Chemistry - Thermodynamics - II Question with Solution | TestHub
Match Column I (Reaction) with Column II (Signs of Δ H, Δ S)
Column I | Column II |
|---|---|
P) CaCO3(s) → CaO(s) + CO2(g) | i) Δ H > 0, Δ S > 0 |
Q) 2H2(g) + O2(g) → 2H2O(l) | ii) Δ H < 0, Δ S < 0 |
R) H2(g) + I2(g) → 2HI(g) | iii) Δ H > 0, Δ S ≈ 0 |
S) C(s) + O2(g) → CO2(g) | iv) Δ H < 0, Δ S ≈ 0 |
Options:
Answer:
Solution:
Thermal decomposition of limestone is endothermic and releases a gas: Δ H > 0, Δ S > 0 (P – i). Burning hydrogen is exothermic, and 3 mol of gas become liquid water: Δ H < 0, Δ S < 0 (Q – ii). HI formation is slightly endothermic (Δ H > 0) with Δ ng = 0, so Δ S ≈ 0 (R – iii). Burning carbon is strongly exothermic with Δ ng = 0 (one mole of gas consumed, one produced), so Δ S ≈ 0 (S – iv). Trap: count moles of gas only — condensed phases barely contribute to the entropy estimate, and HI formation is endothermic, unlike most combination reactions.
