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Types of Electrodes in Electrolysis

Inert vs reactive electrodes: explain how a copper anode can dissolve instead of producing oxygen and how this affects products, mass and concentration.

  • SEC G3 Pure Chemistry 2027
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Learning objectives

  • predict the likely products of the electrolysis of an aqueous electrolyte, given relevant information
  • describe the electrolysis of aqueous copper(II) sulfate with copper electrodes as a means of purifying copper (no technical details are required)

Electrode material is part of the chemical system. An inert anode provides a surface for oxidation, while a reactive metal anode can itself lose electrons and enter the electrolyte as ions.

1. Definition

A. Inert Electrode

An inert electrode does not react. It is just a surface for electron transfer (e.g., graphite or platinum).

B. Reactive (Active) Electrode

A reactive electrode reacts and becomes part of the electrolysis. The required example is a copper anode in copper(II) sulfate solution.

2. Key Ideas

  • In aqueous electrolysis, cathode usually depends on which cation is discharged; anode depends on which anion is discharged unless the electrode is reactive.
  • With a reactive anode, the metal electrode is oxidised instead of discharging anions like OH⁻.
  • With a reactive anode, the electrolyte concentration can stay (nearly) constant because the anode supplies ions as fast as the cathode removes them.
Recall: Redox at electrodes

Cathode = reduction (gain electrons). Anode = oxidation (lose electrons). What Is Electrolysis?

3. Detailed Explanations

Quick Recall (inert vs reactive)
  • Inert electrode: does not react; anions (often OH⁻) are discharged at the anode.
  • Reactive anode: the metal electrode is oxidised and dissolves (e.g., Cu(s) → Cu²⁺(aq) + 2e⁻).
  • In CuSO₄(aq): inert electrodes → blue fades; copper electrodes → blue stays about constant (anode replaces Cu²⁺).

A. Inert vs Reactive (What Actually Changes)

FeatureInert electrodes (graphite/platinum)Reactive copper electrode
Anode reactionAnions (usually OH⁻ or halides) are dischargedThe electrode metal is oxidised and dissolves
Mass of anodeUsually stays about the sameDecreases (dissolves)
Mass of cathodeIncreases if metal is depositedIncreases (metal deposits)
Colour of CuSO₄(aq)Becomes paler (removing Cu²⁺)Stays roughly the same (anode replaces Cu²⁺)

Fast decision (very common exam case):

ElectrolyteAnode is graphite (inert)Anode is copper (reactive)
CuSO₄(aq)O₂(g) formed (from OH⁻)Cu²⁺(aq) formed (copper anode dissolves)
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
With copper electrodes, copper atoms enter the solution at the anode as copper(II) ions while copper(II) ions leave the solution at the cathode. This matched transfer keeps the blue colour approximately constant.

B. Case Study 1: CuSO₄(aq) with Copper Electrodes

Electrolyte dissociation (aq): CuSO₄(aq) → Cu²⁺(aq) + SO₄²⁻(aq)

In aqueous solution, water also provides: H₂O(l) ⇌ H + (aq) + OH-(aq)

Relevant ions present: Cu²⁺, SO₄²⁻, H⁺, OH⁻.

Cathode (reduction): copper is deposited Cu²⁺(aq) + 2e⁻ → Cu(s)

Anode (oxidation): copper dissolves (reactive electrode) Cu(s) → Cu²⁺(aq) + 2e⁻

Overall effect (exam language):

  • Copper transfers from anode to cathode.
  • Cu²⁺ concentration stays about constant, so the solution stays blue.
Key sentence

If the anode is reactive, write: “the anode dissolves (is oxidised) to form metal ions; anions are not discharged.”

4. Common Mistakes

  • Writing 4OH⁻ → O₂ + ... at the anode when the anode is copper. The copper anode dissolves instead.
  • Saying “sulfate ions are discharged”. In aqueous electrolysis with inert electrodes, SO₄²⁻ is usually not discharged; with reactive electrodes, the electrode dissolves anyway.
  • Forgetting observations: mass change at electrodes, colour change (or no change).

5. Exam Tips

What to write when asked 'state what happens'

Mention all 3: anode mass, cathode mass, and solution colour (if coloured ions like Cu²⁺ are present).

