Mini Chemistry
Part of Mini EducationSwitch subject

Electroplating and Copper Purification

Learn K324 / 6092 electroplating and copper purification: electrode roles, electrolyte choice, mass changes, observations, and balanced half-equations.

  • SEC G3 Pure Chemistry 2027
On this page

Learning objectives

  • describe the electrolysis of aqueous copper(II) sulfate with copper electrodes as a means of purifying copper (no technical details are required)
  • describe the electroplating of metals, e.g. copper plating, and state one use of electroplating

Applications questions test whether you can connect a purpose to the correct electrodes, electrolyte and half-equations. For 6092, prioritise electroplating and purifying copper.

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).
  • The plating metal is the anode (it dissolves to replace ions).
  • The electrolyte must contain ions of the plating metal.
  • In copper purification, impure copper is the anode and pure copper is the cathode.
Recall: Electrodes and redox

In electrolysis: cathode is negative (reduction), anode is positive (oxidation). What Is Electrolysis?

3. Detailed Explanations

Quick Recall (industrial setups)
  • Electroplating: object is the cathode; plating metal is the anode; electrolyte contains the plating metal ions.
  • Electrorefining (Cu): impure copper anode dissolves; pure copper cathode gains copper.

A. Electroplating

Electroplating coats one metal with another using electrolysis. Reasons:

  • 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 setupThe 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 setupDC power supply+−ANODE (+)CATHODE (−)plating metalobject to be platedMobjectMⁿ⁺Mⁿ⁺Mⁿ⁺Mⁿ⁺metal ions move to the cathodeAnode: M → Mⁿ⁺ + ne⁻Cathode: Mⁿ⁺ + ne⁻ → M
For electroplating, name all three parts: the object is the cathode, the coating metal is the anode, and the electrolyte contains ions of that metal.

Setup rules (mark-scheme):

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

B. Electrorefining (Purification of Copper)

Electrorefining makes copper purer:

  • Anode: impure copper (dissolves).
  • Cathode: pure copper sheet (copper is deposited).
  • Electrolyte: CuSO₄(aq) (often acidified).

Key idea: copper transfers from anode to cathode.

Electrolytic purification of copperImpure copper is the positive anode and pure copper is the negative cathode in copper(II) sulfate solution. Copper atoms leave the anode as copper(II) ions and copper(II) ions gain electrons at the cathode. Insoluble impurities collect below the anode.Electrolytic purification of copperDC power supply+−ANODE (+)CATHODE (−)impure copperpure copper sheetCuCuCu²⁺Cu²⁺Cu²⁺Cu²⁺metal ions move to the cathodeAnode: Cu → Cu²⁺ + 2e⁻Cathode: Cu²⁺ + 2e⁻ → Cuinsoluble anode sludge
Copper transfers from the impure anode to the pure cathode. Insoluble impurities do not plate onto the cathode; they collect as anode sludge.

At the anode, copper atoms lose electrons: Cu(s) → Cu²⁺(aq) + 2e⁻

At the cathode, copper(II) ions gain electrons: Cu²⁺(aq) + 2e⁻ → Cu(s)

C. Connect the Three Representations

LevelCopper purification
Macroscopic observationThe impure anode becomes smaller; the pure cathode becomes thicker; insoluble impurities collect below the anode.
Particle modelCopper atoms at the anode form Cu²⁺ ions. Copper(II) ions at the cathode gain electrons and form copper atoms.
Symbolic representationAnode: Cu(s) → Cu²⁺(aq) + 2e⁻; cathode: Cu²⁺(aq) + 2e⁻ → Cu(s).

Because Cu²⁺ ions removed at the cathode are replaced at approximately the same rate at the anode, the blue colour of the electrolyte remains approximately unchanged. Do not say that the ions travel directly through the wire: electrons move in the external circuit, while ions move through the electrolyte.

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 (the anode loses mass and the cathode gains mass).

5. Exam Tips

Electroplating marks checklist

State: cathode (object), anode (plating metal), electrolyte (metal ions), and the cathode half-equation (metal ions gain electrons).

  • 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).

Reverse the same nickel change

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⁻.

Common misconception 3

Explain an Observation

Find and correct the mistake

Learner prediction

A student predicts the blue electrolyte becomes steadily paler during copper purification because copper ions deposit at the cathode. Explain why the observed colour remains approximately unchanged.

Track removal and replacement

Cathode
Anode

View solution step by step

  1. Track cathode removal

    Method

    Remove Cu²⁺ as copper deposits.

    Reason

    Reduction consumes copper(II) ions.

    Working

    Cu²⁺ + 2e⁻ → Cu.

  2. Track anode replacement

    Method

    Dissolve copper atoms as new Cu²⁺ ions.

    Reason

    The reactive impure copper anode replaces approximately what is consumed.

    Working

    Cu → Cu²⁺ + 2e⁻; concentration and blue colour stay about constant.

Challenge 4

Copper Purification Half-Equations

Minimal support

Application transfer

Write both half-equations for electrolytic purification of copper and state the direction of copper transfer.

Assign impure and pure electrodes

Impure anode
Pure cathode

Hints

Hint 1: purity direction
Copper leaves the impure anode and builds the pure cathode.
View solution step by step

  1. Oxidise impure copper

    Method

    Dissolve copper at the anode.

    Reason

    Oxidation releases electrons and puts copper ions into solution.

    Working

    Cu(s) → Cu²⁺(aq) + 2e⁻.

  2. Deposit pure copper

    Method

    Reduce copper ions at the pure cathode.

    Reason

    The deposited metal forms the purified copper product.

    Working

    Cu²⁺(aq) + 2e⁻ → Cu(s); copper transfers impure anode → pure cathode.

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.

Mind stretcher 2: Electrorefining ObservationExtension

In copper electrorefining, state what happens to the mass of the anode and cathode, and what happens to insoluble impurities.

Show Answer

Anode mass decreases (copper dissolves). Cathode mass increases (copper deposits). Insoluble impurities fall off and collect as anode sludge.

8. Quiz

Quiz Time!

Ready to test your knowledge? Practice identifying industrial purpose, setup, and products without mixing up electrode roles.

Go to Quiz Page