1. 1
   A chemical plant electrolyses concentrated sodium chloride solution (brine) using inert electrodes, producing three useful products: a gas at the cathode, a gas at the anode, and a solution left behind.

   (a)

   What gas is produced at the cathode in this process?

   [1 mark]

   • AChlorine
   • BOxygen
   • CHydrogen
   • DCarbon dioxide

   (b)

   What gas is produced at the anode during the electrolysis of concentrated sodium chloride solution?

   [1 mark]

   • AHydrogen
   • BChlorine
   • COxygen
   • DSodium vapour

   (c)

   State the name of the useful solution left behind after electrolysis of concentrated sodium chloride solution, and give one industrial use for this solution.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student tests the gas collected at the cathode during electrolysis of brine using a lighted splint, and hears a squeaky pop. This confirms the gas is hydrogen.

   (a)

   Why is hydrogen produced at the cathode instead of sodium?

   [1 mark]

   • ABecause sodium is more reactive than hydrogen, so hydrogen ions are discharged in preference
   • BBecause there are no sodium ions present in the solution
   • CBecause sodium is less reactive than hydrogen
   • DBecause hydrogen ions are negatively charged and are attracted to the cathode

   (b)

   The student then tests the gas collected at the anode using damp blue litmus paper, which turns red and is then bleached white. Which gas has been confirmed by this test?

   [1 mark]

   • AHydrogen
   • BOxygen
   • CAmmonia
   • DChlorine

   (c)

   Write the half-equations for the reactions occurring at both the cathode and the anode during electrolysis of concentrated sodium chloride solution.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A chemical company operates a large-scale plant that electrolyses concentrated sodium chloride solution to produce hydrogen, chlorine and sodium hydroxide solution.

   (a)

   Explain why hydrogen, rather than sodium, is produced at the cathode, and why chlorine, rather than oxygen, is produced at the anode.

   [3 marks]

   (b)

   State one industrial use each for hydrogen gas, chlorine gas and sodium hydroxide solution produced by this process, and explain why this makes the electrolysis of brine an economically important industrial process.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemical manufacturer is expanding its brine electrolysis plant to increase production of chlorine, hydrogen and sodium hydroxide.

   (a)

   Explain in full detail, using ideas about reactivity and ion concentration, why each of these three products forms in this process, including relevant half-equations.

   [6 marks]

   (b)

   Evaluate the industrial importance of this electrolysis process, considering the range of products formed and the economic and safety factors involved in handling chlorine gas on a large scale.

   [6 marks]

   Total for question 4: 12 marks

End of questions