  • Name the competing ions and state which one is discharged. For example: “Cu²⁺ is discharged instead of H⁺.”
  • Link the observation to ions: “blue fades because Cu²⁺ decreases”.

6. Worked Examples

Modelled example 1

Copper(II) Sulfate with Copper Electrodes

Core

Problem

Copper(II) sulfate is electrolysed using copper electrodes. State what happens to both electrode masses and the solution colour.
Study the worked solution

  1. Oxidise the copper anode

    Method

    Describe copper atoms dissolving as Cu²⁺.

    Reason

    A reactive copper anode supplies electrons by oxidation.

    Working

    Cu(s) → Cu²⁺(aq) + 2e⁻; anode mass decreases.

  2. Reduce copper ions at the cathode

    Method

    Deposit copper metal.

    Reason

    Cu²⁺ ions gain electrons at the cathode.

    Working

    Cu²⁺(aq) + 2e⁻ → Cu(s); cathode mass increases.

  3. Track solution colour

    Method

    Keep the blue colour roughly unchanged.

    Reason

    The anode replaces approximately the copper(II) ions removed at the cathode.

    Working

    [Cu²⁺] remains about constant.

Common misconception 2

Copper(II) Sulfate with Inert Electrodes

Find and correct the mistake

Learner claim

A student says carbon electrodes keep copper(II) sulfate blue because “electrodes always replace discharged ions”. Correct the claim, state both products and explain the colour change.

Distinguish inert from reactive anodes

Cathode product
Anode product
Blue colour

View solution step by step

  1. Keep the cathode process

    Method

    Deposit copper at the cathode.

    Reason

    Cu²⁺ gains electrons regardless of the inert anode.

    Working

    Cu²⁺(aq) + 2e⁻ → Cu(s).

  2. Correct the anode process

    Method

    Produce oxygen from hydroxide ions.

    Reason

    Carbon is inert and does not dissolve to supply copper ions.

    Working

    4OH-(aq) → O₂(g) + 2H₂O(l) + 4e⁻.

  3. Correct the colour prediction

    Method

    Predict the blue solution becomes paler.

    Reason

    Copper(II) ions are removed at the cathode without replacement at the anode.

    Working

    [Cu²⁺] decreases.

Challenge 3

Copper anode with a carbon cathode

Minimal support

Electrode-material transfer

Copper(II) sulfate solution is electrolysed using a copper anode and a carbon cathode. State what happens at each electrode and predict the solution colour.

Consider each electrode material separately

Carbon cathode
Copper anode
Blue colour

Hints

Hint 1: separate the electrodes
The carbon cathode is inert, but the copper anode is reactive.
View solution step by step

  1. Deposit copper on carbon

    Method

    Reduce Cu²⁺ at the cathode.

    Reason

    The cathode material provides a conducting surface; copper(II) ions gain electrons there.

    Working

    Cu²⁺(aq) + 2e⁻ → Cu(s); cathode mass increases.

  2. Dissolve the copper anode

    Method

    Oxidise copper atoms at the anode.

    Reason

    The reactive copper anode supplies replacement Cu²⁺ ions.

    Working

    Cu(s) → Cu²⁺(aq) + 2e⁻; anode mass decreases and the blue colour stays approximately constant.

7. Mind Stretchers

Mind stretcher 1: Choose the Electrode Type (and Justify)Extension

A student wants to keep the blue colour of CuSO₄(aq) roughly constant during electrolysis. Should they use copper or carbon electrodes? Explain in one sentence using ions.

Show Answer

Use copper electrodes, because the copper anode dissolves to produce Cu²⁺ ions that replace the Cu²⁺ ions discharged at the cathode, keeping the blue colour roughly constant.

Mind stretcher 2: Electrorefining Logic CheckExtension

In copper electrorefining, impure copper is the anode and pure copper is the cathode. State where the impurities go (simple O-Level description).

Show Answer

Copper dissolves from the anode as Cu²⁺ and is deposited as pure copper on the cathode. Insoluble impurities fall off and collect below the anode as sludge.

8. Quiz

Quiz Time!

Ready to check your understanding? Start by deciding whether the anode is inert or reactive, then connect that choice to the product and mass change.

Go to Quiz Page