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.
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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 conducts electrons and provides a surface for electron transfer without being consumed in the stated reaction. Graphite and platinum are treated as inert in these syllabus examples. “Inert” describes their role under those conditions, not a promise that the material never reacts. For example, graphite can react with oxygen and gradually wear away in some electrolysis systems.
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
- The cathode is where reduction occurs, whether its material is inert or reactive. In copper(II) sulfate, copper ions gain electrons and deposit copper on the cathode.
- At an inert anode in this solution, oxygen forms and no copper ions are added.
- At a copper anode in this solution, copper atoms lose electrons and enter the solution as copper(II) ions.
- With matched copper dissolution and deposition, the copper(II) ion concentration stays approximately constant if the solution volume stays constant. This is a specific consequence of the matched reactions, not a rule for every reactive electrode.
Cathode = reduction (gain electrons). Anode = oxidation (lose electrons).
What Is Electrolysis?
3. Detailed Explanations
- Copper deposition at the cathode removes Cu²⁺ from solution.
- An inert anode does not replace those copper ions, so the blue solution becomes paler.
- A copper anode dissolves to replace Cu²⁺, so the blue colour stays approximately constant in the ideal comparison.
A. Inert vs Reactive (What Actually Changes)
In the following comparison, both cells contain copper(II) sulfate solution and deposit copper at the cathode. Only the anode material changes.
| Feature | Inert anode (graphite/platinum) | Reactive copper anode |
|---|---|---|
| Anode reaction | Water/OH⁻ is oxidised to oxygen | The electrode metal is oxidised and dissolves |
| Mass of anode | Usually stays about the same | Decreases (dissolves) |
| Mass of cathode | Increases (copper deposits) | Increases (copper deposits) |
| Colour of CuSO₄(aq) | Becomes paler (removing Cu²⁺) | Stays roughly the same (anode replaces Cu²⁺) |
Fast decision (very common exam case):
| Electrolyte | Anode is graphite (inert) | Anode is copper (reactive) |
|---|---|---|
| CuSO₄(aq) | O₂(g) formed (from OH⁻) | Cu²⁺(aq) formed (copper anode dissolves) |
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, assuming the two copper reactions dominate and the solution volume is unchanged:
- Copper transfers from anode to cathode.
- Cu²⁺ concentration stays about constant, so the solution stays blue.
For a copper anode in copper(II) sulfate, explain: “Copper atoms lose electrons and enter the solution as copper(II) ions.” Name the material and electrolyte rather than applying this reaction to every metal anode.
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 a copper anode in copper(II) sulfate, copper is oxidised instead.
- Forgetting observations: mass change at electrodes, colour change (or no change).
5. Exam Tips
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
Problem
Study the worked solution
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.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.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
Learner claim
Distinguish inert from reactive anodes
View solution step by step
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).Correct the anode process
Method
Produce oxygen from hydroxide ions.Reason
Carbon is treated as inert under the stated conditions and does not supply copper ions.Working
4OH-(aq) → O₂(g) + 2H₂O(l) + 4e⁻.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.
Guided practice 3
Copper anode with a carbon cathode
Electrode-material transfer
Consider each electrode material separately
Hints
Hint 1: separate the electrodes
View solution step by step
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.Dissolve the copper anode
Method
Oxidise copper atoms at the anode.Reason
The copper anode in this solution 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. Explain how pure copper is obtained, and state what happens to insoluble impurities. Use the copper-purification explanation if you need the full setup.
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. Some other impurities can dissolve and remain in solution rather than deposit as copper; not every impurity becomes sludge.
Try independently: Two cells contain the same copper(II) sulfate solution and both deposit copper at the cathode. One has a platinum anode and the other a copper anode. In the ideal model, each cathode gains 0.32 g of copper. Predict the anode mass change and blue-colour change in each cell. Assume the solution volume remains unchanged.
Show answer and reasoning
With platinum, the inert anode does not supply copper ions: its mass stays approximately unchanged and the blue solution becomes paler. With copper, the anode loses 0.32 g of copper in the ideal matched transfer; its copper atoms replace the copper ions removed at the cathode, so the blue colour stays approximately constant. Each copper atom transferred corresponds to two electrons at either electrode. Equal cathode gains alone do not imply equal anode behaviour.
Practise and check
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.
Open the Redox Chemistry topic checkSyllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
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