Total entropy change and feasibilityEdexcel International A Level Chemistry: Subtopic test

1. 1
   Instant cold packs contain solid ammonium nitrate and a sealed pouch of water. When the pouch is burst, the solid dissolves: NH₄NO₃(s) + aq → NH₄⁺(aq) + NO₃⁻(aq). The enthalpy change of solution is +25.7 kJ mol⁻¹ and the entropy change of the system is +108.5 J K⁻¹ mol⁻¹. The pack is used at 298 K and becomes noticeably cold as the solid dissolves.

   (a)

   Which statement best explains why the dissolving is spontaneous at 298 K even though it is endothermic?

   [1 mark]

   • AEvery endothermic process is spontaneous at room temperature
   • BThe increase in entropy of the system outweighs the decrease in entropy of the surroundings, so ΔStotal is positive
   • CThe entropy of the surroundings increases because heat is taken in from them
   • DThe reaction has a low activation energy so it must be thermodynamically feasible

   (b)

   What is the entropy change of the surroundings when the ammonium nitrate dissolves at 298 K?

   [1 mark]

   • A+86.2 J K⁻¹ mol⁻¹
   • B−0.0862 J K⁻¹ mol⁻¹
   • C−7.66 × 10⁶ J K⁻¹ mol⁻¹
   • D−86.2 J K⁻¹ mol⁻¹

   (c)

   Calculate the total entropy change for the dissolving at 298 K and state what it shows about feasibility.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In the Haber process, nitrogen and hydrogen react to make ammonia: N₂(g) + 3H₂(g) → 2NH₃(g), ΔH = −92.0 kJ mol⁻¹. Standard molar entropies at 298 K are: N₂(g) 191.6, H₂(g) 130.6 and NH₃(g) 192.3 J K⁻¹ mol⁻¹.

   (a)

   What is the entropy change of the system for the reaction as written?

   [1 mark]

   • A−198.8 J K⁻¹ mol⁻¹
   • B+198.8 J K⁻¹ mol⁻¹
   • C−129.9 J K⁻¹ mol⁻¹
   • D+62.4 J K⁻¹ mol⁻¹

   (b)

   Which statement correctly describes the effect of increasing the temperature on ΔSsurroundings for this reaction?

   [1 mark]

   • AIt becomes larger because more heat is released at a higher temperature
   • BIt becomes negative above a certain temperature
   • CIt stays positive but becomes smaller in magnitude, because ΔSsurroundings = −ΔH/T
   • DIt is unchanged, because ΔH is a constant

   (c)

   Explain, with a calculation of ΔSsurroundings at 298 K, why this reaction is feasible even though ΔSsystem is negative.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Limestone is heated in a kiln to make quicklime: CaCO₃(s) → CaO(s) + CO₂(g), ΔH = +178 kJ mol⁻¹. Standard molar entropies at 298 K are: CaCO₃(s) 92.9, CaO(s) 39.7 and CO₂(g) 213.6 J K⁻¹ mol⁻¹. Assume that ΔH and the entropy values do not change with temperature.

   (a)

   Calculate ΔSsystem and ΔSsurroundings at 298 K for this reaction, and use them to show that the decomposition is not feasible at 298 K.

   [3 marks]

   (b)

   Calculate the minimum temperature at which the decomposition becomes feasible, and explain why raising the temperature makes it feasible.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Methane burns in air: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l), with a standard enthalpy change of combustion of −890 kJ mol⁻¹. Standard molar entropies at 298 K are: CH₄(g) 186.3, O₂(g) 205.0, CO₂(g) 213.6 and H₂O(l) 69.9 J K⁻¹ mol⁻¹. A mixture of methane and air can be stored in a closed vessel at 298 K for years without any reaction.

   (a)

   Calculate ΔStotal for the combustion of methane at 298 K and state what the value shows.

   [6 marks]

   (b)

   The methane–air mixture does not react at 298 K. Explain why this does not contradict your answer to (a), and explain why a spark makes it burn.

   [6 marks]

   Total for question 4: 12 marks

End of questions