Spontaneity and Entropy - UNSOLVED PRACTICE SET

Q7. Define entropy. Why is it called a measure of 'disorder' or 'randomness' of a system?

Q8. Explain why the melting of ice is a spontaneous process at room temperature, even though it is endothermic (ΔH > 0).

Q9. Predict the sign of ΔS for the following reactions and give reasons:

(a) CaCO₃(s) → CaO(s) + CO₂(g)

(b) N₂(g) + 3H₂(g) → 2NH₃(g)

Q10. Why does entropy increase when a substance changes from solid to liquid to gas? Explain at the molecular level.

Q11. A student observes that a ball rolls down a hill spontaneously but never rolls back up on its own. Explain this in terms of entropy and spontaneity.

Q12. Why is the entropy of a pure, perfectly ordered crystalline substance zero at absolute zero temperature? What law supports this statement?

Q13. (a) Define spontaneity in thermodynamic terms. Is a spontaneous process always fast? Explain with an example.

(b) Explain why exothermicity alone cannot predict the spontaneity of a process. Give an example of a spontaneous endothermic process.

(c) A student argues that all exothermic reactions are spontaneous and all endothermic reactions are non-spontaneous. Is the student correct? Justify your answer.

Q14. (a) Define entropy and explain its physical significance.

(b) Predict the sign of ΔS for the following processes and justify your prediction:

(i) Dissolution of sugar in water

(ii) Rusting of iron

(iii) Evaporation of perfume from an open bottle

(iv) Photosynthesis: 6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(s) + 6O₂(g)

(c) Explain why the entropy of the universe increases in every spontaneous process.

Q15. (a) State the Third Law of Thermodynamics. How does it help in determining absolute entropy values?

(b) The standard entropy of H₂(g) is 130.6 J/K·mol at 298 K. Explain why this value is not zero, even though H₂ is an element in its standard state.

(c) Arrange the following in order of increasing entropy and justify your answer:

O₂(g) at 298 K, O₂(g) at 500 K, O₂(l) at 90 K, O₃(g) at 298 K

Q16. For the reaction: 2NO(g) + O₂(g) → 2NO₂(g)

Given: ΔS° = –146.4 J/K·mol at 298 K

(a) Predict whether the entropy of the system increases or decreases. Is your prediction consistent with the given value?

(b) Calculate the entropy change when 4.0 g of NO reacts completely with excess O₂.

(c) Explain why this entropy decrease does not violate the Second Law of Thermodynamics.

(Molar mass of NO = 30 g/mol)

Q17. Calculate the entropy change when 36 g of ice at 0°C melts to form water at 0°C. Also calculate the entropy change of the surroundings if the heat is supplied by a reservoir at 25°C.

Given: ΔHfus of ice = 6.01 kJ/mol

(Molar mass of water = 18 g/mol)

Q18. In India, during the summer season, people often sprinkle water on the roof and courtyard in the evening to cool the house.

(a) Explain this practice in terms of entropy and spontaneity.

(b) Calculate the entropy change of the system when 500 g of water evaporates at 373 K.

(c) The surrounding air is at 310 K. Calculate the entropy change of the surroundings when the same amount of water evaporates.

(d) Using your results, show that the total entropy change of the universe is positive, confirming the spontaneity of the process.

Given: ΔHvap of water = 40.7 kJ/mol, Molar mass of water = 18 g/mol