Question

Which of the following conditions allow a reaction to be spontaneous?

• A. $\Delta H^{\circ} = -, \Delta S^{\circ} = +$
• B. $\Delta H^{\circ} = +, \Delta S^{\circ} = +$
• C. $\Delta H^{\circ} = -, \Delta S^{\circ} = -$
• D. $\Delta H^{\circ} = +, \Delta S^{\circ} = -$

Hint:

Nature loves releasing heat (-$\Delta$H) and making a mess (+$\Delta$S). Both together always mean "go!" (spontaneous).

Answer 1

The Correct Option is A

Step 1: Concept
Spontaneity is determined by the Gibbs Free Energy change: $\Delta G^{\circ} = \Delta H^{\circ} - T\Delta S^{\circ}$. A reaction is spontaneous if $\Delta G^{\circ} < 0$.

Step 2: Analysis
For $\Delta G^{\circ}$ to always be negative, we look at the signs of enthalpy ($\Delta H$) and entropy ($\Delta S$).

Step 3: Reasoning
If $\Delta H$ is negative (exothermic) and $\Delta S$ is positive (increased disorder), the term $(-T\Delta S)$ becomes more negative. The sum of two negative terms is always negative regardless of temperature.

Step 4: Conclusion
The condition $\Delta H^{\circ} < 0$ and $\Delta S^{\circ} > 0$ guarantees spontaneity. Final Answer: (A)