Required Practical 21: Electrolysis of Solutions (AQA GCSE Combined Science: Synergy: Physical Sciences): Revision Note

Required practical 21: Electrolysis of aqueous solutions

Objective

To investigate what happens when aqueous solutions are electrolysed using inert electrodes

Hypothesis

1. When a copper salt solution is electrolysed, copper metal will be deposited at the cathode because copper ions (Cu²⁺) are positive and migrate towards the negative electrode.

2. When solutions of sodium salts are electrolysed, hydrogen gas will be produced at the cathode because hydrogen ions from water are preferentially discharged over sodium ions.

Materials

• Test tubes

• Electrolyte solutions

• 100 cm³ beaker

• Stand and clamp

• Two carbon rod electrodes

• Two crocodile / 4 mm plug leads

• Low voltage power supply

• Blue litmus paper

Electrolytic cell 

Practical Tips

1. Make sure the test tubes do not cover the electrodes completely and fall to the bottom of the cell or the conductivity will fall considerably and the rate of electrolysis will be very slow

2. Replacing the test tubes with graduated test tubes or measuring cylinders means that the volume of gas produced over time can be measured

Method

1. Set up the apparatus as shown in the diagram

2. Add the aqueous solution to the beaker

3. Add two graphite rods as the electrodes and connect this to a power pack or battery

4. Turn on the power pack or battery and allow electrolysis to take place

5. Record the results in a suitable table (see below) and repeat for another solution, checking the electrodes in between runs to see if any metal has been deposited

6. The following aqueous solutions are suitable for this investigation: copper chloride, copper sulfate, sodium chloride, sodium bromide, sodium nitrate

7. The gases produced can be collected in the test tubes to be tested later

Results: Record your results in a suitable table:

Electrolysis of aqueous solutions table

Evaluation

The gases and corresponding tests are:

• Hydrogen – lighted splint goes out with a squeaky pop

• Oxygen – a glowing splint relights

• Chlorine – damp blue litmus paper turns red and is then bleached white

Conclusion

The results match the hypothesis for copper salt solutions, copper metal is deposited at the cathode, confirming that metal ions are preferentially discharged.

For sodium salt solutions, hydrogen gas is produced at the cathode instead of sodium metal, because sodium ions are too reactive to be discharged.

At the anode, halide ions (Cl^-, Br^-) are discharged in preference to OH^- ions, producing chlorine or bromine gas. Where no halide is present, oxygen is produced instead.