1. 1
   A chemical engineer studies the gaseous equilibrium reaction A(g) + B(g) <=> C(g), where 2 moles of gaseous reactants form 1 mole of gaseous product, and investigates how changing the pressure of the closed system affects the position of equilibrium.

   (a)

   A reversible gaseous reaction, A(g) + B(g) <=> C(g), has 2 moles of gas on the reactant side and 1 mole of gas on the product side. If the pressure of this system at equilibrium is increased, which side of the equilibrium is favoured?

   [1 mark]

   • AThe reaction becomes irreversible
   • BThe reactant side (A and B), which has the most molecules
   • CNeither side is favoured; nothing changes
   • DThe product side (C), which has the least number of molecules

   (b)

   If the engineer instead decreases the pressure of this system, which side of the equilibrium would be favoured?

   [1 mark]

   • ANeither side; equilibrium is unaffected by pressure
   • BThe product side (C), which has the least number of molecules
   • CThe reactant side (A and B), which has the greatest number of molecules
   • DThe reaction stops completely

   (c)

   Explain, using the symbol equation A(g) + B(g) <=> C(g), why increasing the pressure of this system increases the equilibrium yield of C.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student examines the Haber process equilibrium, N2(g) + 3H2(g) <=> 2NH3(g), and considers the effect of using a high pressure (around 200 atmospheres) in the industrial reactor on the equilibrium yield of ammonia.

   (a)

   In the Haber process, N2(g) + 3H2(g) <=> 2NH3(g), how many moles of gas molecules are on the reactant side and how many on the product side?

   [1 mark]

   • A1 mole on each side
   • B2 moles of reactant gas molecules, 4 moles of product gas molecules
   • C3 moles on each side
   • D4 moles of reactant gas molecules, 2 moles of product gas molecules

   (b)

   Based on the number of gas molecules on each side, what effect would increasing pressure have on the equilibrium yield of ammonia in the Haber process?

   [1 mark]

   • AIt would have no effect on the yield of ammonia
   • BIt would decrease the yield of ammonia
   • CIt would increase the yield of ammonia
   • DIt would convert all the ammonia back into nitrogen and hydrogen

   (c)

   Explain why the Haber process is operated at a high pressure (around 200 atmospheres) in terms of the equilibrium yield of ammonia.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A process chemist is analysing a reversible gaseous reaction, 2X(g) <=> Y(g) + Z(g), where 2 moles of gas X form 1 mole of Y and 1 mole of Z (a total of 2 moles of gaseous products), and must decide what pressure to recommend for maximum yield of X.

   (a)

   State the general rule that describes how changing the pressure of a gaseous reversible reaction affects the position of equilibrium.

   [3 marks]

   (b)

   For the reaction 2X(g) <=> Y(g) + Z(g), explain whether increasing or decreasing the pressure would favour the reverse reaction (increasing the amount of X present at equilibrium), giving your reasoning.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemical engineering student is writing a report justifying the choice of pressure used in two industrial gas-phase equilibrium reactions: the Haber process, and the Contact process reaction 2SO2(g) + O2(g) <=> 2SO3(g), used to manufacture sulfuric acid.

   (a)

   Explain fully the general rule describing how pressure affects the position of equilibrium in gaseous reversible reactions, and apply this rule to explain why the Haber process, N2(g) + 3H2(g) <=> 2NH3(g), is operated at a high pressure of around 200 atmospheres rather than at atmospheric pressure.

   [6 marks]

   (b)

   For the Contact process reaction, 2SO2(g) + O2(g) <=> 2SO3(g), explain what effect increasing the pressure would be expected to have on the equilibrium yield of sulfur trioxide, and discuss why, despite this, the Contact process is actually operated at close to atmospheric pressure rather than at very high pressure.

   [6 marks]

   Total for question 4: 12 marks

End of questions