Electrolysis Of Aqueous Solutions (AQA GCSE Combined Science: Synergy: Physical Sciences): Revision Note

Electrolysis of an aqueous solution using inert electrodes

Ions present in aqueous solutions

• Aqueous solutions will always contain water molecules (H₂O)

• In the electrolysis of aqueous solutions, the water molecules dissociate producing H⁺ and OH^– ions:

H₂O ⇌ H⁺ + OH^–

• These ions are also involved in the electrolysis process and their chemistry must be considered

• We now have an electrolyte that contains ions from the compound plus ions from the water

• Which ions get discharged and at which electrode depends on the relative reactivity of the elements involved

Electrode reactions

What is produced at the positive electrode (anode)?

• Negatively charged OH^– ions and non-metal ions are attracted to the positive electrode

• If halide ions (Cl^-, Br^-, I^-) and OH^- are present:

  • The halide ion is discharged at the anode

  • The halide ion loses electrons and forms a halogen (chlorine, bromine or iodine)

• If no halide ions are present:

  • The OH^- ion is discharged at the anode

  • The OH^- loses electrons and forms oxygen

• In both cases, the other negative ion remains in solution

What is produced at the negative electrode (cathode)?

• Positively charged H⁺ and metal ions are attracted to the negative electrode but only one will gain electrons

• Either hydrogen gas or the metal will be produced

• If the metal is above hydrogen in the reactivity series:

  • Hydrogen is produced at the cathode, seen as bubbling at the cathode

  • This is because the more reactive ions will remain in solution, causing the least reactive ion to be discharged

• If the metal is below hydrogen in the reactivity series:

  • The metal deposits on the cathode

The reactivity series

Determining what gas is produced

• The gas produced can be tested to determine its identity

• If the gas produced at the cathode burns with a ‘pop’ with a lit splint:

  • The gas is hydrogen

• If the gas produced at the anode relights a glowing splint dipped into the gas

  • The gas is oxygen

• If the gas produced at the anode turns damp blue litmus paper red and is then bleached white

  • The gas is chlorine

• The halogen gases all produce their own colours (bromine is red-brown, chlorine is yellow-green)

Half equations for electrolysis of aqueous solutions

Higher Tier Only

• In aqueous electrolysis, half equations can be written for the reactions at each electrode

• The half equation depends on which ion is discharged

• At the cathode (reduction — gain of electrons):

• If the metal is below hydrogen in the reactivity series, the metal ion is discharged:

Cu²⁺ + 2e^- → Cu

• If the metal is above hydrogen in the reactivity series, hydrogen ions are discharged instead:

2H⁺ + 2e^- → H₂

• At the anode (oxidation — loss of electrons):

• If halide ions are present, the halide is discharged to form a halogen:

2Cl^- → Cl₂ + 2e^-

• If no halide ions are present, hydroxide ions are discharged to form oxygen and water:

4OH^- → 2H₂O + O₂ + 4e^-