Investigating Electrolysis (WJEC GCSE Chemistry)

Introduction

• To electrolyse copper(II) sulfate solution using inert(graphite) electrodes followed by the use of copper electrodes

Apparatus

• 250 cm³ beaker
• 2 graphite electrodes
• Beaker suitable for electrolysis
• 12 V DC. power supply  
• Leads and crocodile clips
• 400 cm³ copper(II) sulfate, about 0.5 mol dm^–3  

Diagram

Electrolysis of copper(II) sulfate using graphite electrodes

There are different observations for each electrode

Method

Graphite electrodes

1. Pour copper(II) 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(II) sulfate solution and position a test tube over each electrode as shown in the diagram
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
7. Record observations of what happens at each electrode, including the results of the gas tests

Observations at anode	Observations at cathode

Practical tip

• The anode will always be the one where gas is produced, which means that you will see bubbles

Analysis of Results

Results table

Observations at anode	Observations at cathode
Bubbling seen and gas formed relights a glowing splint	Orange / brown solid forming on electrode

Evaluation

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

Conclusion

• Copper ions are attracted to the negative electrode and are reduced (gain electrons)
  • Cu²⁺ (aq) + 2e^– → Cu (s)
• At the positive electrode, oxygen gas is formed.
  • The equation for this reaction is:
    • 2OH^– (aq) →  O₂ (g) + 2H⁺ (aq) + 4e^–

Using copper electrodes

Apparatus

• 250 cm³ beaker
• 2 copper electrodes
• beaker suitable for electrolysis
• 12 V DC. power supply  
• Leads and crocodile clips
• 400 cm³ copper(II) sulfate, about 0.5 mol dm^–3  

Diagram

Electrolysis of copper sulfate using graphite electrodes

Cooper is used for both the anode and cathode

Method

Copper electrodes

1. Pour copper(II) 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 step 2 and 3 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 and leave for 20 minutes
7. Remove one of the electrodes and wash it with distilled water, then dip it into propanone
8. Lift the electrode out and allow all the liquid to evaporate. Do not wipe the electrodes clean.
9. Measure and record the mass of the electrode
10. Repeat with the other electrode making sure you can identify which electrode is which
11. Repeat the experiment with fresh electrodes and different currents

	Anode	Cathode
Mass before (g)
Mass after (g)

Practical Tip

• If you measure an electrode to see how much copper has been produced (or has been lost), ensure it is dry so you don't get a false reading

Analysis of results

Results

Results table

	Anode	Cathode
Mass before (g)	18.0	18.0
Mass after (g)	18.4	17.6

Evaluation

• Identify whether the mass of the electrodes has changed and by how much
  • The anode has increased in mass by 0.4 g
  • The cathode has decreased in mass by 0.4 g

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