13. Electrolysis

Outcomes:

> Safely carry out an electrolysis experiment.

> Collect and analyse data to answer questions and demonstrate understanding.

Electrolysis of copper sulfate solution

Aims

The supply of copper-rich ores within the world is seriously limited. Scientists are researching efficient methods of extracting copper from low-grade ores. Plants (phytomining) or bacteria (bioleaching) can be used to concentrate copper solutions from low-grade ores. The copper metal can then be extracted from these solutions by electrolysis. In this activity you will use electrolysis to extract copper from a solution of a copper salt.

Safety

• Wear eye protection.

• Wash your hands carefully after this experiment.

• Wipe up any spilt chemicals using the paper towels.

• Use propanone in a fume cupboard.

Equipment and materials

• 250 cm³ beaker

• two copper electrodes

• dc power supply (6 volt)

• light bulb (small, 6 volt, 5 watt)

• leads and crocodile clips

• 100 cm³ 0.5 mol/dm³ copper sulfate solution

• small piece of emery paper

• permanent marker pen

• paper towels

• mass balance

Method

1 Pour approximately 150 cm³ of copper sulfate solution into the beaker.

2 Clean the copper electrodes with emery paper. Then rinse them clean with water and dry thoroughly with a paper towel.

3 Label the two electrodes as electrode 1 and electrode 2. Then record the mass of both electrodes. Write this in the table below.

4 Set up the circuit as shown in the diagram. Use crocodile clips to attach the leads to the copper electrodes. It is important that the electrodes do not touch. So you should also attach the electrodes to opposite sides of the beaker using these crocodile clips.

5 Turn on the power supply but do not take the voltage above 6 V.

6 If the bulb does not light up, check that you have a complete circuit. Change the bulb and the leads before asking your teacher for help.

7 Once you are certain that there is a current flowing in your circuit, leave the apparatus set up for 10 minutes. Write down any observations you see at either electrode.

8 After 10 minutes, turn off the power supply.

9 Remove the two electrodes. Wash them carefully in water and then in propanone (in a fume cupboard).

10 Allow the propanone to evaporate before recording the mass of each electrode again. Write the new masses in the table below.

Results

Record your results in this table.

Questions

1 Describe how the mass of each electrode has changed.

2 Copper sulfate solution contains Cu²⁺ (copper) ions. Do you think that these ions would be attracted to the positive electrode (anode) or negative electrode (cathode)?

3 Suggest what happens to the copper ions at this electrode.

4 Is the electrode that gains mass the positive electrode (anode) or negative electrode (cathode)?

5 What did you observe at this electrode during the experiment? How would this have affected your results?

6 Which electrode lost mass during the experiment: the positive electrode (anode) or negative electrode (cathode)?

7 What do you think has happened to the copper metal lost from this electrode?

8 How could you make your results more reliable?

Next lesson: 12. Phytomining & Bioleaching