Equilibrium IEdexcel A-Level Chemistry: Topic test

1. 1
   Aqueous chromate(VI) ions are yellow and dichromate(VI) ions are orange. They are in equilibrium in aqueous solution: 2CrO₄²⁻(aq) + 2H⁺(aq) ⇌ Cr₂O₇²⁻(aq) + H₂O(l). A student has an orange solution of potassium dichromate(VI) in a beaker.

   (a)

   A few drops of dilute sodium hydroxide are added to the orange solution. Which observation and explanation is correct?

   [1 mark]

   • AThe solution turns yellow, because H⁺ ions are removed and the equilibrium shifts to the left
   • BThe solution turns yellow, because H⁺ ions are removed and the equilibrium shifts to the right
   • CThe solution stays orange, because the position of equilibrium is unaffected by changes in concentration
   • DThe solution turns a darker orange, because the hydroxide ions increase the concentration of dichromate(VI) ions

   (b)

   The solution is left until the colour stops changing. Which statement describes the system once dynamic equilibrium has been reached?

   [1 mark]

   • AThe forward reaction has stopped and the backward reaction continues
   • BThe concentrations of CrO₄²⁻ and Cr₂O₇²⁻ ions are equal
   • CThe rates of the forward and backward reactions are equal and the concentrations no longer change
   • DBoth reactions have stopped, so the concentrations are constant

   (c)

   Dilute sulfuric acid is added to a yellow solution containing mostly chromate(VI) ions. Use Le Chatelier's principle to explain the colour change seen.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Iron and steam are heated in a sealed reaction vessel until equilibrium is established: 3Fe(s) + 4H₂O(g) ⇌ Fe₃O₄(s) + 4H₂(g).

   (a)

   Which is the correct expression for Kc for this equilibrium?

   [1 mark]

   • A$\frac{[\mathrm{Fe_3O_4}][\mathrm{H_2}]^4}{[\mathrm{Fe}]^3[\mathrm{H_2O}]^4}$
   • B$\frac{[\mathrm{H_2}]^4}{[\mathrm{H_2O}]^4}$
   • C$\frac{[\mathrm{H_2}]}{[\mathrm{H_2O}]}$
   • D$\frac{[\mathrm{H_2O}]^4}{[\mathrm{H_2}]^4}$

   (b)

   The reverse reaction, Fe₃O₄(s) + 4H₂(g) ⇌ 3Fe(s) + 4H₂O(g), is studied at the same temperature. Which is the Kc expression for this equation?

   [1 mark]

   • A$\frac{[\mathrm{Fe}]^3[\mathrm{H_2O}]^4}{[\mathrm{Fe_3O_4}][\mathrm{H_2}]^4}$
   • B$\frac{[\mathrm{H_2O}]}{[\mathrm{H_2}]}$
   • C$\frac{[\mathrm{H_2}]^4}{[\mathrm{H_2O}]^4}$
   • D$\frac{[\mathrm{H_2O}]^4}{[\mathrm{H_2}]^4}$

   (c)

   Explain why the mixture in the sealed vessel is described as a dynamic equilibrium, even though the amount of each substance stays constant.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Phosgene is made by passing carbon monoxide and chlorine over a catalyst in a closed vessel at about 400 K: CO(g) + Cl₂(g) ⇌ COCl₂(g), ΔH = −108 kJ mol⁻¹.

   (a)

   Write the expression for Kc for this reaction. Predict and explain the effect on the position of equilibrium of compressing the mixture at constant temperature.

   [3 marks]

   (b)

   Predict the effect of raising the temperature from 400 K to 600 K on the equilibrium yield of phosgene, and justify your answer. Explain why a manufacturer would not choose a very low temperature.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Ethanol is manufactured by the hydration of ethene: C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g), ΔH = −45 kJ mol⁻¹. For an equilibrium mixture at 65 atm, the percentage of ethene converted to ethanol is 12% at 520 K, 7% at 570 K and 4% at 620 K. The industrial process uses about 570 K and 65 atm with a solid catalyst.

   (a)

   Using Le Chatelier's principle, predict and justify the effects of changing the temperature and of changing the pressure on the equilibrium yield of ethanol. Refer to the data in your answer.

