PYQ NEET- Chemical Thermodynamics L-9

Question: For a given reaction, $\Delta \mathrm{H}=35.5 \mathrm{~kJ} \mathrm{~mol}^{-1}$ and $\Delta \mathrm{S}=83.6 \mathrm{~J} \mathrm{~K}^{-1} \mathrm{~mol}^{-1}$. The reaction is spontaneous at (Assume that $\Delta \mathrm{H}$ and $\Delta \mathrm{S}$ do not vary with temperature.)#

A) $\mathrm{T}>425 \mathrm{~K}$

B) all temperatures

C) $T>298 \mathrm{~K}$

D) $T<425 \mathrm{~K}$

Answer: $\mathrm{T}>425 \mathrm{~K}$#

Solution:#

For a spontaneous reaction, $$ \Delta \mathrm{G}<0 \text { i.e. } \Delta \mathrm{H}-\mathrm{T} \Delta \mathrm{S}<0 $$ $$ \begin{aligned} & T>\frac{\Delta H}{\Delta S} \ & T>\left(\frac{35.5 \times 1000}{83.6}=424.6 \approx 425 K\right) \ & \therefore T>425 K \end{aligned} $$