MCQ
Standard entropy of $X_2, Y_2$ and $XY_3$ are $60, 40$ and $50\,J \,K^{-1}\,mol^{-1},$ respectively. For the reaction,

$\frac{1}{2}{X_2} + \frac{3}{2}{Y_2} \to X{Y_3},\,\Delta H =  - 30\,kJ$ , to be at equilibrium, the temperature will be ............... $\mathrm{K}$

  • A
    $1250$
  • B
    $500$
  • $750$
  • D
    $1000$

Answer

Correct option: C.
$750$
c
For a reaction to be at equilibrium $\Delta G=0$

since $\Delta G=\Delta H-T \Delta S$

so at equilibrium $\Delta H-T \Delta S=0$

or $\quad \Delta H=T \Delta S$

For the reaction

$\frac{1}{2} X_{2}+\frac{3}{2} Y_{2} \rightarrow X Y_{3}$

$\Delta H=-30 \,k J$ (given)

Calculating $\Delta S$ for the above reaction, we get

$\Delta S=50-\left[\frac{1}{2} \times 60+\frac{3}{2} \times 40\right] \,J K^{-1}$

$=50-(30+60) \,J K^{-1}$

$=-40 \,J K^{-1}$

At equilibrium, $T \Delta S=\Delta H$

$[\text { as } \quad \Delta G=0]$

$\therefore \quad T \times(-40)=-30 \times 1000$

$[\text { as } 1 \,k J=1000\, J]$

$\quad T=\frac{-30 \times 1000}{-40}=\quad 750 \,K$

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