Q76 / 160 · JIPMER 2006 · Chemistry

The entropy values (in $\mathrm{JK}^{-1} \mathrm{~mol}^{-1}$ ) of $\mathrm{H}_2(\mathrm{~g})=130.6, \quad \mathrm{Cl}_2(\mathrm{~g})=223.0 \quad$ and $\mathrm{HCl}(\mathrm{g})=186.7$ at 298 K and 1 atm pressure. Then entropy change for the reaction : $\mathrm{H}_2(\mathrm{~g})+\mathrm{Cl}_2(\mathrm{~g}) \longrightarrow 2 \mathrm{HCl}(\mathrm{g})$ is :

  • A
    +540.3
  • B
    +727.3
  • C
    -166.9
  • Correct answer
    D
    +19.8

Solution

$\begin{aligned} \Delta S^{\circ} & =2 S^{\circ}{ }_{\mathrm{HCl}}-\left(S_{\mathrm{H}_2}^{\circ}+S^{\circ}{ }_{\mathrm{Cl}_2}\right) \\ & =2 \times 186.7-(130.6+223.0) \\ & =19.8 \mathrm{JK}^{-1} \mathrm{~mol}^{-1}\end{aligned}$

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

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