Effect of conditions on KEdexcel A-Level Chemistry: Subtopic test

1. 1
   The Haber process makes ammonia: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹. A chemical engineer studies how the equilibrium constant Kp, and the composition of the equilibrium mixture, change when the conditions are altered.

   (a)

   The temperature of the equilibrium mixture is raised from 500 K to 700 K. What happens to the value of Kp?

   [1 mark]

   • AKp decreases because the forward reaction is exothermic
   • BKp increases because the rates of both reactions increase
   • CKp is unchanged because only a change in pressure affects Kp
   • DKp increases because the position of equilibrium shifts to the right

   (b)

   The pressure of the equilibrium mixture is doubled at constant temperature. What happens once equilibrium is re-established?

   [1 mark]

   • AKp is unchanged and the position of equilibrium is unchanged
   • BKp is unchanged and the position of equilibrium shifts to the right
   • CKp increases because the position of equilibrium shifts to the right
   • DKp decreases and the position of equilibrium shifts to the left

   (c)

   The iron catalyst is added to the reactor. Explain why this does not change the value of Kp.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Hydrogen is made on a large scale by steam reforming of methane: CH₄(g) + H₂O(g) ⇌ CO(g) + 3H₂(g), ΔH = +206 kJ mol⁻¹. The reformers run at about 1100 K.

   (a)

   Extra steam is added to the equilibrium mixture at constant temperature. What happens to the value of Kp?

   [1 mark]

   • AIt increases because more product forms
   • BIt decreases because the reactant concentration has increased
   • CIt increases and then returns to its original value
   • DIt stays the same

   (b)

   The temperature of the reformer is raised from 900 K to 1100 K. Which statement is correct?

   [1 mark]

   • AKp decreases because the forward reaction is endothermic
   • BKp increases because the rate of reaction increases
   • CKp increases because the forward reaction is endothermic
   • DKp is unchanged because the pressure is constant

   (c)

   Explain why the percentage of hydrogen in the equilibrium mixture is higher at 1100 K than at 900 K.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In hydrogen plants carbon monoxide reacts with steam in the water–gas shift reaction: CO(g) + H₂O(g) ⇌ CO₂(g) + H₂(g), ΔH = −41 kJ mol⁻¹. A first reactor runs at about 650 K and a second reactor at about 470 K.

   (a)

   Explain the effect on Kp, and on the position of equilibrium and the yield of hydrogen, of lowering the temperature from 650 K to 470 K.

   [3 marks]

   (b)

   A student claims that raising the total pressure in the reactors would increase the equilibrium yield of hydrogen. Evaluate this claim.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Dinitrogen tetroxide, which is colourless, forms brown nitrogen dioxide: N₂O₄(g) ⇌ 2NO₂(g), ΔH = +57 kJ mol⁻¹. For this equilibrium Kp is 0.14 atm at 298 K and about 4 atm at 350 K. Sealed syringes containing equilibrium mixtures are used in the experiments below.

   (a)

   A sealed syringe containing an equilibrium mixture at 298 K is heated to 350 K at constant pressure. Explain, in terms of Kp, the change in the composition of the mixture and in its colour.

   [6 marks]

   (b)

   At 298 K the syringe plunger is pushed in so that the total pressure doubles, and a catalyst is then added. Explain, in terms of Kp and partial pressures, the effects on the position of equilibrium and on Kp.

   [6 marks]

   Total for question 4: 12 marks

End of questions