EntropyAQA A-Level Chemistry: Subtopic test

1. 1
   A student discusses entropy using the three physical states of water. She considers how the disorder of the particles in ice, liquid water and steam changes as a sample of water is heated from below 0 °C to above 100 °C at constant pressure.

   (a)

   Which state of water has the greatest entropy at 1 mol?

   [1 mark]

   • AH₂O(s) at 0 °C
   • BH₂O(l) at 25 °C
   • CThey all have the same entropy because they contain the same molecules
   • DH₂O(g) at 100 °C

   (b)

   Which change has a negative entropy change, ΔS?

   [1 mark]

   • AH₂O(s) → H₂O(l)
   • BH₂O(g) → H₂O(l)
   • CH₂O(l) → H₂O(g)
   • DH₂O(s) → H₂O(g)

   (c)

   Explain why the entropy of steam is greater than the entropy of liquid water at the same temperature.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Ammonia is made industrially in the Haber process: N₂(g) + 3H₂(g) → 2NH₃(g). Standard entropies (J K⁻¹ mol⁻¹): N₂(g) = 192; H₂(g) = 131; NH₃(g) = 193.

   (a)

   Calculate the entropy change for the Haber process.

   [1 mark]

   • A−199 J K⁻¹ mol⁻¹
   • B+199 J K⁻¹ mol⁻¹
   • C−130 J K⁻¹ mol⁻¹
   • D+971 J K⁻¹ mol⁻¹

   (b)

   Which statement explains the sign of the entropy change for the Haber process?

   [1 mark]

   • AAmmonia is formed as a liquid, which is more ordered than a gas
   • BThe reaction is exothermic, so the system becomes more disordered
   • CThere are fewer moles of gas in the products than in the reactants
   • DNitrogen has a stronger bond than hydrogen, so the products are more ordered

   (c)

   Predict, with a reason, the sign of the entropy change for the reaction Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g).

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Calcium carbonate decomposes on strong heating: CaCO₃(s) → CaO(s) + CO₂(g), ΔH = +178 kJ mol⁻¹. Standard entropies (J K⁻¹ mol⁻¹): CaCO₃(s) = 93; CaO(s) = 40; CO₂(g) = 214. Relative formula mass of CaCO₃ = 100.1.

   (a)

   Calculate the entropy change for the decomposition of calcium carbonate. Include units.

   [3 marks]

   (b)

   Calculate the entropy change when 10.0 g of calcium carbonate decomposes. Explain why an increase in entropy can allow an endothermic reaction such as this one to occur.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemist studies two changes. In the first, ammonium nitrate dissolves in water, NH₄NO₃(s) + aq → NH₄⁺(aq) + NO₃⁻(aq), with ΔH = +25.7 kJ mol⁻¹; the solution becomes noticeably colder, and the process is used in instant cold packs. In the second, hydrogen burns in oxygen. Standard entropies (J K⁻¹ mol⁻¹): H₂(g) = 131; O₂(g) = 205; H₂O(l) = 70; H₂O(g) = 189.

   (a)

   Ammonium nitrate dissolves spontaneously in water even though the process is endothermic. Explain why the enthalpy change alone cannot be used to predict whether a change will occur, with reference to this example.

   [6 marks]

   (b)

   Calculate the entropy change for the reaction 2H₂(g) + O₂(g) → 2H₂O(l), and for the reaction 2H₂(g) + O₂(g) → 2H₂O(g). Explain why both values are negative and why they differ.

   [6 marks]

   Total for question 4: 12 marks

End of questions