1. 1
   A research chemist seals a mixture of brown nitrogen dioxide gas and colourless dinitrogen tetroxide gas in a tube at equilibrium: 2NO2(g) ⇌ N2O4(g), where the forward reaction (forming N2O4) is exothermic. The chemist then increases the temperature of the sealed tube.

   (a)

   Which statement correctly describes Le Chatelier's Principle?

   [1 mark]

   • AIf a system at equilibrium is subjected to a change, the system responds in a way that counteracts the change
   • BA system at equilibrium always returns exactly to its original conditions no matter what change is made
   • CA system at equilibrium cannot be affected by any external change
   • DEquilibrium can only be reached in reactions that do not involve gases

   (b)

   The chemist then increases the temperature of the sealed tube. Given that the forward reaction (forming N2O4) is exothermic, in which direction does the equilibrium position shift, and what colour change would be observed?

   [1 mark]

   • AThe equilibrium shifts in the forward (exothermic) direction, and the mixture becomes more colourless
   • BThe equilibrium shifts in the reverse (endothermic) direction, and the mixture becomes more brown
   • CThe equilibrium position does not change, and the colour stays exactly the same
   • DThe reaction stops completely, and the colour fades to nothing

   (c)

   Using Le Chatelier's Principle, explain why increasing the temperature shifts this equilibrium towards the endothermic (reverse) direction.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   An ammonia production plant runs the Haber process at equilibrium: N2(g) + 3H2(g) ⇌ 2NH3(g), where the forward reaction is exothermic. Plant engineers increase the operating pressure of the reactor.

   (a)

   How many moles of gas are present on the reactant side of the equation compared with the product side?

   [1 mark]

   • A3 moles of reactant gas and 3 moles of product gas
   • B2 moles of reactant gas and 4 moles of product gas
   • C1 mole of reactant gas and 3 moles of product gas
   • D4 moles of reactant gas and 2 moles of product gas

   (b)

   Plant engineers increase the operating pressure of the reactor. What effect does this have on the equilibrium position and the yield of ammonia?

   [1 mark]

   • AThe equilibrium position is completely unaffected by pressure changes
   • BThe equilibrium shifts towards the side with more gas moles, decreasing the yield of ammonia
   • CThe equilibrium shifts towards the side with fewer gas moles, increasing the yield of ammonia
   • DIncreasing pressure causes the reaction to stop entirely

   (c)

   Using Le Chatelier's Principle, explain why increasing the pressure in the reactor increases the yield of ammonia.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A fertiliser manufacturer operates a Haber process plant to produce ammonia, using purified nitrogen and hydrogen gas passed over an iron catalyst at a compromise temperature of 450 degrees Celsius.

   (a)

   Describe how the raw materials for the Haber process are obtained, and outline the reversible reaction and the conditions used in the reactor.

   [3 marks]

   (b)

   The forward reaction to produce ammonia is exothermic. Explain the trade-off between rate of reaction and equilibrium yield that leads the manufacturer to use a compromise temperature of about 450 degrees Celsius, rather than a much higher or much lower temperature.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A fertiliser manufacturer operates a Haber process plant. Once ammonia is formed, the manufacturer must separate it from the unreacted gases before deciding what to do with the leftover nitrogen and hydrogen.

   (a)

   The Haber process is operated at a pressure of about 200 atmospheres. Explain why using a high pressure favours a higher equilibrium yield of ammonia, and explain why the manufacturer does not choose to use an even higher pressure than 200 atmospheres, despite this benefit to yield.

   [6 marks]

   (b)

   Explain how ammonia is separated from the unreacted nitrogen and hydrogen gases leaving the reactor, and explain why the manufacturer recycles the unreacted gases rather than releasing them or discarding them.

   [6 marks]

   Total for question 4: 12 marks

End of questions