Copper sulfate: inert and copper electrodes
Predict how changing the electrode material changes the anode reaction and solution colour.
Extended only (Supplement) · Chemistry 0620, 2026–2028.
Before you begin
- Describe electrode oxidation and reduction.
- Recognise aqueous Cu²⁺ ions and write charge-balanced half-equations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which species gives aqueous copper(II) sulfate its blue colour?
What you will learn
- Identify products and observations for aqueous copper(II) sulfate with graphite or copper electrodes.
- Explain the changes using balanced electrode half-equations.
With inert carbon or graphite electrodes
In aqueous copper(II) sulfate with inert electrodes, copper deposits as a reddish-brown solid at the negative cathode. The half-equation is Cu²⁺+2e⁻→Cu. Oxygen gas forms as bubbles at the positive anode; an oxygen-producing half-equation is 4OH⁻→O₂+2H₂O+4e⁻. Sulfate is not converted into sulfur in the required prediction.
As copper ions leave solution and are not replenished by an inert anode, the blue colour becomes paler with continued electrolysis. The cathode gains deposited copper while the anode does not supply copper ions. If asked for observations, state the deposit, gas bubbles and colour change rather than only writing product formulas.
With copper electrodes
The cathode still gains copper by Cu²⁺+2e⁻→Cu. At a soluble copper anode, copper atoms become ions: Cu→Cu²⁺+2e⁻. The anode gets thinner and loses mass while the cathode gains mass. In the ideal model, copper ions supplied at the anode replace those removed at the cathode, so their concentration and the blue solution colour remain approximately unchanged.
Copper is effectively transferred from the anode to the cathode. Oxygen is not the predicted main anode product in this copper-electrode arrangement. Equal ideal anode loss and cathode gain assume the same copper reaction carries the current at each electrode without losses or side reactions; a real measurement that differs should be interpreted using those assumptions rather than hidden by the model.
Pause and explain
What happens to a soluble copper anode in the ideal copper-electrode cell?
Worked example
Compare the products and observations when aqueous CuSO₄ is electrolysed first with inert graphite electrodes, then with copper electrodes.
Show the worked solution
- Both arrangements deposit reddish-brown copper at the cathode by Cu²⁺+2e⁻→Cu.
- The inert anode produces oxygen bubbles, and the blue colour fades as copper ions are removed.
- The copper anode dissolves by Cu→Cu²⁺+2e⁻; its mass falls, the cathode mass rises and the blue colour stays approximately constant in the ideal model.
Answer Inert anode: oxygen and fading blue. Copper anode: dissolving copper and approximately unchanged blue.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Predict the cathode product, anode product and solution-colour trend for CuSO₄(aq) with inert graphite electrodes.
Give me a hint
The inert anode cannot replace copper ions removed at the cathode.
Compare my reasoning
- Copper forms a reddish-brown cathode deposit.
- Oxygen forms bubbles at the anode.
- The solution's blue colour becomes paler as its copper-ion concentration decreases.
Cathode copper; anode oxygen; blue solution becomes paler.
Look for these in your work
- Each product has a matching observation.
- The colour trend follows copper-ion removal.
Write both half-equations for CuSO₄(aq) with copper electrodes and explain why the solution can remain blue.
Give me a hint
Compare copper ions produced with copper ions removed.
Compare my reasoning
- Cathode: Cu²⁺+2e⁻→Cu.
- Anode: Cu→Cu²⁺+2e⁻.
- The ideal reactions replenish copper ions at the same rate as they are deposited, maintaining approximate concentration and colour.
Cathode Cu²⁺+2e⁻→Cu; anode Cu→Cu²⁺+2e⁻; copper ions are replenished.
Look for these in your work
- The electron sides identify reduction and oxidation correctly.
- The unchanged colour is explained with an ion balance.
In an ideal copper-electrode cell, the anode loses 0.64 g copper. Predict the cathode gain and total copper mass change across both electrodes. What would a smaller measured cathode gain prompt you to investigate?
Give me a hint
The model transfers the same copper amount rather than creating metal.
Compare my reasoning
- The ideal cathode gain is 0.64 g, equal to anode loss.
- Combined electrode copper mass change is −0.64+0.64=0 g.
- A smaller measured gain suggests checking incomplete collection of deposit, measurement error or side reactions rather than claiming the ideal prediction was measured.
Ideal gain 0.64 g; combined copper mass change zero; investigate losses, measurements and side reactions if they disagree.
Look for these in your work
- The model's mass conservation is explicit.
- A measured discrepancy is kept separate from the prediction.
Common mistakes
- Predicting oxygen at a copper anode without considering its material.
- Predicting unchanged blue colour with an inert anode throughout removal of copper ions.
- Describing the deposit as blue copper metal rather than reddish-brown copper.
What can you explain now?
Write the half-equation for a soluble copper anode during electrolysis of aqueous CuSO₄.
Compare with the explanation
Cu → Cu²⁺ + 2e⁻
Copper atoms lose two electrons and enter solution as Cu²⁺; the anode reaction is oxidation.
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Make it stick
Draw two copper sulfate cells differing only in electrode material. Write the anode reaction for each and predict the deposit, bubbles, mass trends and blue colour.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.