Topic 7 of 7
Copper purification and aluminium anodising
Connect electrode reactions to their industrial purpose.
A-Level 9476 (2026-2027)
Use electrode reactions to explain the industrial purpose
Copper purification transfers metal; anodising grows an oxide layer.
| Part | What happens | Why it purifies |
|---|---|---|
| Positive anode: impure copper | Cu(s) → Cu2+(aq) + 2e-. | Copper leaves the impure material as dissolved ions. |
| Negative cathode: pure copper sheet | Cu2+(aq) + 2e- → Cu(s). | Copper deposits on the pure sheet. |
| Electrolyte: acidified copper(II) sulfate | Copper ions are replaced at the anode as they are removed at the cathode. | The copper-ion concentration is approximately maintained during normal operation. |
| Impurities | Less readily oxidised metals such as Ag and Au collect as anode sludge; more readily oxidised metals can dissolve. | Under the chosen conditions, dissolved impurities such as Zn2+ are not preferentially deposited with copper. |
The net intended process is transfer of copper from the impure anode to the pure cathode. The power supply controls this electrolytic process. Swapping the electrodes would defeat the purpose; simply filtering the original copper cannot remove metallic impurities within it.
In anodising aluminium, the aluminium object is connected as the positive anode in a suitable acidic electrolyte, such as dilute sulfuric acid. Oxidation builds a thicker protective aluminium oxide layer on its surface. An idealised oxide-forming anode equation is 2Al(s) + 3H2O(l) → Al2O3(s) + 6H+(aq) + 6e-.
At the cathode, hydrogen ions can be reduced: 6H+(aq) + 6e- → 3H2(g). The oxide layer adheres to and protects the object; anodising is not deposition of aluminium metal onto a cathode. Detailed plant design and operating parameters are outside this syllabus.