Electrolysis of Copper(II) Sulfate
The electrolysis of copper(II) sulfate (CuSO₄) is one of the most important electrolysis investigations in IGCSE Chemistry because its products depend entirely on the type of electrodes used. When a direct current is passed through aqueous copper(II) sulfate solution, Cu²⁺ ions migrate to the cathode (the negative electrode) and SO₄²⁻ ions migrate to the anode (the positive electrode), but what actually forms at each electrode is determined by whether inert or copper electrodes are employed. This system perfectly illustrates the principle of selective discharge: at the anode, the ion that is most easily oxidised is discharged first, while at the cathode the ion that is most easily reduced is discharged. The experiment with inert electrodes demonstrates the electrolysis of water alongside metal deposition, since hydroxide ions in water provide the competing anodic species. With copper electrodes, the process becomes electroplating — a commercially vital application of electrolysis used to purify blister copper ores to over 99.99% purity in industry.
With Inert Electrodes (Carbon/Graphite/Platinum)
When inert electrodes (carbon rods, graphite rods, or platinum foil) are dipped into copper(II) sulfate solution, the products at the two electrodes differ dramatically. Inert electrodes do not participate chemically in the reaction — they merely provide a surface for electron transfer to occur. The identity of the products is therefore determined solely by the ions present in the electrolyte and the selectivity rules of electrolysis.
Species Present in Solution
The following ions are present in aqueous copper(II) sulfate:
- Cations: Cu²⁺(aq) from the salt, and H⁺(aq) from the self-ionisation of water
- Anions: SO₄²⁻(aq) from the salt, and OH⁻(aq) from water
At the Cathode (Negative Electrode)
At the cathode, reduction occurs — electrons are gained. Two cations compete for discharge: Cu²⁺ and H⁺. Copper(II) ions are lower in the reactivity series than hydrogen ions, meaning Cu²⁺ is more easily reduced (it has a more positive standard electrode potential). Therefore, copper(II) ions are discharged in preference to hydrogen ions.
Half-equation at the cathode (reduction):
Cu²⁺(aq) + 2e⁻ → Cu(s)
Observation: A pink-brown solid deposit of copper metal forms on the surface of the cathode. Over time, this deposit thickens and may become a distinct reddish-brown coating. The copper metal appears pink when freshly deposited but darkens to a characteristic copper-brown colour upon exposure.
At the Anode (Positive Electrode)
At the anode, oxidation occurs — electrons are lost. Two anions compete for discharge: SO₄²⁻ and OH⁻. Sulfate ions are very stable and difficult to oxidise, so hydroxide ions are discharged in preference to sulfate ions.
Half-equation at the anode (oxidation):
4OH⁻(aq) → O₂(g) + 2H₂O(l) + 4e⁻
Observation: Bubbles of colourless oxygen gas are evolved at the anode. These can be tested with a glowing splint, which relights in the presence of oxygen. The gas production is steady and visible throughout the electrolysis.
Overall Observations
The blue colour of the solution fades gradually as Cu²⁺ ions (which give the solution its characteristic blue colour) are removed from the solution by deposition at the cathode. Crucially, the copper(II) ions are not replaced — unlike in the case with copper electrodes — so the intensity of the blue colour diminishes. Concurrently, the solution becomes increasingly acidic because, as OH⁻ ions are discharged at the anode, the remaining H⁺ ions from water accumulate in the solution, effectively forming sulfuric acid (H₂SO₄) along with the leftover SO₄²⁻ ions.
Summary of Inert Electrode Electrolysis
| Feature | Detail |
|---|---|
| Electrolyte | Aqueous copper(II) sulfate, CuSO₄(aq) — blue solution |
| Cathode material | Carbon, graphite, or platinum (inert) |
| Anode material | Carbon, graphite, or platinum (inert) |
| Cathode product | Copper metal, Cu(s) — pink-brown solid |
| Cathode half-equation | Cu²⁺ + 2e⁻ → Cu(s) |
| Anode product | Oxygen gas, O₂(g) — colourless gas |
| Anode half-equation | 4OH⁻ → O₂ + 2H₂O + 4e⁻ |
| Colour change | Blue solution fades (Cu²⁺ ions removed) |
| pH change | Solution becomes acidic (H₂SO₄ forms) |
| Gas test at anode | Glowing splint relights |
With Copper Electrodes (Reactive Electrodes)
When both electrodes are made of copper metal, the electrolysis behaviour changes fundamentally at the anode. Unlike inert electrodes, copper electrodes are reactive — the anode itself participates in the reaction by dissolving. This is the principle underlying copper electroplating and electrorefining.
At the Cathode (Negative Electrode)
The cathode reaction is identical to that observed with inert electrodes. Copper(II) ions from the solution migrate to the cathode and are reduced to copper metal.
Half-equation at the cathode (reduction):
Cu²⁺(aq) + 2e⁻ → Cu(s)
Observation: A pink-brown deposit of pure copper metal plates onto the cathode. The cathode increases in mass as copper accumulates.
At the Anode (Positive Electrode)
This is where the difference from inert electrode electrolysis is most profound. The copper anode is not inert — it can be oxidised. Copper atoms in the anode lose electrons and enter the solution as Cu²⁺ ions, effectively dissolving the anode. This occurs because the oxidation of copper metal to Cu²⁺ ions requires less energy than the oxidation of hydroxide ions. Therefore, the copper anode dissolves in preference to any anion being discharged.
Half-equation at the anode (oxidation):
Cu(s) → Cu²⁺(aq) + 2e⁻
Observation: The copper anode gradually dissolves and loses mass. The surface of the anode becomes pitted and eroded as copper atoms enter the solution. No gas is evolved at the anode.
