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Electroplating: Growing a Metal Coating

Explain how a metal coating grows on the cathode, choose the electrolyte and anode, and connect electron transfer to electrode changes.

  • SEC G3 Pure Chemistry 2027

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Electroplating uses electrolysis to grow a metal coating on an object. Start with the object: it must receive electrons so that dissolved metal ions can become metal on its surface. Copper purification is developed in the next application lesson.

1. Definition

A. Electrolysis (Industrial Use)

Electrolysis is used in industry to:

  • coat objects with a metal (electroplating),
  • purify copper (electrorefining).

Reactive metals can also be extracted from molten compounds by electrolysis. That idea is covered in electrolysis of molten compounds.

2. Key Ideas

  • In electroplating, the object is the cathode (it gains a metal layer).
  • In the soluble-anode setups used here, the plating metal is the anode and dissolves to replace ions.
  • The electrolyte must contain ions of the plating metal.
  • Electroplating coats an object; copper purification instead separates copper from impurities.
Recall: Electrodes and redox

In electrolysis: cathode is negative (reduction), anode is positive (oxidation).

What Is Electrolysis?

3. Detailed Explanations

Follow the coating metal
  • The object is the negative cathode, where metal ions gain electrons.
  • The electrolyte contains ions of the coating metal.
  • A suitable soluble metal anode loses electrons and replenishes those ions.
  • Use the anode stated in the question: not every industrial plating process uses a dissolving anode of the coating metal.

A. Electroplating

Electroplating coats a conducting object with a metal using electrolysis. For the metal objects in these examples, the coating can:

  • improve appearance (shiny surface),
  • prevent corrosion while the protective coating remains intact,
  • improve wear resistance.

For example, nickel or chromium plating gives taps and other fittings an attractive, corrosion-resistant surface; silver plating gives cutlery the appearance and surface properties of silver while using less of the expensive metal.

Electroplating with a soluble metal anodeIn this soluble-anode setup, the plating metal is the positive anode and the object to be plated is the negative cathode. The electrolyte contains ions of the plating metal. Metal ions gain electrons and form a coating on the object while the metal anode replaces the ions.Electroplating with a soluble metal anodeDC power supply+−electrons to supplyelectrons to cathodeANODE (+)CATHODE (−)plating metalobject to be platedMobjectMⁿ⁺Mⁿ⁺Mⁿ⁺Mⁿ⁺metal ions move to the cathodeAnode: M → Mⁿ⁺ + ne⁻Cathode: Mⁿ⁺ + ne⁻ → M

Anode (+): The soluble coating-metal anode loses electrons and supplies metal ions.

Cathode (−): Metal ions gain electrons and coat the object.

Charge paths: electrons move in the wires and electrodes; ions move in the solution.

In this soluble-metal-anode model, the object is the cathode, a strip of the coating metal is the anode, and the electrolyte supplies ions of that metal. Other plating processes can use a different anode reaction. This schematic is not to scale. Ion colours and sizes are symbolic; other electrolyte ions are omitted. Coating and sludge sizes are exaggerated for visibility.

For the soluble-metal-anode model shown:

  • Cathode: object to be plated.
  • Anode: a suitable strip of the plating metal, which dissolves.
  • Electrolyte: solution containing ions of the plating metal (e.g., Ag⁺, Ni²⁺, Cu²⁺).

Some industrial processes use an anode that does not supply the coating metal; the electrolyte must then be replenished separately. For syllabus questions, follow the stated materials rather than assuming that every plating anode dissolves.

Use a different setup to purify copper

Electroplating coats a chosen object. Purifying Copper by Electrolysis uses an impure copper anode and a pure copper cathode to obtain purer copper. The electrode reactions are similar, but the purpose and treatment of impurities need their own explanation.

Continue the observation–particle–equation comparison in Purifying Copper by Electrolysis.

4. Common Mistakes

  • Swapping anode/cathode roles in electroplating (object must be cathode).
  • Using an electrolyte that does not contain the plating metal ions (no ions, no plating).
  • Saying copper ions move through the wire (ions move in the electrolyte; electrons move in the wire).
  • Describing both electrodes as gaining mass in a soluble-anode setup: the anode loses metal while the cathode gains the coating.

5. Exam Tips

Electroplating marks checklist

Identify the object as cathode, the stated anode material, the coating-metal ions in the electrolyte, and the cathode half-equation. Explain the purpose of each part.

  • Use the word discharged for ions at electrodes.
  • Link each observation to a half-equation. For example, cathode mass increases because Cu²⁺(aq) gains electrons and forms Cu(s).

6. Worked Examples

Modelled example 1

Choose Electrodes for Electroplating

Core

Problem

A steel spoon is to be electroplated with nickel using NiSO₄(aq). Identify the anode and cathode and explain the choices.
Study the worked solution

  1. Assign the object

    Method

    Connect the steel spoon as the cathode.

    Reason

    Metal ions gain electrons and deposit at the cathode.

    Working

    Cathode: steel spoon.

  2. Assign the plating metal

    Method

    Use a nickel strip as the anode.

    Reason

    The reactive anode can replenish nickel ions removed from solution.

    Working

    Anode: nickel strip.

  3. State the coating mechanism

    Method

    Reduce Ni²⁺ onto the spoon.

    Reason

    The deposited metal is the required nickel coating.

    Working

    Ni²⁺ + 2e⁻ → Ni on the spoon.

Guided practice 2

Electroplating Half-Equations (Nickel Plating)

About 6 min

Problem

Write both half-equations for nickel electroplating with a nickel anode in NiSO₄(aq).

Write both equations on paper, then compare the choices

Cathode equation
Anode equation

Hints

Hint 1: cathode reduction
The coating grows where nickel ions gain electrons.
Hint 2: anode oxidation
Reverse the equation at the nickel strip.
View solution step by step

  1. Deposit nickel

    Method

    Reduce nickel ions at the cathode.

    Reason

    The + 2 ion must gain two electrons.

    Working

    Ni²⁺(aq) + 2e⁻ → Ni(s).

  2. Replenish nickel ions

    Method

    Oxidise nickel at the anode.

    Reason

    The reactive plating metal supplies replacement ions.

    Working

    Ni(s) → Ni²⁺(aq) + 2e⁻.

Continue this question in Purifying Copper by Electrolysis.

Continue this question in Purifying Copper by Electrolysis.

7. Mind Stretchers

Mind stretcher 1: Fix the Wrong SetupExtension

A student sets the object to be plated as the anode and uses a graphite cathode. They say “the object will gain a coating”. Explain why they are wrong.

Show Answer

Metal is deposited at the cathode (reduction). If the object is the anode, it tends to oxidise/dissolve instead of being plated. The object must be the cathode.

Continue this question in Purifying Copper by Electrolysis.

Try independently: Two copper-plating cells have identical cleaned objects as cathodes. Both deposit copper. Cell A has a soluble copper anode; cell B has a platinum anode treated as inert. Predict whether copper ions are replenished by each anode, and explain why the object must remain the cathode in both cells.

Show answer and reasoning

A copper anode supplies Cu²⁺ by oxidation. Platinum does not supply copper ions, so those removed by deposition must be replaced separately if their concentration is to be maintained. In both cells, Cu²⁺ gains electrons at the cathode and forms the coating on the object. The coating requirement fixes the object’s role; the anode reaction depends on its material.

Practise and check

Practise and check

Practise choosing the electrodes and electrolyte for a stated coating.

Open the Redox Chemistry topic check
Syllabus and review details

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