Question

A reaction has a value of $\Delta H-20Kcal$ at $200K$, the reaction is spontaneous, below this temperature, it is not. The values $\Delta G$ and $\Delta S$ at $400K$ are, respectively:

• A. $10,-0.1cal{K}^{-1}$
• B. $-10Kcal,-100cal{K}^{-1}$
• C. $0,10.0cal{K}^{-1}$
• D. $20Kcal,-100cal{K}^{-1}$

Answer 1

The correct option is D $20Kcal,-100cal{K}^{-1}$

$\Delta G=\Delta H-T\Delta S$ equation.

$\Delta H=-20Kcal$

Since, the reaction is not spontaneous below 200K, the reaction is in equilibrium at 200K So $\Delta G=0$

Hence, $\Delta S=-20000/200=-100cal/K$

At, 400K $\Delta G=(-20+400\ast 0.1)Kcal=20Kcal$

So, $\Delta G=20Kcaland\Delta S=-100cal/K$