Core practical

Investigating the electrolysis of copper sulfate solution

There are a number of ways to investigate the electrolysis 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) electrodes.

Method

Core practical showing the different effects of a positively charged carbon electrode and negatively charged electrod when placed in water.
  1. Pour some copper sulfate solution into a beaker.
  2. 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.
  3. Completely fill two small test tubes with copper sulfate solution and position a test tube over each electrode as shown in the diagram.
  4. Turn on the power supply and observe what happens at each electrode.
  5. Test any gas produced with a glowing splint and a burning splint.
  6. 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 electrodePositive electrode
ObservationsA brown/pink solid formsBubbles of a colourless gas form
Gas testThe gas relights a glowing splint

Analysis

Use your observations and results to draw conclusions on the changes that occur at each electrode:

  1. copper metal is formed at the negative electrode
  2. 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 atoms lose electrons and form copper ions, Cu2+
  • at the negative electrode, copper ions gain electrons and form copper atoms

This process is used industrially to purify copper.

Method

Core practical of two 12v charged copper electrodes in water, with a variable resistor and ammeter
  1. Pour some copper sulfate solution into a beaker.
  2. 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.
  3. Repeat step 2 with another piece of copper foil, but this time attach it to the positive terminal.
  4. 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.
  5. After 20 minutes, turn off the dc supply.
  6. 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.
  7. Repeat step 6 with the other electrode. Make sure you know which is which.
  8. 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)

Mass of positive electrode

Current (A)At start (g)At end (g)Change (g)

Analysis

  1. calculate the change in mass of each electrode
  2. 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:

HazardHarmPrecaution
Copper sulfate solutionCauses skin and serious eye irritationWear gloves and eye protection
PropanoneHighly flammable liquid and vapour - may cause drowsiness or dizzinessKeep away from naked flames - use it in a fume cupboard

Fran Scott demonstrates how to perform practical experiments with electrolysis