1. 1
   A chemistry technician heats blue hydrated copper(II) sulfate crystals in a test tube, and they turn white as steam is given off. When water is later added back to the white powder, it turns blue again and becomes warm.

   (a)

   What does this behaviour show about the reaction?

   [1 mark]

   • AIt is an irreversible reaction, since a permanent colour change has occurred
   • BIt is a reversible reaction, since the direction can be changed by heating or adding water
   • CIt shows that copper(II) sulfate cannot combine with water at all
   • DIt shows that no chemical reaction has taken place

   (b)

   What term describes the white powder remaining after all the water has been driven off the copper(II) sulfate crystals?

   [1 mark]

   • AHydrated copper(II) sulfate
   • BCopper(II) oxide
   • CAnhydrous copper(II) sulfate
   • DCopper(II) sulfate solution

   (c)

   Write the symbol used to represent a reversible reaction, and use it to describe, in words, the two directions of change the technician observed with the copper(II) sulfate.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A forensic investigator uses strips of paper coated with cobalt(II) chloride to test a spilled liquid at a crime scene, to check whether the liquid contains water.

   (a)

   Dry cobalt(II) chloride paper is blue; what colour would it turn if the spilled liquid contains water?

   [1 mark]

   • AIt would turn pink
   • BIt would turn black
   • CIt would remain blue
   • DIt would turn colourless

   (b)

   If the investigator later dries the pink paper in an oven, what would be expected to happen, given that this is a reversible change?

   [1 mark]

   • AThe paper would remain pink permanently
   • BThe paper would turn back to blue as the water is driven off
   • CThe paper would char and turn black
   • DThe paper would dissolve completely

   (c)

   Explain why cobalt(II) chloride paper is a useful test for the presence of water, referring to the reversibility of the reaction.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A fertiliser plant runs the Haber process, in which nitrogen and hydrogen gases are combined to form ammonia in a closed reaction vessel: N2(g) + 3H2(g) ⇌ 2NH3(g).

   (a)

   State the sources of the nitrogen and hydrogen used, and explain what is meant by saying the reaction reaches equilibrium in the closed vessel.

   [3 marks]

   (b)

   The plant engineers increase the pressure inside the reaction vessel while keeping the temperature constant. Predict and explain the effect this has on the position of equilibrium and the yield of ammonia.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A fertiliser plant on an industrial site runs the Haber process, N2(g) + 3H2(g) ⇌ 2NH3(g), at 450 °C, about 200 atm pressure, using an iron catalyst, even though a lower temperature would give a higher equilibrium yield of ammonia (the forward reaction is exothermic). A neighbouring plant on the same site produces sulfuric acid using the Contact process, 2SO2(g) + O2(g) ⇌ 2SO3(g), operating at 450 °C, about 200 kPa and using a vanadium(V) oxide catalyst.

   (a)

   The fertiliser plant operates the Haber process at 450 °C and about 200 atm pressure, using an iron catalyst, even though a lower temperature would give a higher equilibrium yield of ammonia (the forward reaction is exothermic). Explain why these particular conditions are chosen, discussing the trade-off between rate of reaction and position of equilibrium.

   [6 marks]

   (b)

   State the sources of the sulfur dioxide and oxygen used in the neighbouring plant's Contact process, and explain, using similar reasoning to the Haber process, why the conditions chosen for the Contact process represent a sensible compromise for the plant.

   [6 marks]

   Total for question 4: 12 marks

End of questions