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Purifying Copper by Electrolysis

Explain how copper transfers from an impure anode to a pure cathode, track copper ions and electrode masses, and distinguish impurity paths.

  • SEC G3 Pure Chemistry 2027

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Copper purification, or electrorefining, uses electrolysis to obtain purer copper from impure copper. The goal is to build a copper cathode without transferring the anode’s impurities along with it. This differs from electroplating, whose goal is to coat a chosen object.

Choose the electrodes and electrolyte

  • Positive anode: impure copper, which supplies copper ions by oxidation.
  • Negative cathode: a pure copper sheet, on which more copper is deposited by reduction.
  • Electrolyte: aqueous copper(II) sulfate, commonly acidified, which conducts through mobile ions and supplies copper(II) ions for deposition.
  • DC supply: provides the electrical energy needed for electrolysis.
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+−electrons to supplyelectrons to cathodeANODE (+)CATHODE (−)impure copperpure copper sheetCuCuCu²⁺Cu²⁺Cu²⁺Cu²⁺metal ions move to the cathodeAnode: Cu → Cu²⁺ + 2e⁻Cathode: Cu²⁺ + 2e⁻ → Cuinsoluble anode sludge

Anode (+): Impure copper loses electrons and supplies copper(II) ions.

Cathode (−): Copper(II) ions gain electrons and build the pure copper sheet.

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

Impurities: insoluble material collects as sludge below the anode; other impurities can dissolve and remain in solution.

Copper transfers from the impure anode to the pure cathode. Insoluble impurities do not plate onto the cathode; they collect as anode sludge. 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.

Track atoms, ions and electrons

At the anode, copper atoms lose electrons and enter the solution:

Cu(s) → Cu²⁺(aq) + 2e⁻

At the cathode, copper(II) ions gain electrons and form copper atoms:

Cu²⁺(aq) + 2e⁻ → Cu(s)

Copper therefore transfers from the impure anode to the pure cathode through the electrolyte. Copper ions do not travel through the wire; electrons carry charge in the external circuit. The cathode can gain copper already present as ions in the electrolyte, so you do not need to follow one particular atom from the anode to the cathode.

Explain why the cathode is purer

Insoluble impurities detach from the dissolving anode and collect below it as anode sludge. They do not have to pass through the solution as ions before settling. Other impurities can dissolve and remain in the electrolyte rather than deposit with copper under the operating conditions. Do not claim that every impurity becomes sludge or that an impure anode dissolves entirely as copper ions.

The process separates copper by selecting electrode reactions. It is not filtration of a solid copper mixture, and it does not turn impurity atoms into copper atoms.

Connect observations, particles and equations

ObservationExplanation
The impure anode loses massCopper atoms are oxidised; insoluble material can also detach.
The pure cathode gains massCopper(II) ions gain electrons and deposit copper.
Insoluble material collects below the anodeThese impurities form anode sludge rather than the copper deposit.
The blue colour stays approximately constant in the ideal modelCopper(II) ions added by copper dissolution replace those removed by deposition, assuming unchanged solution volume and no significant competing reaction.

The amount of copper transferred can match at the two electrodes without the total mass lost by an impure anode matching the cathode gain: anode mass can also leave as sludge or dissolved impurities.

Worked equations and observations

Guided practice 1

Copper Purification Half-Equations

About 7 min

Application transfer

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

Write the equations on paper, then compare the choices

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.

Common misconception 2

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.

Independent practice

Mind stretcher 1: 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.

Try independently: An impure anode loses 1.30 g while a pure copper cathode gains 1.20 g. In this simplified case, no impurities dissolve, all copper dissolved at the anode deposits at the cathode, and all detached insoluble material is collected. Calculate the sludge mass and explain why unequal electrode mass changes do not contradict equal electron transfer in the copper half-equations.

Show answer and reasoning

Sludge mass = 1.30-1.20 = 0.10 g. The anode loses 1.20 g of copper by oxidation and 0.10 g of insoluble material by detachment. The cathode gains only the 1.20 g of copper. The matched electron transfer applies to the copper reactions, not to material that falls off without that reaction.

Practise and check

Use the Redox Chemistry topic check to practise connecting the purpose, electrodes, observations and equations.

Syllabus and review details

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