Section 1 of 5The Electroplating Set-Up
Electroplating is electrolysis in which the object is coated with a thin layer of a different metal.
Anode
- The anode is made from the pure metal that will coat the object, and it dissolves to replace ions removed from solution.
Cathode
- The cathode is the object to be plated, which receives a deposited layer of the plating metal.
Electrolyte
- The electrolyte is an aqueous solution of a soluble salt containing ions of the plating metal.
Power supply
- A direct current supply drives the reaction, with the anode connected to the positive terminal.
Section 2 of 5Plating Iron With Tin
Tin plating of an iron strip shows how both electrodes change during the process.
1
At the tin anode, tin atoms lose electrons and are oxidised to form tin ions: Sn(s) → Sn²⁺(aq) + 2e⁻.
2
At the iron cathode, tin ions gain electrons and are reduced to form tin atoms: Sn²⁺(aq) + 2e⁻ → Sn(s).
3
The tin atoms deposit as a thin, even layer on the iron strip, coating it.
4
The concentration of tin ions in the electrolyte stays about the same, because ions are removed and replaced at the same rate.
Section 3 of 5Why Electroplate?
Why Electroplate?
Corrosion resistance
- Chromium and nickel plating protect the underlying metal from corrosion and damage.
Appearance
- Silver plating of cutlery and jewellery improves their look at far lower cost than solid silver.
Cost
- Only a thin layer of an expensive metal is needed, which reduces material costs.
Surface durability
- A plated layer can make the surface of an object harder wearing than the metal beneath it.
Section 4 of 5Worked Example
Question Describe how the masses of the anode and the cathode change during the electroplating of an iron strip with tin.
- The tin anode is oxidised and dissolves, so its mass decreases.
- The iron cathode receives tin atoms by reduction, so its mass increases.
- The concentration of tin ions in the electrolyte stays the same, because the same amount of tin is removed and replaced.
Answer The tin anode loses mass while the iron cathode gains mass, and the electrolyte concentration remains constant.
Section 5 of 5Exam Checklist & Tips
3.4.5 – Key Points for the Exam
- Electroplating Coating an object with a thin layer of a metal using electrolysis.
- Corrosion Gradual destruction of a metal by reaction with substances in its environment.
- Electrolyte An ionic solution or molten compound that conducts electricity during electrolysis.
- Plating metal The metal that forms the coating on the object being electroplated.
Common Mistakes
- Placing the object to be plated at the anode; it is always the cathode.
- Choosing an electrolyte that does not contain ions of the plating metal.
- Saying the tin anode gains mass; it loses mass as it dissolves.
- Forgetting that the coating forms by reduction of metal ions at the cathode.