Electrolysis of aqueous solutions

• 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
• Electrolysis of Aqueous Solutions

  1. 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 H* (a) + OH()

  2. 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). 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.

  3. At the anode, if OH and halide ions (Cl, Br, 1-) 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.
• Copper(II) sulfate (Cu80) solution


  • Contains four different ions: Cu2+, SO,, H and OH
  • Copper metal is less reactive than hydrogen. So at the cathode, copper metal is produced and coats the electrode.
  • There aren't any halide ions present. So at the anode oxygen and water are produced. The oxygen can be seen as bubbles.
• Sodium chloride (NaCl) solution

  • Contains four different ions: Na+, CH, OH and H
  • Sodium metal is more reactive than hydrogen. So at the cathode, hydrogen gas is produced.
  • Chloride ions are present in the solution. So at the anode chlorine gas is produced.
• You can set up an electrolysis experiment in the lab like the set-up on page 236
• • Chlorine bleaches damp litmus paper, turning it white
  • Hydrogen makes a "squeaky pop" with a lighted splint
  • Oxygen will relight a glowing splint
• 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
• Half equations for electrolysis of aqueous sodium chloride

  1. Negative Electrode: 2H + 2e → H₂

  2. Positive Electrode: 2Cl- → Cl₂ + 2e
• When a halide isn't present in the aqueous solution, the half equation for the anode is: 4OH- → O₂ + 2H₂O + 4e