}_{2}+3 \mathrm{H}_{2} \mathrm{O}$
Heat of combustion of Ethane $\Delta \mathrm{x}=2(-94.1)$
$\begin{array}{l}
+3(-68.3)-(-21.1) \\
=(-188.2)+(-204.9)-21.1 \\
=-372 \mathrm{kcal}
\end{array}$
Hence, the correct answer is option: 'A'.