1. 1
   A student electrolyses a concentrated solution of sodium chloride using inert electrodes. The solution contains both sodium ions and hydrogen ions (from water), as well as chloride ions and hydroxide ions (from water). At the cathode, hydrogen gas is produced instead of sodium metal.

   (a)

   Why does this happen?

   [1 mark]

   • ABecause sodium ions are not present in the solution
   • BBecause sodium is more reactive than hydrogen, so hydrogen ions are discharged in preference
   • CBecause sodium is less reactive than hydrogen
   • DBecause the cathode only attracts negative ions

   (b)

   At the anode in this experiment, chlorine gas is produced rather than oxygen, because the sodium chloride solution is concentrated. What does this show about the factor that determines the product at the anode when a mixture of ions is present?

   [1 mark]

   • AThe product at the anode depends on the relative concentrations of the ions present
   • BThe product depends only on the reactivity of the metal ions present
   • CThe anode always produces oxygen regardless of the solution used
   • DThe product depends on the colour of the solution

   (c)

   Summarise the general rule that determines which ion is discharged at the cathode, and the general rule that determines which ion is discharged at the anode, when a mixture of ions is present in an electrolyte.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A technician electrolyses a solution of potassium bromide using inert electrodes. Bromine (seen as an orange-brown colour in the solution) is produced at the anode, and hydrogen gas is produced at the cathode.

   (a)

   Why is potassium metal not produced at the cathode?

   [1 mark]

   • ABecause potassium ions are not present in the solution
   • BBecause potassium is more reactive than hydrogen, so hydrogen ions are discharged instead
   • CBecause potassium ions are negatively charged and move to the anode
   • DBecause the electrodes used are made of potassium

   (b)

   Why is bromine, rather than oxygen, produced at the anode in this experiment?

   [1 mark]

   • ABecause oxygen cannot be produced by electrolysis of any solution
   • BBecause bromide ions are present in a much higher concentration than hydroxide ions in this solution
   • CBecause bromide ions are less reactive than hydroxide ions
   • DBecause the anode attracts positive ions only

   (c)

   Write half-equations for the reactions occurring at both electrodes in this electrolysis of potassium bromide solution.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A chemical company electrolyses a concentrated solution containing a mixture of copper(II) sulfate and sodium chloride, using inert electrodes.

   (a)

   Explain which product would form at the cathode, referring to the reactivity of the metals involved.

   [3 marks]

   (b)

   Explain which product would form at the anode of this mixed solution, referring to the concentrations of the ions present, and write the half-equation for the reaction occurring there.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A laboratory technician electrolyses a concentrated aqueous solution containing a mixture of magnesium chloride and potassium iodide, using inert electrodes.

   (a)

   Explain, with reference to reactivity and ion concentration, what products would form at each electrode, including relevant half-equations.

   [6 marks]

   (b)

   Discuss how the results of this electrolysis experiment would change if the technician instead used a very dilute solution of the same mixture of magnesium chloride and potassium iodide, and explain the underlying reason for the change.

   [6 marks]

   Total for question 4: 12 marks

End of questions