1. 1
   A museum conservator is examining an old chemistry demonstration kit containing hydrated copper(II) sulfate crystals, which turn from blue to white on heating and back to blue again when water is added, and wants to understand and demonstrate this reversible reaction, as well as a similar reversible reaction involving ammonium chloride, to museum visitors.

   (a)

   A museum conservator notices that blue hydrated copper(II) sulfate crystals in an old chemistry demonstration turn white on gentle heating, and can be turned blue again by adding water. Which symbol is used in a chemical equation to show that this reaction is reversible?

   [1 mark]

   • A→
   • B⇌
   • C=
   • D↔

   (b)

   The conservator also demonstrates the reversible reaction that occurs when solid ammonium chloride is heated. What is observed when ammonium chloride is heated strongly?

   [1 mark]

   • AIt decomposes into ammonia and hydrogen chloride gases, which recombine as they cool
   • BIt melts and stays a liquid permanently
   • CIt burns with a bright flame and produces ash
   • DIt dissolves instantly without any visible change

   (c)

   Describe the reversible reaction that occurs when hydrated copper(II) sulfate crystals are heated, and explain what happens when water is subsequently added to the product.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   An industrial chemist working at an ammonia production plant is studying the reversible Haber process reaction, N2 + 3H2 ⇌ 2NH3, carried out in a sealed reactor vessel, and is investigating how dynamic equilibrium is reached and how a catalyst is used to speed up the process.

   (a)

   An industrial chemist at an ammonia production plant runs the Haber process reaction, N2 + 3H2 ⇌ 2NH3, in a sealed reactor vessel until dynamic equilibrium is reached. Which statement correctly describes dynamic equilibrium?

   [1 mark]

   • AThe forward reaction stops completely and only the reverse reaction continues
   • BThe forward and reverse reactions both stop
   • CThe forward and reverse reactions occur at the same rate, and concentrations of reactants and products remain constant
   • DThe concentration of products continuously increases while reactants continuously decrease

   (b)

   The plant uses an iron catalyst to speed up the approach to equilibrium in the Haber process reactor. What effect does adding this catalyst have on the position of equilibrium?

   [1 mark]

   • AIt shifts the equilibrium position further towards the products
   • BIt shifts the equilibrium position further towards the reactants
   • CIt removes the reverse reaction entirely
   • DIt has no effect on the position of equilibrium

   (c)

   Explain why a catalyst does not affect the position of equilibrium in the reversible Haber process reaction, even though it increases the rate at which equilibrium is reached.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A process engineer at a chemical plant is optimising the temperature and pressure conditions used in a sealed reactor for a reversible, exothermic gas-phase reaction, aiming to maximise the yield of a valuable product while balancing costs and reaction rate for the company's production process.

   (a)

   A process engineer at a different chemical plant is optimising conditions for a reversible, exothermic gas-phase reaction carried out in a sealed reactor. Explain the general effect of increasing the temperature on the position of equilibrium for an exothermic reversible reaction, and explain the general effect of increasing the pressure on the position of equilibrium for a reaction where the forward reaction produces fewer gas molecules than the reverse reaction.

   [3 marks]

   (b)

   The engineer decides to compromise on a moderate temperature and moderate-to-high pressure rather than using the theoretically ideal conditions for maximum equilibrium yield. Explain why a real industrial process might use compromise conditions rather than the conditions that would give the absolute highest possible equilibrium yield.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemistry teacher is planning a classroom demonstration of the reversible dehydration and rehydration of hydrated copper(II) sulfate crystals, intending to use it as a starting point for a wider class discussion comparing simple reversible reactions with true dynamic equilibrium systems used in industry, such as the Haber process.

   (a)

   A chemistry teacher wants to demonstrate the reversible dehydration of hydrated copper(II) sulfate to a class, alongside a class discussion of dynamic equilibrium. Describe, in full detail, how the teacher could carry out and explain the demonstration of heating hydrated copper(II) sulfate and then rehydrating it, and explain how this reaction illustrates the general concept of a reversible reaction, even though it does not reach a true dynamic equilibrium in an open test tube.

   [6 marks]

   (b)

   A student in the class asks why the Haber process, carried out in a sealed reactor, is described as reaching dynamic equilibrium, while the copper(II) sulfate demonstration in an open test tube is not. Evaluate the key differences between the two systems that explain why one reaches true dynamic equilibrium and the other does not, and explain what would need to change about the copper(II) sulfate demonstration for it to reach a genuine dynamic equilibrium.

   [6 marks]

   Total for question 4: 12 marks

End of questions