Lesson 7 of 7
Copper purification and aluminium anodising
Connect electrode reactions to their industrial purpose.
A-Level 9476 (2026-2027)
I can explain industrial electrolysis through electrode reactions.Syllabus
Syllabus 9476, 12(o). Explain industrial electrolysis through electrode reactions.
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.
Check your understanding
A student connects an aluminium object to the negative terminal to anodise it. What is wrong?
Show the answer
Anodising needs oxidation, so the object must be the positive anode.
The negative electrode is the cathode, where reduction happens. Thickening the oxide layer needs aluminium to be oxidised, which happens at the anode.