Rank these systems in order of decreasing entropy. Rank from highest to lowest entropy. To rank items as equivalent overlap them.
1 mol of hydrogen peroxide gas at 273K and 40L
½ mol of helium gas at 100 K and 20 L
1 mol of chlorine gas at 273K and 40 L
½ mol of helium gas at 273K and 20 L
1 mol of helium gas at 273K and 20 L
½ mol of liquid helium at 100K
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
Question 10:
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.
