Question

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

• A. $\Delta H = 0 $ and $ \Delta S < 0$
• B. $\Delta H > 0 $ and $\, \Delta S > 0 $
• C. $\Delta H < 0 \,$ and $\, \Delta S < 0$
• D. $\Delta H > 0 \,$ and $\, \Delta S < 0$

Answer 1

Answer: (B)

For the reaction
$PCl_{5} (g) \rightleftharpoons PCl_{3} (g) + Cl_{2} (g)$
The reaction given is an example of decomposition reaction and we know that decomposition reactions are endothermic in nature, i.e, $\Delta H > 0$.
Further
$\Delta n = (1 + 1) - 1= + 1$
Hence more number of molecules are present in products which shows more randomness
i.e. $\Delta S > 0 (\Delta S $ is positive)