Electrolysis of Copper Sulfate Solution

Aim

To investigate the electrolysis of copper(II) sulfate solution using (a) carbon (graphite) electrodes and (b) copper electrodes, and to observe the products at each electrode.

Apparatus

• Beaker (250 cm3)
• Two carbon (graphite) electrodes
• Two copper electrodes (for the second part)
• D.C. power supply or battery (6V)
• Connecting wires with crocodile clips
• Ammeter (optional, to monitor current)
• Copper(II) sulfate solution (approximately 1 mol/dm3)
• Electronic balance (to weigh electrodes before and after)
• Emery paper/sandpaper (to clean electrodes)

Method

Part A: Carbon electrodes

1. Pour approximately 150 cm3 of copper(II) sulfate solution into the beaker.
2. Clean two carbon electrodes with emery paper and connect them to the D.C. power supply using wires and crocodile clips. Note which electrode is connected to the positive terminal (anode) and which to the negative terminal (cathode).
3. Place both electrodes in the solution, ensuring they do not touch each other.
4. Switch on the power supply and set the voltage to approximately 4-6V.
5. Observe both electrodes for 10-15 minutes. Record all observations.
6. Switch off, remove the electrodes, and examine them.

Part B: Copper electrodes

1. Clean two copper electrodes with emery paper. Weigh each one separately and record the masses.
2. Set up the circuit as before, but using the copper electrodes instead of carbon.
3. Electrolyse the copper sulfate solution for 15-20 minutes.
4. Switch off, carefully remove both electrodes, rinse gently with distilled water, and allow to dry.
5. Reweigh each electrode and calculate the change in mass.

Expected results

Part A: Carbon electrodes

• Cathode (negative electrode): A brown/pink coating of copper metal forms on the surface. Cu2+ ions migrate to the cathode and are reduced: Cu2+(aq) + 2e- → Cu(s)
• Anode (positive electrode): Bubbles of gas form. The gas is oxygen, produced by the discharge of hydroxide ions from water: 4OH-(aq) → 2H2O(l) + O2(g) + 4e-
• Solution: The blue colour gradually fades because Cu2+ ions are removed from solution and deposited as copper metal at the cathode.

Part B: Copper electrodes

• Cathode: Gains mass — copper is deposited. Cu2+(aq) + 2e- → Cu(s)
• Anode: Loses mass — the copper anode dissolves. Cu(s) → Cu2+(aq) + 2e-
• Solution: The blue colour remains constant because Cu2+ ions are being replaced at the anode at the same rate they are removed at the cathode.
• Mass change: The mass gained by the cathode should approximately equal the mass lost by the anode.

Analysis

With carbon electrodes, sulfate ions (SO4 2-) are not discharged because they are more stable than hydroxide ions. The hydroxide ions from water are preferentially discharged. Oxygen is released, and the solution becomes more acidic over time (H+ ions are left in solution as OH- ions are discharged).

With copper electrodes, the anode itself dissolves because copper is more easily oxidised than either OH- or SO4 2- ions in this situation. This is the principle behind copper purification: impure copper is used as the anode, pure copper is deposited at the cathode, and impurities fall to the bottom of the cell as “anode sludge.”

Sources of error

• Electrode cleaning: If electrodes are not cleaned thoroughly, impurities affect the mass readings and the appearance of deposits.
• Drying electrodes: After electrolysis, if copper electrodes are not rinsed and dried carefully, the deposited copper may flake off or residual solution may add to the mass.
• Current variation: If the current fluctuates, the rate of deposition changes. Using an ammeter to monitor a steady current improves consistency.
• Short electrolysis time: If the experiment runs for too short a time, the mass changes may be too small to measure accurately.

Exam tips

The most common exam question asks you to compare the results with carbon and copper electrodes. State clearly: with carbon electrodes, the solution fades (Cu2+ removed); with copper electrodes, the colour stays constant (Cu2+ replaced). Examiners also ask for half-equations at each electrode — practise writing these from memory.

Remember that cations go to the cathode (both start with C for positive — wait, cations are positive, cathode is negative, so positive ions go to the negative electrode). The memory aid: Cathode = reduCtion (gain of electrons), Anode = oxidAtion (loss of electrons). Copper purification is a high-value application question worth 3-4 marks.