Question

A reaction has $\Delta H=-33\text{kJ}$ and $\Delta S=-58\text{J/K}$. This reaction would be:

• A. Spontaneous at all temperatures
• B. Non-spontaneous at all temperatures
• C. Spontaneous above a certain temperature
• D. Spontaneous below a certain temperature

Answer 1

The correct option is D Spontaneous below a certain temperature

$\Delta G=\Delta H-T\Delta S$
For a reaction to be spontaneous, $\Delta G$ should be negative.

a) For a reaction to be spontaneous at all temperatures, $\Delta H$ should be negative and $\Delta S$ should be positive.

b) For a reaction to be non spontaneous at all temperatures, $\Delta H$ should be positive and $\Delta S$ should be negative.

c) For a reaction to be spontaneous above a certain temperature, $\Delta H$ should be positive and $\Delta S$ should be positive.

d) For a reaction to be spontaneous below a certain temperature, $\Delta H$ should be negative and $\Delta S$ should be negative.

In the give data, both $\Delta H$ and $\Delta S$ are negative. Hence, the reaction is spontaneous below a certain temperature.