Practical activity

Electrolysis

There are a number of ways that you could carry out electrolysis in Chemistry. This is an outline of the required steps to undertake one of these methods.

Aims

To investigate the electrolysis of copper(II) sulfate solution, and of sodium chloride 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.

Copper(II) sulfate solution

  1. Pour some copper(II) 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. Use a dropping pipette to completely fill two small test tubes with copper(II) sulfate solution. Quickly turn them upside down, keeping their mouth under the surface of the copper(II) sulfate solution and positioning one over the cathode and one over the anode.
  4. Make sure the electrodes do not touch each other, then turn on the power supply. If no bubbles are observed, check all the electrical connections and the dc supply.
  5. Continue to collect any gases in the test tubes, then turn off the dc supply.
  6. Oxygen should be produced at the positive electrode. Confirm this by holding a glowing splint just inside the open mouth of its test tube. Record whether the splint relights.

Sodium chloride solution

  1. Repeat steps 1 to 5, but using sodium chloride solution instead of copper(II) sulfate solution.
  2. Hydrogen should be produced at the negative electrode. Confirm this by holding a lighted splint near the open mouth of its test tube. Record whether a squeaky pop is obtained.
  3. Chlorine should be produced at the positive electrode. Confirm this by holding a piece of damp blue litmus paper just inside the open mouth of its test tube. Record whether this turns red then white.

Results

Record what happens at each electrode, including the results of the gas tests.

Analysis

Identify the gases produced during the two experiments.

Question

Name the brown substance produced at the negative electrode during the electrolysis of copper(II) sulfate.

The brown substance was copper.

Evaluation

Question

Explain why it may be difficult to obtain positive results in the tests for gases.

The volumes of gas collected are very small, making it difficult to carry out the tests.

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:

HazardPossible harmPossible precaution
Copper(II) sulfate solutionCauses skin irritation. Causes serious eye irritation Wear gloves; wear eye protection
Chlorine gasToxic if inhaledMake sure the lab is well ventilated and avoid inhaling the gas; do not run the experiment any longer than is necessary to collect a sample
dc electricity supplyElectric shockMake sure electrodes do not touch; make sure that electricity supply is switched off before handling apparatus

Fran Scott demonstrates how to perform practical experiments with electrolysis

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