Core Practical: Electrolysis of Copper(II)Sulfate (Edexcel GCSE Chemistry: Combined Science)

Part 1- Electrolysis with Passive Electrodes

Aim:

To electrolyse copper(II) sulfate solution using inert(graphite) electrodes

Diagram:

Apparatus for the electrolysis of copper(II)sulfate using passive(inert) electrodes

Method: (Graphite electrodes)

• Pour copper sulfate solution into a beaker
• Place two graphite rods into the copper sulfate solution. Attach one electrode to the negative terminal of a DC supply, and the other electrode to the positive terminal
• Completely fill two small test tubes with copper sulfate solution and position a test tube over each electrode as shown in the diagram
• Turn on the power supply and observe what happens at each electrode
• Test any gas produced with a glowing splint and a burning splint
• Record your observations and the results of your tests

Analysis of results:

• Record observations of what happens at each electrode, including the results of the gas tests

Conclusion:

• Copper metal is formed at the negative electrode and oxygen gas is formed at the positive electrode

Part 2: Electrolysis with Active Electrodes

Aim:

Apparatus for the electrolysis of copper(II)sulfate using active electrodes

Method: (copper electrodes)

• Pour copper sulfate solution into a beaker
• Measure and record the mass of a piece of copper foil. Attach it to the negative terminal of a DC supply, and dip the copper foil into the copper sulfate solution
• Repeat with another piece of copper foil, but this time attach it to the positive terminal
• Make sure the electrodes do not touch each other, then turn on the power supply
• Adjust the power supply to achieve a constant current and leave for 20 minutes
• Remove one of the electrodes and wash it with distilled water, then dip it into propanone
• Lift the electrode out and allow all the liquid to evaporate. Do not wipe the electrodes clean. Measure and record the mass of the electrode
• Repeat with the other electrode making sure you can identify which electrode is which
• Repeat the experiment with fresh electrodes and different currents.

• Record the currents used and the masses of each electrode in suitable table format
• Calculate the change in mass of each electrode

Conclusion:

• The cathode increases in mass while the anode decreases
• This occurs as copper atoms are oxidised at the anode and form ions while copper ions are reduced at the cathode, forming copper atoms
• The gain in mass by the negative electrode is the same as the loss in mass by the positive electrode
• Therefore the copper deposited on the negative electrode must be the same copper ions that are lost from the positive electrode
• That implies that the concentration of the Cu²⁺ ions in the solution remains constant

Hazards, risks and precautions

Hazard symbols to show substances that are corrosive, harmful to health and flammable

• Copper(II) sulfate solution is corrosive and harmful to health as it is a skin irritant and can cause serious eye damage 
• Propanone, which is often used to clean the electrodes, is flammable
• Avoid contact with the skin and use safety goggles when handling copper(II) sulfate solution
• Propanone should be kept away from naked flames, e.g. a Bunsen burner

Copper refining

• The electrolysis of CuSO₄ using graphite rods produces oxygen and copper
• By changing the electrodes from graphite to pure and impure copper, the products can be changed at each electrode
• Electrolysis can be used to purify metals by separating them from their impurities
• In the set-up, the impure metal is always the anode, in this case the impure copper
• The cathode is a thin sheet of pure copper
• The electrolyte used is an aqueous solution of a soluble salt of the pure metal at the anode, e.g. CuSO₄
• Copper atoms at the anode lose electrons, go into solution as ions:

Cu ⟶ Cu²⁺  +  2e

• The anode thus becomes thinner due to loss of atoms and the impurities fall to the bottom of the cell as sludge
• The copper(II) ions are attracted to the cathode where they gain electrons and form now purified copper atoms
• The cathode gradually becomes thicker

Cu²⁺  +  2e^- ⟶ Cu

• The anode sludge is a highly valuable material and is further refined as it often contains small quantities of precious metals like silver which are found as impurities in the unrefined copper