1. 1
   Hydrogen gas and purple iodine vapour are sealed in a flask at 450 °C and allowed to reach equilibrium: H₂(g) + I₂(g) ⇌ 2HI(g). At this temperature Kc = 50.0.

   (a)

   What is the value of Kc for 2HI(g) ⇌ H₂(g) + I₂(g) at 450 °C?

   [1 mark]

   • A0.141
   • B0.0200
   • C25.0
   • D−50.0

   (b)

   The volume of the flask is halved at constant temperature. Which statement is correct once the system has settled?

   [1 mark]

   • AThe position of equilibrium is unchanged, the colour is darker and Kc is unchanged.
   • BThe position of equilibrium shifts to the right and the colour becomes paler.
   • CThe position of equilibrium shifts to the left and Kc decreases.
   • DThe position of equilibrium and the colour are both unchanged.

   (c)

   Describe two characteristics of this system once it has reached equilibrium.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Phosphorus pentachloride decomposes when heated: PCl₅(g) ⇌ PCl₃(g) + Cl₂(g). At 250 °C, Kc = 0.042. Two mixtures are prepared in sealed vessels at 250 °C. Mixture 1: [PCl₅] = 0.200, [PCl₃] = 0.050, [Cl₂] = 0.100 mol dm⁻³. Mixture 2: [PCl₅] = 0.100, [PCl₃] = 0.080, [Cl₂] = 0.080 mol dm⁻³.

   (a)

   What is the value of the reaction quotient, Q, for mixture 1?

   [1 mark]

   • A0.0050
   • B0.75
   • C40
   • D0.025

   (b)

   Which statement describes what happens as mixture 1 moves towards equilibrium?

   [1 mark]

   • AA net reverse reaction occurs, so [PCl₅] increases.
   • BNo net change occurs because Q and Kc are both less than 1.
   • CA net forward reaction occurs, so [Cl₂] increases and [PCl₅] decreases.
   • DA net forward reaction occurs and Kc increases until it equals Q.

   (c)

   Determine the direction in which mixture 2 will react to reach equilibrium.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   The water–gas shift reaction is used to make hydrogen: CO(g) + H₂O(g) ⇌ CO₂(g) + H₂(g). Experiment 1: 1.00 mol of CO and 1.00 mol of H₂O are sealed in a 2.00 dm³ vessel at temperature T₁, at which Kc = 4.00. Experiment 2: 0.600 mol of CO and 0.400 mol of H₂O are sealed in a 1.00 dm³ vessel at a higher temperature, T₂; at equilibrium the vessel contains 0.250 mol of CO₂.

   (a)

   Calculate the equilibrium concentrations of all four species in experiment 1.

   [3 marks]

   (b)

   Determine Kc at T₂ and deduce whether the forward reaction is exothermic or endothermic.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   The Deacon process converts waste hydrogen chloride into chlorine: 4HCl(g) + O₂(g) ⇌ 2Cl₂(g) + 2H₂O(g), ΔH° = −114 kJ mol⁻¹. The standard Gibbs energy change, ΔG°, is −76.0 kJ mol⁻¹ at 298 K and −24.7 kJ mol⁻¹ at 700 K. The industrial reactor runs at about 700 K over a catalyst. Use R = 8.31 J K⁻¹ mol⁻¹.

   (a)

   Calculate the equilibrium constant at 298 K and at 700 K, and discuss what the values show about the reaction and why the process is run at 700 K.

   [6 marks]

   (b)

   The system is at equilibrium at 700 K. Use the reaction quotient, Q, to explain how the system responds to each of the following, and state whether K changes: (1) water vapour is removed; (2) the volume is halved at constant temperature; (3) the temperature is raised.

   [6 marks]

   Total for question 4: 12 marks

End of questions