Electrolysis of Aqueous Solutions

• When you electrolyse an aqueous solution, you also have to factor in the ions in the water
• It may be easier to discharge ions from water than the solute
• In aqueous solutions, as well as the ions from the ionic compound, there will be hydrogen ions (H+) and hydroxide ions (OH-) from the water: H2O(l) ⇌ H+(aq) + OH-(aq)
• At the cathode, if H+ ions and metal ions are present, hydrogen gas will be produced if the metal ions form an elemental metal that is more reactive than hydrogen (e.g. sodium ions)
• At the cathode, if the metal ions form an elemental metal that is less reactive than hydrogen (e.g. copper ions), a solid layer of the pure metal will be produced instead
• At the anode, if OH- and halide ions (Cl-, Br-, I-) are present, molecules of chlorine, bromine or iodine will be formed. If no halide ions are present, then the OH- ions are discharged and oxygen will be formed
• A solution of copper(II) sulfate (CuSO4) contains 4 different ions: Cu2+, SO4^2-, H+ and OH-

  • Copper metal is less reactive than hydrogen. So at the cathode, copper metal is produced and coats the electrode

  Cu2+ + 2e- -> Cu

  • There aren't any halide ions present. So at the anode oxygen and water are produced. The oxygen can be seen as bubbles

  4OH- -> O2 + 2H2O + 4e-

  
• A solution of sodium chloride (NaCl) contains four different ions: Na+, Cl-, OH- and H+

  • Sodium metal is more reactive than hydrogen. So at the cathode, hydrogen gas is produced

  2H+ + 2e- -> H2

  • Chloride ions are present in the solution. So at the anode chlorine gas is produced

  2Cl- -> Cl2 + 2e-

  
• If you're drawing the apparatus for an electrolysis experiment, remember to include a d.c. power supply, wires and labels for the anode and cathode. The anode is the electrode on the same side as the longer line of the d.c. power supply symbol

  
• Half equations show the reactions at the electrodes. The important thing to remember when you're combining half equations is that the number of electrons needs to be the same for each half equation
• For the electrolysis of aqueous sodium chloride the half equations are:

  • Negative electrode: 2H+ + 2e- -> H2
  • Positive electrode: 2Cl- -> Cl2 + 2e-
  • These combine to form the ionic equation: 2H+ + 2Cl- -> H2 + Cl2

  The electrons on each side of the half equations balance, so they can be cancelled out in the full ionic equation
• When a halide isn't present in the aqueous solution, the half equation for the anode is:

  4OH- -> O2 + 2H2O + 4e- or 4OH- - 4e- -> O2 + 2H2O