Answer
📚 Concept: Standard Molar Entropy (S°)
Standard molar entropy refers to the absolute entropy of 1 mole of a substance at 25°C and 1 atm. The magnitude of entropy generally follows these trends:
- Gases > Liquids > Solids (in terms of entropy)
- More complex or heavier molecules tend to have higher entropy than simpler or lighter ones in the same phase.
- Aqueous ions tend to have lower entropy than gaseous species due to solvation ordering.
❌ Incorrect Options Explained
H₂S(aq) > H₂S(g) ❌ — Gases have more entropy than aqueous forms.
Li(s) > Pb(s) ❌ — Heavier atoms like Pb(s) have more microstates and greater entropy than lighter ones like Li(s).
H₂(g) > H₂O(g) ❌ — H₂O is a polyatomic molecule with more vibrational modes, hence higher entropy than diatomic H₂.
NaCl(s) > NaCl(aq) ❌ — Dissolving a salt increases disorder, so NaCl(aq) has higher entropy.
✅ Correct Option
CO₂(g) > CO(g) — Correct ✔️
CO₂ is a heavier and more complex molecule than CO, so it has greater standard molar entropy at 25°C.
CO₂ is a heavier and more complex molecule than CO, so it has greater standard molar entropy at 25°C.
🧠 Final Answer:
CO₂(g) > CO(g)