Electroplating
Section: 4. Electrochemistry | Syllabus: Cambridge IGCSE Chemistry (0620)
Why Electroplate Objects?
Electroplating uses electrolysis to coat an object with a thin layer of another metal.
- To improve appearance - a shiny, attractive finish (e.g. gold- or silver-plated jewellery)
- To improve resistance to corrosion - protecting a reactive base metal with an unreactive coating (e.g. chromium-plated steel)
How Electroplating Works
| Component | What it is |
|---|---|
| Electrolyte | a solution containing ions of the plating metal, e.g. CuSO₄ to copper-plate |
| Cathode | the object being plated |
| Anode | a bar of the pure plating metal (an active electrode) |
- Cathode: metal ions from solution gain electrons and deposit onto the object as a coating
- Anode: the pure metal bar loses electrons and dissolves, replacing the ions removed from solution
Because metal lost at the anode replaces metal gained at the cathode, the electrolyte's concentration stays constant throughout.
Worked Example: Copper Plating
- Question: an iron spoon is copper-plated using a copper sulfate electrolyte. Describe what happens at each electrode. [4]
- Answer - cathode (the spoon): Cu²⁺ + 2e⁻ → Cu; a pink/brown copper coating forms on the spoon [2]
- Answer - anode (pure copper bar): Cu → Cu²⁺ + 2e⁻; the copper bar gradually gets smaller/thinner [2]
The object to be plated is always the cathode; the pure plating metal forms the anode
Preparing the Object
Before plating, the object's surface must be thoroughly cleaned (grease and dirt removed, any oxide layer removed with acid, then rinsed) so the coating adheres well and evenly.
Common Electroplated Objects
| Object | Plating metal | Reason |
|---|---|---|
| Car bumpers/trim | Chromium | shiny, corrosion-resistant |
| Cheap jewellery | Gold/silver | attractive appearance |
| Food cans | Tin | prevents corrosion, food-safe |
| Cutlery | Silver | attractive, hygienic |
| Circuit boards | Gold/tin | good electrical contact |
Common Mistakes
- ❌ Making the object being plated the anode
- ✓ the object must be the cathode so that metal ions deposit onto it; if reversed, the object would dissolve instead
- ❌ Using an inert electrode as the anode in electroplating
- ✓ electroplating specifically uses an active anode made of the plating metal, so it dissolves to keep replenishing the electrolyte
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