Question

For $A \longrightarrow B, \Delta H=4\, Cal\, mol ^{-1}\,\Delta S=10\, Cal\, mol ^{-1}\, K ^{-1}$ reaction is spontaneous when temperature can be :

• A. 400 K
• B. 300 K
• C. 500 K
• D. None of these

Answer 1

Answer: (C)

500 K

Answer 2

For $A \longrightarrow B$. This reaction is spontaneous, so $\Delta G$ should be negative but for temperature $400\, K$ and $300\, K$, $\Delta G$ value would not be negative and hence will not be spontaneous. $A \longrightarrow B$ $\Delta H=4\, kcal\, mol ^{-1}=4000\, cal\, mol ^{-1}$ $\Delta S=10\, cal\, mol ^{-1} K ^{-1}$ $\Delta G=\Delta H-T \Delta S=4000-500 \times 10$ $=4000-5000=-1000$ for $500\, K , \Delta G$ will be negative and hence it will be spontaneous process.