Electrolysis of Aqueous Solutions (Cambridge O Level Chemistry)

• Aqueous solutions will always have water present (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 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
• Concentrated and dilute solutions of the same compound give different products
• For anions, the more concentrated ion will tend to get discharged over a more dilute ion

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 then the halide ion is discharged at the anode, loses electrons and forms a halogen (chlorine, bromine or iodine)
• If no halide ions are present, then OH^- is discharged at the anode, loses electrons and forms oxygen gas
• In both cases the other negative ion remains in solution
• The concentration of the solution also affects which ion is discharged:
  • If a concentrated halide solution is being electrolysed, the halogen forms at the anode
  • If a dilute halide solution is being electrolysed, oxygen is formed
• For example:
  • For a concentrated solution of barium chloride, the Cl^- ions are discharged more readily than the OH^- ions, so chlorine gas is produced at the anode
  • If the solution is dilute however only the OH^- ion is discharged and so oxygen would be formed

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 metal will be produced
• If the metal is above hydrogen in the reactivity series, then hydrogen will be produced and bubbling will be seen at the cathode
• This is because the ions of the more reactive metal will remain in the solution, causing the ions of the least reactive metal to be discharged
• Therefore, at the cathode, hydrogen gas will be produced unless the positive ions from the ionic compound are less reactive than hydrogen, in which case the metal is produced

The reactivity series of metals including hydrogen and carbon

Products formed for Common Aqueous Solutions

Electrolysis of Aqueous Copper Sulfate

• Aqueous copper sulfate contains the following ions:
  • Cu²⁺, SO₄^2-, H⁺ and OH^-

Using graphite electrodes:

Apparatus for the electrolysis of copper(II)sulfate using inert / passive graphite electrodes

Product at the Cathode: 

• Cu²⁺ and H⁺ will both be attracted to the cathode but the less reactive ion will be discharged
  • In this case, copper is less reactive than hydrogen 
  • Copper ions are discharged at the cathode, gain electrons and are reduced to form copper metal
  • The half equation for the reaction at the electrode is:

Cu²⁺   +   2e^-   →   Cu

Product at the Anode: 

• SO₄^2- and OH^- are both attracted to the anode
  • OH- ions lose electrons more readily than SO₄^2-
  • OH- lose electrons and are oxidised to form oxygen gas 
  • The half equation for the reaction at the anode is 

4OH^–    ⟶    O₂   +   2H₂O   +   4e^– 

Using copper electrodes:

Apparatus for the electrolysis of copper(II)sulfate using active copper electrodes

Observations at the anode and cathode

• The cathode increases in mass while the anode decreases
• This occurs as copper atoms are oxidised at the anode and form ions while copper ions are reduced at the cathode, forming copper atoms
• The gain in mass by the negative electrode is the same as the loss in mass by the positive electrode
• Therefore the copper deposited on the negative electrode must be the same copper ions that are lost from the positive electrode
• That implies that the concentration of the Cu²⁺ ions in the solution remains constant