5.2-electrolysis with copper electrodes

• Electrolysis

  The decomposition (breakdown) of a compound using an electric current
• Electrolysis

  • The products of electrolysis can be predicted for a given electrolyte
  • Copper can be purified using electrolysis
• Electrolysis of copper sulfate solution

  1. Pour copper sulfate solution into a beaker

  2. Place two graphite rods into the copper sulfate solution

  3. Attach one electrode to the negative terminal of a dc supply, and the other electrode to the positive terminal

  4. Completely fill two small test tubes with copper sulfate solution and position a test tube over each electrode

  5. Turn on the power supply and observe what happens at each electrode

  6. Test any gas produced with a glowing splint and a burning splint
• At the negative electrode, a brown/pink solid forms
• At the positive electrode, bubbles of a colourless gas form
• The gas relights a glowing splint
• Copper metal is formed at the negative electrode
• Oxygen gas is formed at the positive electrode
• The volumes of gas collected are very small, making it difficult to carry out the tests
• Electrolysis of copper sulfate solution using copper electrodes

  1. Pour some copper sulfate solution into a beaker

  2. Measure and record the mass of a piece of copper foil

  3. Attach it to the negative terminal of a dc supply, and dip the copper foil into the copper sulfate solution

  4. Repeat with another piece of copper foil, but this time attach it to the positive terminal

  5. Make sure the electrodes do not touch each other, then turn on the power supply

  6. Adjust the power supply to achieve a constant current as directed by your teacher

  7. After 20 minutes, turn off the dc supply

  8. Carefully remove one of the electrodes, gently wash it with distilled water, then dip it into propanone, lift the electrode out and allow all the liquid to evaporate

  9. Measure and record the mass of the electrode

  10. Repeat with the other electrode
• The gain in mass by the negative electrode is the same as the loss in mass by the positive electrode
• The copper deposited on the negative electrode must be the same copper ions that are lost from the positive electrode
• For a given time, the gain in mass by the negative electrode

  Should be directly proportional to the current used
• Copper sulfate solution causes skin and serious eye irritation
• Propanone is highly flammable liquid and vapour - may cause drowsiness or dizziness
• Wear gloves and eye protection when handling copper sulfate solution
• Use propanone in a fume cupboard, keep away from naked flames