Question

A certain reaction is non spontaneous at 298K. The entropy change during the reaction is 121 $JK^{-1}.$ Is the reaction is endothermic or exothermic ? The minimum value of $\Delta$H for the reaction is

• A. endothermic, $\Delta H=36.06 KJ$
• B. exothermic, $\Delta H=-36.06 KJ$
• C. endothermic, $\Delta H=60.12KJ$
• D. exothermic, $\Delta H=-60.12KJ$

Answer 1

Answer: (A)

endothermic, $\Delta H=36.06 KJ$

Answer 2

For non spontaneous reaction
$\Delta G=+Ve$
$\Delta G=\Delta H-T \Delta S and$
$\Delta S=121 JK^{-1}$
$For\, \Delta G=+Ve$
$\Delta H$ has to be positive. Hence the reaction is endothermic.
The minimum value of $\Delta H$ can be obtained by putting $\Delta$G = 0
$\Delta H=T\Delta S=298\times121J$
= 36.06 kJ