Electrolysis of aqueous copper(II) sulfate using inert (graphite) electrodes
- In aqueous copper(II) sulfate solution, we have copper(II) and hydrogen cations, and sulfate and hydroxide anions. The cations are attracted to the cathode, and anions are attracted to the anode.
- According to the electrochemical series, copper(II) ions are easier to discharge compared to hydrogen ions, so copper(II) cations are preferentially discharged and form copper metal on the cathode.
- At the anode, sulfate ions do not get discharged, so only the hydroxide ions get discharged to form oxygen gas.
- Thus, reddish brown copper metal deposits will be found on the cathode, and colourless oxygen bubbles are observed at the anode.
- As more copper(II) ions are discharged, the solution becomes more dilute and the blue colour fades. Also, since hydroxide ions are discharged while hydrogen ions remain, the solution gets more acidic over time.
Electrolysis of aqueous copper(II) sulfate using reactive electrodes (copper)
- Similarly, we have copper(II) and hydrogen cations, and sulfate and hydroxide anions in this solution. Solid copper electrodes are also present.
- Just like the previous experiment, copper(II) cations get discharged and reddish brown copper metal is deposited on the cathode.
- Unlike the previous experiment, instead of oxygen gas formed at the anode, the copper anode gets oxidised. There is no gas formed.
- The neutral copper atoms of the anode get oxidised and lose electrons to become copper(II) ions, which then dissolves into the solution. As copper(II) ions are constantly added to the solution, it continues to remain blue and does not fade.
- Thus, copper moves from the anode to the cathode, resulting in the cathode increasing in size due to the copper metal deposits, while the anode shrinks in size and impurities fall below the anode. This technique can be used to purify copper metal.
Electroplating
- Metals like copper, silver, gold, nickel and chromium can be used to plate other metals.
- Metals are electroplated to improve their appearance and increase resistance to corrosion. Cheaper metals can be made to look much better by electroplating a layer of precious metal like silver or gold onto it, such as silverware and gold plated jewellery.
- For this process, a thin layer of metal is plated electrolytically on the surface of another metal. This forms a solid metallic film on the surface of the object.
- The object to be plated is used as the cathode, while the metal to be used as plating is used as the anode. Then, the electrolytic cell is switched on to begin the process.
- Copper(II) sulfate solution is used as the electrolyte to plate copper, while silver nitrate solution is used to plate silver.

4.03 - Electrolysis (III)

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