Core practical - electrolysis of copper sulfate solution
Investigating the electrolysis of copper sulfate solution
There are a number of ways to investigate the The decomposition (breakdown) of a compound using an electric current. of copper sulfate solution in GCSE Chemistry. This is an outline of the required steps to undertake one of these methods.
Part 1 - Investigation using inert electrodes
Aims
To investigate the electrolysis of copper sulfate solution using inert (unreactive) A conductor used to establish electrical contact with a circuit. The electrode attached to the negative terminal of a battery is called a negative electrode, or cathode. The electrode attached to the positive terminal of a battery is the positive electrode, or anode..
Method
- Pour some 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.
Results
Record observations of what happens at each electrode, including the results of the gas tests.
| Negative electrode | Positive electrode | |
| Observations | A brown/pink solid forms | Bubbles of a colourless gas form |
| Gas test | The gas relights a glowing splint |
| Observations | |
|---|---|
| Negative electrode | A brown/pink solid forms |
| Positive electrode | Bubbles of a colourless gas form |
| Gas test | |
|---|---|
| Negative electrode | |
| Positive electrode | The gas relights a glowing splint |
Analysis
Use your observations and results to draw conclusions on the changes that occur at each electrode:
- copper metal is formed at the negative electrode
- oxygen gas is formed at the positive electrode
Evaluation
Question
Suggest an explanation for why it may be difficult to obtain positive results for the tests on gases.
The volumes of gas collected are very small, making it difficult to carry out the tests.
Part 2 - Investigation using copper electrodes
Aims
To investigate the electrolysis of copper sulfate solution using non-inert electrodes.
Copper electrodes take part in the reactions and are described as non-inert:
- at the positive electrode, copper The smallest part of an element that can exist. lose Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons. and form copper Electrically charged particle, formed when an atom or molecule gains or loses electrons., Cu2+
- at the negative electrode, copper ions gain electrons and form copper atoms
This process is used industrially to purify copper.
Method
- Pour some 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 step 2 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 as directed by your teacher.
- After 20 minutes, turn off the dc supply.
- 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. Do not wipe the electrodes clean. Measure and record the mass of the electrode.
- Repeat step 6 with the other electrode. Make sure you know which is which.
- Repeat the experiment with fresh electrodes and different currents.
Results
Record the currents used and the masses of each electrode in suitable tables:
Mass of negative electrode
| Current (A) | At start (g) | At end (g) | Change (g) |
| Current (A) | |
|---|---|
| At start (g) | |
| At end (g) | |
| Change (g) |
Mass of positive electrode
| Current (A) | At start (g) | At end (g) | Change (g) |
| Current (A) | |
|---|---|
| At start (g) | |
| At end (g) | |
| Change (g) |
Analysis
- calculate the change in mass of each electrode
- plot a graph to show:
- change in mass of the negative electrode on the vertical axis
- current on the horizontal axis
Make sure you choose suitable scales so that at least 50% of the graph area includes plotted points. Draw a line of best fit through these points.
As the current is increased the change in mass of the electrodes becomes greater.
Evaluation
Draw conclusions about what is happening during the electrolysis:
The gain in mass by the negative electrode is the same as the loss in mass by the positive electrode. So the copper deposited on the negative electrode must be the same copper ions that are lost from the positive electrode.
Question
For a given time, the gain in mass by the negative electrode should be directly proportional to the current used. Explain how the graph should show this.
The graph should show a straight line, passing through the origin and with a positive gradient.
Hazards, risks and precautions
It is important in this practical activity to use appropriate apparatus and methods. This includes the safe use and careful handling of substances.
Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:
| Hazard | Harm | Precaution |
| Copper sulfate solution | Causes skin and serious eye irritation | Wear gloves and eye protection |
| Propanone | Highly flammable liquid and vapour - may cause drowsiness or dizziness | Keep away from naked flames - use it in a fume cupboard |
| Hazard | Copper sulfate solution |
|---|---|
| Harm | Causes skin and serious eye irritation |
| Precaution | Wear gloves and eye protection |
| Hazard | Propanone |
|---|---|
| Harm | Highly flammable liquid and vapour - may cause drowsiness or dizziness |
| Precaution | Keep away from naked flames - use it in a fume cupboard |
Fran Scott demonstrates how to perform practical experiments with electrolysis