Entropy and spontaneityEdexcel A-Level Chemistry: Subtopic test

1. 1
   In a demonstration, solid barium hydroxide octahydrate is mixed with solid ammonium chloride in a flask. The mixture becomes a liquid, ammonia gas is given off and the flask becomes so cold that it freezes a wet wooden block to the bench. The reaction is Ba(OH)₂·8H₂O(s) + 2NH₄Cl(s) → BaCl₂(s) + 2NH₃(g) + 10H₂O(l). It happens spontaneously at room temperature even though it is endothermic.

   (a)

   Which statement best explains why this endothermic reaction occurs spontaneously at room temperature?

   [1 mark]

   • AEndothermic reactions always occur spontaneously
   • BThe entropy of the surroundings increases because heat is absorbed
   • CThe increase in entropy of the system outweighs the decrease in entropy of the surroundings, so the total entropy increases
   • DThe reaction has no activation energy

   (b)

   In which of these changes does the entropy of the system decrease?

   [1 mark]

   • AIce melting to form liquid water
   • B2H₂(g) + O₂(g) → 2H₂O(l)
   • CAmmonium nitrate dissolving in water
   • DCaCO₃(s) → CaO(s) + CO₂(g)

   (c)

   Explain, in terms of disorder, why the entropy of the system increases in this reaction.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The Haber process for making ammonia, N₂(g) + 3H₂(g) → 2NH₃(g), is exothermic with ΔH = −92 kJ mol⁻¹. A student uses it to practise calculating the entropy change of the surroundings, and assumes that ΔH does not change with temperature.

   (a)

   What is the entropy change of the surroundings for this reaction at 298 K?

   [1 mark]

   • A+309 J K⁻¹ mol⁻¹
   • B−309 J K⁻¹ mol⁻¹
   • C+0.309 J K⁻¹ mol⁻¹
   • D+92 J K⁻¹ mol⁻¹

   (b)

   The reaction is repeated at 500 K. Which statement about the entropy change of the surroundings is correct?

   [1 mark]

   • AIt is greater than +309 J K⁻¹ mol⁻¹ because the surroundings are hotter
   • BIt is unchanged at +309 J K⁻¹ mol⁻¹
   • CIt is negative, because the temperature is higher
   • DIt is smaller than +309 J K⁻¹ mol⁻¹, at about +184 J K⁻¹ mol⁻¹

   (c)

   Explain why the entropy change of the surroundings is positive for an exothermic reaction.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Calcium carbonate decomposes on heating: CaCO₃(s) → CaO(s) + CO₂(g). The enthalpy change is ΔH = +178 kJ mol⁻¹. Standard entropies in J K⁻¹ mol⁻¹: CaCO₃(s) 92.9; CaO(s) 39.7; CO₂(g) 213.6.

   (a)

   Calculate the entropy change of the system, ΔS system, for the decomposition.

   [3 marks]

   (b)

   Calculate the entropy change of the surroundings and the total entropy change at 298 K. Hence state whether the decomposition is feasible at 298 K.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A teacher demonstrates three changes in which entropy plays a part: ice melting in a room at 20 °C; magnesium ribbon burning in oxygen, 2Mg(s) + O₂(g) → 2MgO(s); and solid ammonium carbonate reacting with ethanoic acid, which gives off carbon dioxide gas and makes the mixture cool. A further endothermic reaction in the class has ΔH = +60 kJ mol⁻¹ and ΔS(system) = +280 J K⁻¹ mol⁻¹, measured at 293 K.

   (a)

   Explain, in terms of entropy, why each of the three changes occurs.

   [6 marks]

   (b)

   Calculate the total entropy change for the endothermic reaction at 293 K, and use your answer to explain why it occurs even though it is endothermic.

   [6 marks]

   Total for question 4: 12 marks

End of questions