   [6 marks]

   (b)

   Evaluate the conditions of about 570 K and 65 atm used in the industrial process.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   At the high temperatures inside a petrol engine, nitrogen and oxygen react to form nitrogen monoxide in a closed cylinder: N₂(g) + O₂(g) ⇌ 2NO(g), ΔH = +180 kJ mol⁻¹.

   (a)

   The temperature of the equilibrium mixture is raised. What happens to the position of equilibrium?

   [1 mark]

   • AIt moves left, because the forward reaction is exothermic
   • BIt moves right, because the forward reaction is endothermic
   • CIt does not move, because there are equal numbers of moles of gas on both sides
   • DIt moves right, because the number of molecules of gas increases

   (b)

   The pressure on the equilibrium mixture is doubled by reducing the volume at constant temperature. What happens to the position of equilibrium?

   [1 mark]

   • AIt moves right, to the side with more moles of gas
   • BIt moves left, to the side with fewer moles of gas
   • CIt moves right, because the forward reaction is endothermic
   • DIt does not change, because there are equal numbers of moles of gas on both sides

   (c)

   Extra nitrogen is injected into the cylinder at constant temperature. Use the rates of the forward and backward reactions to explain how a new equilibrium is established.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   Ammonium chloride decomposes when heated in a sealed tube: NH₄Cl(s) ⇌ NH₃(g) + HCl(g). A student also considers the gas-phase equilibrium PCl₅(g) ⇌ PCl₃(g) + Cl₂(g).

   (a)

   Which is the correct expression for Kc for the equilibrium involving phosphorus(V) chloride?

   [1 mark]

   • A$\frac{[\mathrm{PCl_3}][\mathrm{Cl_2}]}{[\mathrm{PCl_5}]}$
   • B$\frac{[\mathrm{PCl_5}]}{[\mathrm{PCl_3}][\mathrm{Cl_2}]}$
   • C$\frac{[\mathrm{PCl_3}] + [\mathrm{Cl_2}]}{[\mathrm{PCl_5}]}$
   • D$[\mathrm{PCl_3}][\mathrm{Cl_2}]$

   (b)

   Which is the correct expression for Kc for the equilibrium involving ammonium chloride?

   [1 mark]

   • A$\frac{[\mathrm{NH_3}][\mathrm{HCl}]}{[\mathrm{NH_4Cl}]}$
   • B$\frac{[\mathrm{NH_4Cl}]}{[\mathrm{NH_3}][\mathrm{HCl}]}$
   • C$[\mathrm{NH_3}][\mathrm{HCl}]$
   • D$[\mathrm{NH_3}]$

   (c)

   Explain why the ammonium chloride equilibrium is described as heterogeneous, and why ammonium chloride does not appear in the Kc expression.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   Iodine is only slightly soluble in water but dissolves readily in aqueous potassium iodide, forming a brown solution containing triiodide ions: I₂(aq) + I⁻(aq) ⇌ I₃⁻(aq).

   (a)

   Write the expression for Kc for this equilibrium. A small volume of concentrated potassium iodide solution is added to the brown mixture at equilibrium. Predict, with a reason, the effect on the position of equilibrium.

   [3 marks]

   (b)

   Silver nitrate solution is now added to the equilibrium mixture. Silver ions react with iodide ions: Ag⁺(aq) + I⁻(aq) → AgI(s). Explain the effect on the position of equilibrium and on the concentration of triiodide ions, referring to the rates of the forward and backward reactions.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   Chlorine is made from hydrogen chloride by the Deacon process: 4HCl(g) + O₂(g) ⇌ 2Cl₂(g) + 2H₂O(g), ΔH = −114 kJ mol⁻¹. Data for equilibrium mixtures at 1 atm show that the percentage conversion of HCl to Cl₂ is 90% at 600 K, 80% at 700 K and 65% at 800 K. The industrial process operates at about 700 K and close to atmospheric pressure, using a copper(II) chloride catalyst.

   (a)

   Deduce the expression for Kc for the Deacon process. Predict and justify the effects of increasing the temperature and of increasing the pressure on the equilibrium yield of chlorine, using the data where appropriate.

   [6 marks]

   (b)

   Evaluate the use of about 700 K and close to atmospheric pressure for the industrial process.

   [6 marks]

   Total for question 8: 12 marks

End of questions