Q95 / 200 · NEET 2008 (Screening) · Chemistry

For the gas phase reaction, $\mathrm{PCl}_5(g) \rightleftharpoons \mathrm{PCl}_3(g)+\mathrm{Cl}_2(g)$ which of the following conditions are correct?

  • A
    $\Delta H=0$ and $\Delta S < 0$
  • Correct answer
    B
    $\Delta H>0$ and $\Delta S>0$
  • C
    $\Delta H < 0$ and $\Delta S < 0$
  • D
    $\Delta H>0$ and $\Delta S < 0$

Solution

Key Idea : $\Delta H=\Delta E+\Delta n R T$ $\Delta n=$ number of moles of product - number of moles of reactants $\Delta G=\Delta H-T \Delta S$ For a spontaneous process $\Delta G$ must be negative. $\mathrm{PCl}_5(\mathrm{~g}) \rightleftharpoons \mathrm{PCl}_3(\mathrm{~g})+\mathrm{Cl}_2(\mathrm{~g})$ In this reaction $\Delta n=2-1=1$ Thus, $\Delta H$ is positive, $i e,>0$ If $\Delta H$ is positive, then to maintain the value of $\Delta G$ negative, $\Delta S$ should be positive, $i e, \Delta S>0$.

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

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