Q37 / 120 · NEET 2010 (Mains) · Chemistry
The following two reactions are known
$$
\begin{aligned}
& \mathrm{Fe}_2 \mathrm{O}_3(\mathrm{~s})+3 \mathrm{CO}(\mathrm{g}) \longrightarrow 2 \mathrm{Fe}(\mathrm{s})+3 \mathrm{CO}_2(\mathrm{~g}), \\
& \Delta \mathrm{H}=-26.8 \mathrm{~kJ} \\
& \mathrm{FeO}(\mathrm{s})+\mathrm{CO}(\mathrm{g}) \longrightarrow \mathrm{Fe}(\mathrm{s})+\mathrm{CO}_2(\mathrm{~g}) \\
& \Delta \mathrm{H}=-16.5 \mathrm{~kJ}
\end{aligned}
$$
The value of $\Delta \mathrm{H}$ for the following reaction $\mathrm{Fe}_2 \mathrm{O}_3(\mathrm{~s})+\mathrm{CO}(\mathrm{g}) \longrightarrow 2 \mathrm{FeO}(\mathrm{s})+\mathrm{CO}_2(\mathrm{~g})$ is
- A$+10.3 \mathrm{~kJ}$
- B$-43.3 \mathrm{~kJ}$
- C$-10.3 \mathrm{~kJ}$
- Correct answerD$+6.2 \mathrm{~kJ}$
Solution
Given,
(I) $\mathrm{Fe}_2 \mathrm{O}_3(\mathrm{~s})+3 \mathrm{CO}(\mathrm{g}) \longrightarrow 2 \mathrm{Fe}(\mathrm{s})+3 \mathrm{CO}_2(\mathrm{~g})$;
$\Delta \mathrm{H}=-26.8 \mathrm{~kJ}$
(II) $\mathrm{FeO}$ (s) $+\mathrm{CO}(\mathrm{g}) \longrightarrow \mathrm{Fe}(\mathrm{s})+\mathrm{CO}_2(\mathrm{~g})$;
$\Delta \mathrm{H}=-16.5 \mathrm{~kJ}$
On multiplying Eq (II) with 2, we get
(III) $2 \mathrm{FeO}(\mathrm{s})+2 \mathrm{CO}(\mathrm{g}) \longrightarrow 2 \mathrm{Fe}(\mathrm{s})+2 \mathrm{CO}_2(\mathrm{~g})$;
$\Delta \mathrm{H}=-33 \mathrm{~kJ}$
On subtracting Eq (III) from I, we get
$$
\begin{array}{r}
\mathrm{Fe}_2 \mathrm{O}_3(\mathrm{~s})+\mathrm{CO}(\mathrm{g}) \longrightarrow 2 \mathrm{FeO}(\mathrm{s})+\mathrm{CO}_2(\mathrm{~g}) ; \\
\Delta \mathrm{H}=-26.8-(-33) \\
=+6.2 \mathrm{~kJ}
\end{array}
$$
Hence, the correct answer is option: 'D'.