Electrolysis of Copper SulfateOxford AQA IGCSE Chemistry: Revision notes
Section 1
What ions are present in copper sulfate solution?
Copper sulfate solution (CuSO₄) contains Cu²⁺ and SO₄²⁻ ions from the dissolved salt, plus H⁺ and OH⁻ ions from the water itself. During electrolysis, these ions move to opposite electrodes: Cu²⁺ and H⁺ move to the cathode, and SO₄²⁻ and OH⁻ move to the anode.
Section 2
What is the required practical investigating?
The required practical investigates the products formed at the anode and cathode when copper sulfate solution is electrolysed using inert (unreactive) electrodes, typically carbon or platinum. A direct current power supply is connected to two electrodes dipped in copper sulfate solution, and the changes at each electrode are observed and tested.
Examiners may ask you to describe the practical method — mention using inert electrodes, a DC power supply, and observing colour/mass changes at each electrode.
Section 3
What happens at the cathode?
At the cathode, Cu²⁺ ions are discharged in preference to H⁺ ions, because copper is less reactive than hydrogen. The copper ions gain electrons (reduction) and a layer of pinkish-brown copper metal deposits on the electrode.
Half-equation: Cu²⁺ + 2e⁻ → Cu
As the reaction proceeds, the blue colour of the solution fades slightly as Cu²⁺ ions are removed from solution.
Don't say hydrogen forms at the cathode here — copper is less reactive than hydrogen, so copper is discharged instead, unlike in dilute sodium chloride solution.
Section 4
What happens at the anode?
At the anode, OH⁻ ions are discharged in preference to SO₄²⁻ ions, because sulfate ions are not discharged under these conditions (they remain in solution). The hydroxide ions lose electrons (oxidation), producing oxygen gas, which can be seen bubbling at the electrode, and water.
Overall half-equation: 4OH⁻ → O₂ + 2H₂O + 4e⁻
Must Know
- Copper sulfate solution contains Cu²⁺, SO₄²⁻, H⁺ and OH⁻ ions
- The required practical uses inert electrodes (e.g. carbon) and a DC power supply
- At the cathode, copper is deposited: Cu²⁺ + 2e⁻ → Cu (copper is less reactive than hydrogen)
- At the anode, oxygen gas is produced: 4OH⁻ → O₂ + 2H₂O + 4e⁻ (sulfate ions are not discharged)
- The blue colour of the solution fades slightly as Cu²⁺ ions are removed
- This links to the general 'competing ions' rules: reactivity decides the cathode product, concentration/availability decides the anode product
That's the notes covered.
Carry on to the next subtopic.