Question

Which of the following conditions will always lead to non-spontaneous change?

• A. $\Delta H$ and $\Delta S$ both $+$ ve
• B. $\Delta H$ is -ve and $\Delta S$ is +ve
• C. $\Delta H$ and $\Delta S$ both -ve
• D. $\Delta H$ is +ve and $\Delta S$ is -ve

Answer 1

Answer: (D)

A non-spontaneous reaction is a reaction that does not favor the formation of products at the given set of conditions.
For a reaction to be non-spontaneous, it must be a Endothermic reaction accomponted by a decsease in Entropy.
for an endothermic reaction $\Delta H=+$ ve because the heat is absorbed from the surroundings.
Decrease in entropy impltes that $\Delta S=-ve$
$\Delta G=\Delta H=T\Delta S$
for non-spontaneous change $\Delta G>0$
if $\Delta H=+ve$ and $\Delta S=-ve$
$\Delta G=\Delta H+T\Delta S>0$
So if $\Delta H=+ve$ and $\Delta S=-ve$ the change is always non-spontaneous.