1. 1
   An industrial chemist melts pure lead(II) bromide, PbBr2, in a crucible and passes an electric current through it using two inert graphite electrodes, producing molten lead at one electrode and bromine gas at the other.

   (a)

   What term describes this decomposition of an ionic compound, when molten, by the passage of an electric current?

   [1 mark]

   • AElectrolysis
   • BDistillation
   • CNeutralisation
   • DSublimation

   (b)

   During the electrolysis, molten lead forms at one electrode and orange bromine vapour forms at the other. At which electrode does the molten lead form, and why?

   [1 mark]

   • AThe anode (positive electrode), because positive lead ions are attracted there and gain electrons
   • BThe cathode (negative electrode), because positive lead ions are attracted there and gain electrons
   • CThe cathode, because negative bromide ions are attracted there and lose electrons
   • DNeither electrode, since lead remains dissolved in the melt

   (c)

   Explain, in terms of the movement of ions and electrons, how bromine gas is produced at the anode during the electrolysis of molten lead(II) bromide.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A laboratory carries out electrolysis of dilute sulfuric acid, using inert platinum electrodes, and observes bubbles of gas forming at both electrodes.

   (a)

   Which gas is produced at the cathode during this electrolysis?

   [1 mark]

   • AChlorine
   • BOxygen
   • CSulfur dioxide
   • DHydrogen

   (b)

   Bubbles of a different gas are also observed at the anode of the same electrolysis of dilute sulfuric acid. Which general rule explains why a non-metal gas, rather than a metal, is produced at the anode?

   [1 mark]

   • AOnly hydrogen can ever be produced at the anode
   • BMetals are always formed at the anode, never at the cathode
   • CNon-metals (other than hydrogen) are formed at the anode during electrolysis
   • DNo gases can ever form at the anode during electrolysis

   (c)

   Construct the ionic half-equation for the reaction occurring at the cathode during the electrolysis of dilute sulfuric acid, and state whether this reaction is oxidation or reduction.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A chlor-alkali plant carries out electrolysis of concentrated aqueous sodium chloride solution using inert electrodes, producing chlorine gas at the anode, hydrogen gas at the cathode, and leaving a solution of sodium hydroxide.

   (a)

   Identify the products formed at each electrode during this electrolysis, and describe the observations that would be made at each electrode.

   [3 marks]

   (b)

   Explain, in terms of the movement of ions and electrons, the transfer of charge that occurs throughout the electrolysis cell during this process, referring to the external circuit, the electrodes and the electrolyte.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An electroplating company wants to coat a steel spoon with a thin, even layer of pure copper to improve its appearance and resistance to corrosion, using an electrolytic cell containing aqueous copper(II) sulfate solution.

   (a)

   Explain fully, including relevant ionic half-equations, how the company should set up the electrolysis cell (identifying which electrode should be the steel spoon and which should be pure copper), describe what happens at each electrode during the process, and explain why using copper electrodes instead of inert electrodes is necessary to achieve the electroplating.

   [6 marks]

   (b)

   The same electroplating company also investigates what happens if inert carbon/graphite electrodes are used instead of copper electrodes during the electrolysis of aqueous copper(II) sulfate solution. Predict and explain fully the products formed at each electrode in this case, including relevant ionic half-equations, and explain how the ions present in aqueous copper(II) sulfate solution (Cu2+, SO4^2-, H+ and OH−, the latter two from water) determine which ions are discharged at each electrode.

   [6 marks]

   Total for question 4: 12 marks

End of questions