Investigating the electrolysis of aqueous solutions and electroplating

It is important in this specified practical to use appropriate apparatus to make and record a range of measurements accurately, including mass and time. This includes the safe use of a range of equipment and substances.

There are a number of ways that you could investigate the electrolysis of aqueous solutions. 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(II) sulfate solution using inert (unreactive) electrodes.

Method

Two carbon electrodes are places in a copper(II) sulfate solution and attached to a 12 V DC power supply.
  1. Pour some copper(II) sulfate solution into a beaker.
  2. Place two graphite rods into the copper(II) sulfate solution. Attach one electrode to the negative terminal of a dc supply, and the other electrode to the positive terminal.
  3. Make sure the electrodes do not touch each other, then turn on the power supply.
  4. Leave the electrical supply switched on for about five minutes, then turn off and disconnect the dc supply.
  5. Remove the electrodes from the copper(II) sulfate solution.

Results

Record your observations at each electrode.

Analysis

Discuss, with reasons, which product would be expected at each electrode in this experiment.

Evaluation

Question

Suggest an explanation for why it may be difficult to test for the gas produced in this experiment.

It would be hard to collect the gas and the volume collected would be small, so it would be difficult to carry out a test. Also, the gas may have reacted with the graphite electrode to produce carbon dioxide.

Part 2 - Investigation using copper electrodes

Aims

To investigate the electrolysis of copper(II) 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(II) ions gain electrons and form copper atoms

This process is used industrially to purify copper.

Method

Diagram of copper electrodes in a copper(II) sulfate solution, with a variable resistor on one side of the power supply and an ammeter the other.
  1. Pour some copper(II) sulfate solution into a beaker.
  2. Clean two pieces of copper foil. Measure and record the mass of one piece of copper foil. Attach it to the negative terminal of a dc supply, and dip the copper foil into the copper(II) sulfate solution.
  3. Repeat step 2 with the other 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.

Evaluation

Copper is deposited on the negative electrode while copper ions are lost from the positive electrode. The gain in mass by the negative electrode should be the same as the loss in mass by the positive electrode. Explain whether your results show this.

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(II) sulfate solutionCauses skin irritationWear gloves
Copper(II) sulfate solutionCauses serious eye irritationWear eye protection
PropanoneHighly flammable liquid and vapourKeep away from naked flames
PropanoneMay cause drowsiness or dizzinessUse it in a fume cupboard
Direct current electricity supplyElectric shockMake sure that electricity supply is switched off before handling apparatus

Fran Scott demonstrates how to perform practical experiments with electrolysis