Question

A reaction's Gibbs free energy change (\(\Delta G\)) is found to be positive at 298 K but becomes negative at 350 K. What does this imply about the reaction?

• A. It is non-spontaneous at all temperatures.
• B. It is spontaneous only above 350 K.
• C. It is spontaneous only below 298 K.
• D. It becomes spontaneous as temperature increases.

Hint:

Temperature Dependence of Spontaneity (\(\Delta G = \Delta H - T\Delta S\)). If \(\Delta G\) changes from positive (non-spontaneous) to negative (spontaneous) as T increases, it implies both \(\Delta H\) and \(\Delta S\) are positive.

Answer 1

The Correct Option is D

The spontaneity of a reaction is determined by the sign of the Gibbs free energy change (\(\Delta G\)): - \(\Delta G<0\): Reaction is spontaneous.
- \(\Delta G>0\): Reaction is non-spontaneous.
- \(\Delta G = 0\): Reaction is at equilibrium.
We are given that \(\Delta G>0\) at 298 K (non-spontaneous) and \(\Delta G<0\) at 350 K (spontaneous).
This shows that the reaction's spontaneity changes from non-spontaneous to spontaneous as the temperature increases from 298 K to 350 K.
Option (4) accurately describes this trend.
Option (2) might also be true, implying a transition temperature between 298 K and 350 K, but Option (4) is the most direct conclusion from the given data.
The relationship \(\Delta G = \Delta H - T\Delta S\) governs this behavior.
For \(\Delta G\) to decrease (become more negative) as T increases, the entropy change (\(\Delta S\)) must be positive.
The switch from positive \(\Delta G\) to negative \(\Delta G\) implies \(\Delta H\) is also positive (endothermic reaction becoming spontaneous at higher T due to favorable entropy term).