The principles of electrolysis

Electrolysis Revision Questions And Answers (GCE O level Chemistry)

Question 1 โ€“ Electrolysis of aqueous sodium chloride

Chlorine, hydrogen and sodium hydroxide are made by the electrolysis of concentrated aqueous sodium chloride.

  1. List the ions in aqueous sodium chloride.
  2. Explain why hydrogen, not sodium is discharged at the cathode.
  3. Write down the anode and cathode reactions and comment on the redox nature of each reaction.
  4. Suggest why, as the electrolysis proceeds, the concentration of sodium hydroxide in the electrolyte increases.

Solution 1

  1. Na+, Clโ€“, H+ and OHโ€“.

    Na+, Clโ€“ are from the sodium chloride. H+ and OHโ€“ are from water since the sodium chloride is in solution.

  2. H+ is lower than Na+ in the electrochemical series (reactive series) so it is preferentially discharged.

    Ions of less reactive elements, lower in reactivity series, are preferentially discharged during electrolysis.

  3. Anode reaction: 2Clโ€“(aq) โ€“ 2e โ†’ Cl2(g)

    Oxidation is the loss of electrons by a species, so the anode reaction is an oxidation reaction because it involves loss of electrons.

    Both OHโ€“ and Clโ€“ move to the anode during electrolysis. However since the solution is concentrated, there is a higher concentration of Clโ€“ ions than of OHโ€“. So the ions of higher concentration, Clโ€“, are discharged first. Each Clโ€“ ion lose 1 electron to form a Cl atom. However chlorine is a diatomic gas which means two Clโ€“ ions should lose two electrons to form a Cl2 molecule.

    Cathode reaction: 2H+(aq) + 2e โ†’ H2(g)

    Reduction is the gain of electrons by a species, so the cathode reaction is a reduction reaction because it involves gain of electrons.

  4. As the electrolysis proceeds, H+ ions are discharged at the cathode and Clโ€“ ions are discharged at the anode, leaving Na+ and OHโ€“ in solution to combine and form sodium hydroxide. So the concentration of sodium hydroxide increases.