- Indicate which of the following has the lowest standard molar entropy (Sº)? a. CH4 (g) c. He (g) d. Na(s) b. CH3CH2OH(1) e. H2O(s) 11. Which of the following has the highest entropy at 298 K? a. 0.5 g of HCN b. 1 mol of HCN c. 2 kg of HCN d. 2 mol of HCN e. All of the above have the same entropy at 298 K 12. Predict the sign of the entropy change of a reaction (S positive or negative) a. Caco: (s) Cad(s) + CO2 (g) b. CS (1) CS, (g) c. 2 Hg (1) + O2(g) 2 HgQ (s) d. 2 Na,O2 (s) + 2 H2O (1) 4 NaOH(aq) + O2(g)
The Correct Answer and Explanation is:
Answers:
10. The correct answer is d. Na(s)
11. The correct answer is d. 2 mol of HCN
12.
- a. Positive (ΔS > 0)
- b. Positive (ΔS > 0)
- c. Negative (ΔS < 0)
- d. Positive (ΔS > 0)
Explanation
Standard molar entropy (Sº) refers to the entropy content of one mole of a substance at standard conditions (298 K and 1 atm). Entropy generally increases from solids to liquids to gases. Among the options:
- CH₄(g), He(g): gases → high entropy
- CH₃CH₂OH(l): liquid → moderate entropy
- H₂O(s): solid → low entropy
- Na(s): solid metal → lowest entropy, since metals are highly ordered in crystal form
Thus, Na(s) has the lowest standard molar entropy.
Question 11:
Entropy depends on amount (moles), not mass alone. While all options refer to HCN, they differ in quantity:
- 0.5 g and 2 kg of HCN are masses, but entropy is extensive—it increases with the number of particles.
- 1 mol vs. 2 mol: more moles = more microstates = higher entropy.
So, 2 mol of HCN (d) has the highest entropy.
Question 12:
Entropy change (ΔS) depends on the change in disorder during a reaction:
- a. CaCO₃(s) → CaO(s) + CO₂(g): A gas forms from solids → ΔS > 0 (positive)
- b. CS₂(l) → CS₂(g): Liquid to gas = more disorder → ΔS > 0 (positive)
- c. 2 Hg(l) + O₂(g) → 2 HgO(s): Liquids and gas forming solid = decrease in entropy → ΔS < 0 (negative)
- d. 2 Na₂O₂(s) + 2 H₂O(l) → 4 NaOH(aq) + O₂(g): More particles, especially gas and aqueous ions formed = more disorder → ΔS > 0 (positive)
In summary, entropy is influenced by phase, number of particles, and state changes. Gases and more moles usually mean higher entropy.
