Partial pressures and the equilibrium constant KpAQA A-Level Chemistry: Subtopic test

1. 1
   Dinitrogen tetroxide decomposes reversibly: N₂O₄(g) ⇌ 2NO₂(g). A sample of 1.00 mol of N₂O₄ is allowed to reach equilibrium in a sealed vessel at constant temperature. At equilibrium the mixture contains 0.60 mol of N₂O₄ and 0.80 mol of NO₂ and the total pressure is 200 kPa.

   (a)

   How is the partial pressure of N₂O₄ in the equilibrium mixture calculated?

   [1 mark]

   • ATotal pressure ÷ mole fraction of N₂O₄
   • BMole fraction of N₂O₄ ÷ total pressure
   • CMole fraction of N₂O₄ × total pressure
   • DMoles of N₂O₄ × total pressure

   (b)

   Which is the correct expression for Kp for this equilibrium?

   [1 mark]

   • AKp = p(NO₂) ÷ p(N₂O₄)
   • BKp = p(NO₂)² ÷ p(N₂O₄)
   • CKp = p(N₂O₄) ÷ p(NO₂)²
   • DKp = 2p(NO₂) ÷ p(N₂O₄)

   (c)

   Calculate the partial pressure of each gas in the equilibrium mixture.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Ammonia is made in the Haber process: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹. An iron catalyst is used.

   (a)

   The total pressure of an equilibrium mixture is increased at constant temperature. Which statement is correct?

   [1 mark]

   • AThe position of equilibrium shifts to the right and the value of Kp increases
   • BThe position of equilibrium shifts to the left and the value of Kp is unchanged
   • CThe position of equilibrium does not change but the value of Kp increases
   • DThe position of equilibrium shifts to the right and the value of Kp is unchanged

   (b)

   The temperature of the equilibrium mixture is increased. Which statement about Kp is correct?

   [1 mark]

   • AKp decreases because the forward reaction is exothermic
   • BKp increases because the forward reaction is exothermic
   • CKp is unchanged because the number of moles of gas is unchanged
   • DKp increases because the rate of reaction increases

   (c)

   State and explain the effect of the iron catalyst on the value of Kp and on the position of equilibrium.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In a sealed vessel, 1.00 mol of phosphorus(V) chloride is heated at constant temperature until equilibrium is reached: PCl₅(g) ⇌ PCl₃(g) + Cl₂(g). The equilibrium mixture contains 0.40 mol of PCl₅, 0.60 mol of PCl₃ and 0.60 mol of Cl₂, and the total pressure is 150 kPa.

   (a)

   Calculate the value of Kp for this equilibrium, and state its units.

   [3 marks]

   (b)

   At the same temperature, a different equilibrium mixture has a partial pressure of PCl₅ of 20.0 kPa. The PCl₃ and Cl₂ come only from the decomposition of PCl₅. Calculate the partial pressure of Cl₂ and the total pressure of this mixture.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In the Contact process, sulfur dioxide is oxidised to sulfur trioxide: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH = −196 kJ mol⁻¹, using a vanadium(V) oxide catalyst.

   (a)

   Explain how changes in temperature and in pressure affect the position of equilibrium and the value of Kp for this reaction, and why a compromise temperature of about 450 °C is used in industry. State the effect of the catalyst.

   [6 marks]

   (b)

   A mixture of 2.00 mol of SO₂ and 1.00 mol of O₂ is allowed to reach equilibrium at constant temperature and a total pressure of 200 kPa. At equilibrium, 0.40 mol of SO₂ remains. Calculate the value of Kp, including its units.

   [6 marks]

   Total for question 4: 12 marks

End of questions