Electrolysis of copper(II) sulfate solution

The electrolysis of copper(II) sulfate solution is the experiment that shows electrolysis is not one fixed process, the products depend on which ions are present and, crucially, on the electrode material. It appears in the Form 5 Redox Equilibrium chapter because it explains selective discharge and sets up copper purification and electroplating. Run both versions below, compare them, then rehearse the Paper 3 answers.

Aim

To investigate the electrolysis of copper(II) sulfate solution using carbon (inert) electrodes and then copper electrodes, to identify the products at each electrode, and to explain the effect of the electrode material.

Apparatus and materials

• Beaker or electrolytic cell
• Two carbon (graphite) electrodes and two copper electrodes
• Battery or d.c. power supply, ammeter, switch and connecting wires
• Copper(II) sulfate solution
• Test tube and a wooden splint (for the gas test)
• Safety goggles

Procedure

1. Pour copper(II) sulfate solution into the beaker.
2. Fit two carbon electrodes and connect them through the switch, ammeter and battery.
3. Close the switch and let the current flow for several minutes.
4. Observe each electrode and note any change in the colour of the solution.
5. Collect the gas released at the anode in a test tube and test it with a glowing splint.
6. Switch off, then replace the carbon electrodes with two copper electrodes.
7. Repeat steps 3 and 4, watching the anode, the cathode and the colour of the solution.
8. Record all observations for both sets of electrodes in a table.

Expected observations

• With carbon electrodes: a brown (pinkish) solid deposits on the cathode; gas bubbles are released at the anode and relight a glowing splint, showing oxygen; and the blue colour of the solution gradually fades.
• With copper electrodes: the cathode becomes coated with fresh copper and appears thicker; the copper anode dissolves and becomes thinner; and the blue colour of the solution stays about the same.

Describe the deposits, the gas and the colour of the solution; do not invent a mass gained or a volume of gas.

Inference and conclusion

With inert carbon electrodes, the products are set by selective discharge. The solution contains copper(II) and hydrogen ions at the cathode; copper(II) is lower in the electrochemical series, so it is discharged in preference and copper is deposited by reduction. At the anode, hydroxide ions are discharged in preference to sulfate ions, and oxygen is released by oxidation. Because copper ions are removed and not replaced, the blue colour fades. With copper electrodes, the cathode reaction is the same, but the copper anode itself is oxidised and dissolves, releasing copper(II) ions into the solution as fast as they are deposited at the cathode. The concentration of copper(II) ions, and so the blue colour, stays constant. In both cases reduction occurs at the cathode and oxidation at the anode, so electrolysis is a redox process; the difference is that a copper anode takes part in the reaction while a carbon anode does not.

Science process skills (Paper 3 style)

• Hypothesis. “The type of electrode used changes the products of the electrolysis of copper(II) sulfate solution, especially the reaction at the anode.” State a testable relationship.
• Variables. The manipulated variable is the type of electrode (carbon or copper); the responding variable is the change at each electrode and the colour of the solution; the controlled variables are the concentration and volume of the solution, the current, the time and the size of the electrodes.
• Tabulating data. Use columns for the electrode material, the cathode observation, the anode observation and the colour change, so the two cases sit side by side.
• Making an inference. From oxygen at the anode and copper at the cathode with carbon electrodes, infer that hydroxide and copper(II) ions are discharged in preference to sulfate and hydrogen ions.
• Operational definition. “An inert electrode is one that conducts the current but does not itself take part in the reaction, shown here by carbon giving oxygen at the anode while copper dissolves instead.”

Safety precautions

• Wear safety goggles, because copper(II) sulfate solution is irritant and harmful if splashed into the eyes.
• Keep the gas test away from other flames, because the oxygen released supports combustion strongly.
• Use only a low-voltage d.c. supply and dry hands, so there is no risk of electric shock.
• Dispose of the copper(II) sulfate solution in the labelled waste container, because it contains copper ions harmful to the environment.

Common errors

• Letting the electrodes touch, which short-circuits the cell so no proper discharge occurs.
• Testing the anode gas with a lighted rather than a glowing splint, which confuses the oxygen test.
• Reporting that the blue fades with copper electrodes; it stays the same because the anode replaces the copper ions.
• Mixing up which electrode gains and which loses mass in the copper-electrode case.
• Using a very concentrated or unusual solution, which can change which ion is discharged and give unexpected products.

How we help

In our online one-to-one SPM Chemistry lessons, taught in English, our teachers set the two versions side by side so you can explain, in one clear paragraph, why the electrode material decides the anode reaction and the fate of the blue colour. Fees start from RM50 per hour, and a paid one-hour trial lesson lets you see the comparison taught before you commit. Because this experiment is the direct basis of copper purification and electroplating elsewhere in SPM Chemistry, mastering the selective-discharge reasoning here pays off across the whole electrochemistry topic in Paper 2 and Paper 3.