Q. In which of the following reactions, standard reaction entropy change (ΔS°) is positive and standard Gibbs energy change (ΔG°) decreases sharply with increasing temperature?
A) (1/2)C(graphite) + (1/2)O₂(g) → (1/2)CO₂(g)
B) C(graphite) + (1/2)O₂(g) → CO(g)
C) CO(g) + (1/2)O₂(g) → CO₂(g)
D) Mg(s) + (1/2)O₂(g) → MgO(s)
Explanation
[Solution from source] In C(graphite) + ½O₂(g) → CO(g), gaseous moles increase from ½ to 1, so ΔS° is clearly positive and ΔG° = ΔH° − TΔS° falls steeply as T rises. Forming CO₂ from C keeps gas moles unchanged (ΔS° ≈ 0), while CO → CO₂ and Mg → MgO decrease gas moles (ΔS° < 0). Answer (2).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.