Mass and Concentration Changes
A critically important feature of this setup is the dynamic equilibrium established between the two electrodes:
- Cathode mass: Increases (copper deposited from solution)
- Anode mass: Decreases (copper dissolves into solution)
- Net effect on solution concentration: The concentration of CuSO₄ remains approximately constant because Cu²⁺ ions are removed from solution at the cathode at the same rate as they are added to the solution at the anode. This is a key distinction from the inert electrode case, where the solution fades in colour.
- Net effect on solution colour: The blue colour intensity stays constant throughout the electrolysis.
Electroplating and Electrorefining
This arrangement is precisely how copper electroplating works: a thin layer of copper is deposited onto a cathode object from a copper anode and copper(II) sulfate electrolyte. In electrorefining (copper purification), an impure copper anode is used; pure copper deposits at the cathode while impurities — including precious metals such as silver and gold — fall to the bottom of the cell as anode sludge or anode slime. The copper produced can exceed 99.99% purity, making this process economically vital.
Comparison Table: Inert Electrodes vs Copper Electrodes
| Aspect | Inert Electrodes (Carbon/Graphite/Pt) | Copper Electrodes |
|---|---|---|
| Cathode reaction | Cu²⁺ + 2e⁻ → Cu(s) | Cu²⁺ + 2e⁻ → Cu(s) |
| Cathode product | Copper metal (pink-brown solid) | Copper metal (pink-brown solid) |
| Anode reaction | 4OH⁻ → O₂ + 2H₂O + 4e⁻ | Cu(s) → Cu²⁺ + 2e⁻ |
| Anode product | Oxygen gas (colourless, relights glowing splint) | Copper(II) ions (anode dissolves) |
| Anode mass | No change (inert) | Decreases (dissolves) |
| Cathode mass | Increases (copper deposits) | Increases (copper deposits) |
| Solution colour | Fades from blue to colourless | Remains blue (constant concentration) |
| Solution concentration | Decreases (Cu²⁺ removed, not replaced) | Stays constant (Cu²⁺ removed = Cu²⁺ added) |
| pH of solution | Decreases (becomes acidic, H₂SO₄ forms) | Stays approximately neutral |
| Gas evolved? | Yes — O₂ at anode | No gas evolved |
| Practical application | Demonstrating selective discharge | Copper electroplating / electrorefining |
| Overall ionic equation | 2Cu²⁺ + 2H₂O → 2Cu + O₂ + 4H⁺ | No net chemical change (Cu transfers between electrodes) |
Key IGCSE Practical Details
- The blue colour of copper(II) sulfate solution is due to the presence of hydrated Cu²⁺ ions. When these ions are discharged and deposited as copper metal, the colour fades proportionally.
- When testing for oxygen at the anode during inert electrode electrolysis, use a glowing splint (a wooden splint with a glowing ember at its tip). The splint relights in oxygen gas.
- If universal indicator is added to the solution during inert electrode electrolysis, the indicator turns red near the anode, confirming the production of acid (H⁺ ions from H₂SO₄ formation).
- In the IGCSE practical, carbon (graphite) rods are the most commonly used inert electrodes because they are inexpensive, readily available, and sufficiently conductive. Platinum electrodes are used in more precise quantitative investigations.
- The mass changes of the electrodes can be measured with a balance before and after electrolysis. For copper electrodes, the loss in mass of the anode equals the gain in mass of the cathode in an ideal system with 100% current efficiency.
- Students should be able to write balanced ionic half-equations for all four electrode reactions, identifying each as either oxidation or reduction in terms of electron transfer.
Sources
- Cambridge IGCSE Chemistry Coursebook (5th Edition), Richard Harwood and Ian Lodge, Cambridge University Press, 2021, Chapter 5: Electricity and Chemistry, pp. 102–115.
- Cambridge IGCSE Chemistry Syllabus 0620, Topic 4: Electrochemistry, Core and Supplement objectives.
- Edexcel IGCSE Chemistry Student Book, Jim Clark, Pearson, 2017, Section 1(i): Electrolysis.
- “Electrolysis of Copper(II) Sulfate,” Royal Society of Chemistry (RSC) Education Resources, accessed via rsc.org/learn-chemistry.
- AQA GCSE Chemistry (8462) Required Practical Handbook, Practical 3: Electrolysis, 2022.
Common Misconceptions
- Misconception 1: “Sulfate ions are discharged at the anode.” This is incorrect. SO₄²⁻ ions are very stable and require a very high oxidation potential. OH⁻ ions from water are discharged preferentially, producing oxygen gas. The sulfate ions remain in solution unchanged.
- Misconception 2: “Copper forms at the anode in the copper electrode experiment.” This is incorrect. Copper dissolves at the anode (oxidation: Cu → Cu²⁺ + 2e⁻) and deposits at the cathode (reduction: Cu²⁺ + 2e⁻ → Cu). Reduction always occurs at the cathode and oxidation always occurs at the anode.
- Misconception 3: “Both electrodes gain mass when using copper electrodes.” Only the cathode gains mass. The anode loses mass because it dissolves into the solution.
- Misconception 4: “The solution turns colourless with copper electrodes because copper is removed.” In fact, the solution concentration stays approximately constant with copper electrodes because Cu²⁺ ions enter the solution from the dissolving anode at the same rate as they are removed at the cathode. The colour therefore does not fade.
- Misconception 5: “Hydrogen gas is produced at the cathode.” While H⁺ ions are present in the solution, copper is lower in the reactivity series and its ions are more easily reduced. Cu²⁺ is therefore discharged in preference to H⁺, so copper metal — not hydrogen gas — forms at the cathode.