The enthalpies of formation of $\mathrm{Al}_{2} \mathrm{O}_{3}$ and $\mathrm{Cr}_{2} \mathrm{O}_{3}$ are…

The enthalpies of formation of $\mathrm{Al}_{2} \mathrm{O}_{3}$ and $\mathrm{Cr}_{2} \mathrm{O}_{3}$ are $-1596 \mathrm{~kJ}$ and $-1134 \mathrm{~kJ}$ respectively. $\Delta \mathrm{H}$ for the reaction $2 \mathrm{Al}+\mathrm{Cr}_{2} \mathrm{O}_{3} ightarrow 2 \mathrm{Cr}+\mathrm{Al}_{2} \mathrm{O}_{3}$ is
  1. $-2730 \mathrm{~kJ}$
  2. $-462 \mathrm{~kJ}$
  3. $-1365 \mathrm{~kJ}$
  4. $+2730 \mathrm{~kJ}$

Solution

$2 \mathrm{Al}+\frac{3}{2} \mathrm{O}_{2} ightarrow \mathrm{Al}_{2} \mathrm{O}_{3}, \Delta \mathrm{H}=-1596 \mathrm{~kJ}$
$\cdots \cdots$
$2 \mathrm{Cr}+\frac{3}{2} \mathrm{O}_{2} ightarrow \mathrm{Cr}_{2} \mathrm{O}_{3}, \Delta \mathrm{H}=-1134 \mathrm{~kJ}$
$\mathrm{By}(\mathrm{i})-(\mathrm{ii})$
$2 \mathrm{Al}+\mathrm{Cr}_{2} \mathrm{O}_{3} ightarrow 2 \mathrm{Cr}+\mathrm{Al}_{2} \mathrm{O}_{3}, \Delta \mathrm{H}=-462 \mathrm{~kJ}$

Asked in: JEE-TOPICTESTS-CHEMISTRY

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