Electroplating: growing a metal coating
Key idea: Electroplating deposits metal on the object at the cathode as metal ions gain electrons. A soluble anode of the plating metal replaces those ions as its atoms lose electrons.
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Guided lessons · coming soon
Guided lesson
Guided steps
- Predict which terminal makes copper deposit on the key.
- Assign object, plating metal and electrolyte by the electron-transfer roles.
- Follow the nickel-spoon setup before changing metal and ion charge.
- Calculate the final mass gained after some deposited nickel dissolves.
- Explain the reversal with the two nickel half-equations.
- Choose the reduction and oxidation changes for silver plating.
- Write both equations and explain how silver ions are replaced.
- Compare copper and platinum anodes while keeping the cathode the same.
- Connect a thin coating to appearance, corrosion resistance and cost.
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ContinueCheck your understanding 1 Which connection grows the coating?
A brass key is to be coated with copper. Which power-supply terminal should connect to the key?
Show answer
Connect the key to the negative terminal. Cu²⁺(aq) + 2e⁻ → Cu(s) builds the coating there.
Shiny chrome taps, silver-plated cutlery and nickel-coated spoons usually have a cheaper metal underneath. A thin layer of a more attractive or more corrosion-resistant metal has been grown on the surface by electrolysis. This is electroplating. It works because metal ions are reduced to metal atoms at the cathode, so the object you want to coat must be the cathode.
How electroplating works
Electroplating is the use of electrolysis to coat an object with a thin layer of metal. A typical setup has three parts:
| Part | What to use | Why |
|---|---|---|
| Cathode | the object to be plated | Metal ions gain electrons here and form the coating. |
| Anode | a piece of the plating metal | It dissolves and replaces the metal ions used up. |
| Electrolyte | a solution containing ions of the plating metal | It supplies the ions that form the coating. |
When the anode is made of the plating metal M, which forms Mⁿ⁺ ions, the two half-equations are the same change running in opposite directions:
- at the cathode (the object), metal ions are reduced: Mⁿ⁺(aq) + n e⁻ → M(s);
- at the anode, metal atoms are oxidised: M(s) → Mⁿ⁺(aq) + n e⁻.
The metal moves from the anode, through the solution as ions, onto the object. The anode gets thinner, the object gains a coating, and the concentration of metal ions in the electrolyte stays about the same.
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Worked example 1
Choose electrodes for nickel plating
Problem
Show full solution
Assign the object
Method
Connect the steel spoon as the cathode.Reason
Metal ions gain electrons and deposit at the cathode.Working
Answer
Cathode: steel spoon.Assign the plating metal
Method
Use a nickel strip as the anode.Reason
The nickel anode dissolves and replaces the nickel ions removed from solution.Working
Answer
Anode: nickel strip.State the coating mechanism
Method
Reduce Ni²⁺ onto the spoon.Reason
The deposited metal is the required nickel coating.Working
Answer
Ni²⁺(aq) + 2e⁻ → Ni(s) on the spoon.
Quick check
Check your understanding 2 Fix the wrong setup
An object first gains 0.24 g of nickel during plating. The power-supply connections are then reversed, and 0.06 g of nickel dissolves from its coating. A learner predicts a total mass gain of 0.30 g because current flowed during both stages. Calculate the final mass change, using a positive value for a gain.
Show answer
The final mass gain is 0.24-0.06 = 0.18 g. Initially the object is the cathode, where nickel ions gain electrons and deposit. Reversing the connections makes it the anode: nickel in the coating loses electrons and dissolves as ions. Current flowing does not always mean metal is added; the object’s electrode role decides whether deposition or dissolution occurs.
Check your understanding 3 Explain the result
Explain why reversing the connections removes nickel from the coating rather than adding more. Use both electrode roles and half-equations.
Show answer
Initially the object is the cathode: Ni²⁺(aq) + 2e⁻ → Ni(s). Reversing makes it the anode: Ni(s) → Ni²⁺(aq) + 2e⁻. Hence the final gain is 0.24-0.06 = 0.18 g, not 0.24 + 0.06.
Guided practice 2
Half-equations for silver plating
Problem
Write both equations on paper, then compare the choices
Hints
Hint 1
cathode reduction
What is the charge on a silver ion? Check that the total charge balances in each choice.Hint 2
anode oxidation
At the silver anode, the same change runs in reverse.Show solution step by step
Deposit silver on the key
Method
Reduce silver ions at the cathode.Reason
The + 1 ion needs to gain one electron.Working
Answer
Ag⁺(aq) + e⁻ → Ag(s)Replace the silver ions
Method
Oxidise silver at the anode.Reason
The silver anode supplies replacement ions.Working
Answer
Ag(s) → Ag⁺(aq) + e⁻
Quick check
Check your understanding 4 Explain silver transfer to the key
Write both silver-plating half-equations with states, name the electrode where each occurs and explain why silver-ion concentration stays about constant.
Show answer
At the key, the negative cathode:
At the silver strip, the positive anode:
Each transfers one electron per silver atom. The anode replaces the silver ions deposited on the key, so their concentration stays about constant. Check electrode identity, electron side, charge balance and states in both equations.
Check your understanding 5 A copper anode or a platinum anode?
Two copper-plating cells contain aqueous copper(II) sulfate and have identical objects as cathodes. Both initially deposit copper. Cell A has a copper anode; cell B has an inert platinum anode. Explain whether each anode replaces the copper ions removed from solution, and why the object must be the cathode in both cells.
Show answer
The copper anode in cell A dissolves, Cu(s) → Cu²⁺(aq) + 2e⁻, and replaces the copper ions used up. Platinum does not supply copper ions, so in cell B the solution gradually runs out of them and oxygen forms at the anode instead.
In both cells, Cu²⁺ gains electrons at the cathode and forms the coating, so the object must be the cathode. The anode material only decides what happens at the anode.
Why objects are electroplated
Electroplating puts a thin layer of one metal on top of another. The coating can:
- make an object look better, as in shiny chromium or nickel plating on taps and bicycle handlebars;
- protect the metal underneath from corrosion, as long as the coating is not broken. Tin-plated steel cans and nickel-plated steel are examples;
- let a small amount of an expensive metal do the job of a solid one, as in silver-plated cutlery and gold-plated jewellery.
Common mistakes
State that the object is the cathode, name the anode material and the electrolyte, and write the cathode half-equation. Give a reason for each.
- Connecting the object as the anode. It must be the cathode.
- Choosing an electrolyte without ions of the plating metal.
- Saying that metal ions travel through the wires. Ions move through the electrolyte; electrons move through the wires.
- Saying both electrodes gain mass. An anode made of the plating metal loses metal while the object gains the coating.
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Syllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
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