Question

$\Delta$G is the net energy available to do useful work and is a measure of free energy. If a reaction has positive enthalpy change and positive entropy change, under what conditions will the reaction be spontaneous?

• A. $\Delta$G will be positive at low temperature hence reaction is spontaneous at low temperature.
• B. $\Delta$G will be negative at high temperature hence reaction is spontaneous at high temperature.
• C. $\Delta$G will be negative at low temperature hence reaction is spontaneous at low temperature.
• D. $\Delta$G will be negative at all temperature hence reaction is spontaneous at all temperatures.

Answer 1

Answer: (B)

$\Delta$G will be negative at high temperature hence reaction is spontaneous at high temperature.

$\Delta H>0$ and $\Delta S>0$

Now, $\Delta G=\Delta H-T\Delta S$

For a spontaneous process, $\Delta G<0,i.e,T\Delta S>\Delta H$, i.e., temperature should be high.

Therefore, the reaction will be spontaneous for negative value of $\Delta G$, i.e., high temperature is required.