Q11 / 160 · TS EAMCET 2022 (20 Jul Shift 2) · Chemistry

The bond enthalpies of heavy hydrogen, $\mathrm{O}-\mathrm{O}$ and $\mathrm{D}-\mathrm{O}$ are $+400,+498$ and $+490 \mathrm{~kJ} \mathrm{~mol}^{-1}$, respectively. The $\Delta_{\mathrm{r}} \mathrm{H}^{\circ}$ of the reaction to produce $\mathrm{D}_2 \mathrm{O}$ is

  • A
    $-300 \mathrm{~kJ} \mathrm{~mol}^{-1}$
  • Correct answer
    B
    $-331 \mathrm{~kJ} \mathrm{~mol}^{-1}$
  • C
    $29.1 \mathrm{~kJ} \mathrm{~mol}^{-1}$
  • D
    $2.91 \mathrm{~kJ} \mathrm{~mol}^{-1}$

Solution

$\mathrm{D}_2+\frac{1}{2} \mathrm{O}_2 \rightarrow \mathrm{D}_2 \mathrm{O}$ $\Delta_{\mathrm{r}} \mathrm{H}^{\circ}=$ Bond enthalpies of $$ \begin{aligned} & {\left[\left[\left[\mathrm{D}-\mathrm{D}+\frac{1}{2}(\mathrm{O}-\mathrm{O})\right]-2(\mathrm{D}-\mathrm{O})\right]\right.} \\ & =400+\left(\frac{1}{2} \times 498\right)-(2 \times 490)=-331 \mathrm{~kJ} / \mathrm{mol} \end{aligned} $$

Hence, the correct answer is option: 'B'.

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