4.2.1Electrochemistry
Electroplating — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 5 parts Lesson 20 of 20 Unit quiz
Section 1 of 5The Electroplating Cell
Copper plating an iron key is a typical example, using copper(II) sulfate solution.
Cathode
- The object to be plated is made the cathode, which is the negative electrode connected to the negative terminal of the supply.
Anode
- The anode is a piece of the pure plating metal and is connected to the positive terminal.
Electrolyte
- The electrolyte is an aqueous solution of a soluble salt containing ions of the plating metal.
Direct current
- A direct current is passed through the circuit so that metal ions are attracted to the cathode.
| Part | Material for copper plating |
|---|---|
| Cathode (negative) | Object to be plated |
| Anode (positive) | Pure copper |
| Electrolyte | Aqueous copper(II) sulfate |
Section 2 of 5What Happens at the Electrodes
1
At the anode, copper atoms lose electrons and dissolve into the solution as copper(II) ions: Cu(s) → Cu²⁺(aq) + 2e⁻.
2
This loss of electrons is oxidation, and it makes the anode slowly get smaller.
3
Copper(II) ions in the solution move to the cathode, which is the negative electrode.
4
At the cathode the ions gain electrons and form copper atoms: Cu²⁺(aq) + 2e⁻ → Cu(s).
5
This gain of electrons is reduction, and the copper atoms build up as a thin layer on the object.
Section 3 of 5Why Objects Are Plated
Why Objects Are Plated
Corrosion resistance
- A coating such as chromium or nickel keeps air and water away from the metal underneath, so it does not rust or corrode.
Appearance
- Silver plating on cutlery or jewellery gives a shiny, attractive surface that is cheaper than making the whole object from silver.
Zinc coating
- Steel can be coated with zinc, which is called galvanising, to protect it from rusting.
Cost saving
- A thin layer of an expensive metal on a cheap metal gives the benefits of the surface metal at lower cost.
Section 4 of 5Controlling the Result
Electrolyte stays constant
- Copper ions removed at the cathode are replaced from the anode, so the blue colour of the solution stays the same.
Mass changes
- The cathode gains mass and the anode loses mass, by about the same amount.
Clean surface
- The object must first be cleaned of grease so that the metal layer sticks firmly.
Different plating metals
- Silver plating uses a silver anode and silver nitrate solution, and tin plating uses a tin anode with a tin salt solution.
Section 5 of 5Exam Checklist & Tips
4.2.1 – Key Points for the Exam
- Electroplating Coating an object with a thin layer of metal using electrolysis.
- Cathode The negative electrode where reduction takes place.
- Anode The positive electrode where oxidation takes place.
- Electrolyte A liquid or solution containing ions that conducts electricity and is broken down.
- Galvanising Coating iron or steel with zinc to prevent rusting.
Common Mistakes
- Making the object to be plated the anode instead of the cathode.
- Using an unrelated metal salt as the electrolyte rather than a salt of the plating metal.
- Writing the electrode equations the wrong way round or leaving out the electrons.
- Saying the electrolyte becomes paler; in copper plating the concentration stays almost the same.
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