Question

An endothermic reaction is non-spontaneous at freezing point of water and becomes feasible at its boiling point, then

• A. $\Delta H=+ve;\Delta S=+ve$
• B. $\Delta H=–ve;\Delta S=+ve$
• C. $\Delta H=–ve;\Delta S=–ve$
• D. $\Delta H=+ve;\Delta S=+ve$

Answer 1

The correct option is D $\Delta H=+ve;\Delta S=+ve$

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

Since the reaction is endothermic, $\Delta H$ = +ve

As the temperature increases from freezing to boiling, the reaction becomes spontaneous, i.e. $\Delta G$ decreases, i.e. $T\Delta S$ becomes more negative. This is possible only if $\Delta S$ is positive.