Q7 / 180 · KCET 2010 · Chemistry

For the reversible reaction. $\mathrm{A}(\mathrm{s})+\mathrm{B}(\mathrm{g}) \rightleftharpoons \mathrm{C}(\mathrm{g})+\mathrm{D}(\mathrm{g});$ $\Delta \mathrm{G}^{\circ}=-350 \mathrm{~kJ}$ Which one of the following statements is true?

  • A
    The entropy change is negative
  • Correct answer
    B
    Equilibrium constant is greater than one
  • C
    The reaction should be instantaneous
  • D
    The reaction is thermodynamically not feasible

Solution

In the reversible reaction, $$ \begin{aligned} \mathrm{A}(\mathrm{s})+\mathrm{B}(\mathrm{g}) \rightleftharpoons \mathrm{C}(\mathrm{g})+\mathrm{D}(\mathrm{g}) \\ \Delta \mathrm{G}^{\circ}=-350 \mathrm{~kJ} \end{aligned} $$ Since, the randomness increases (because solid is changing into gas), entropy will increase and thus, $\Delta \mathrm{S}$ is positive. Reversible reaction never undergo to completion (ie, never be instantaneous). $\because$ For the given reaction, $\Delta \mathrm{G}^{\circ}=-350 \mathrm{~kJ}$ and we know that $$ \begin{aligned} \Delta G^{\circ} &=-R T \log K \\ -350 &=-R T \log K \end{aligned} $$ ie, $\mathrm{K}$ (equilibrium constant) is greater than one. Moreover, the reaction is thermodynamically feasible.

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

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