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Electrolysis of Concentrated Sodium Chloride

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A series of free IGCSE Chemistry Activities and Experiments (Cambridge IGCSE Chemistry).

Electrolysis of Brine
The aim of this experiment is to investigate the products formed when a concentrated solution of sodium chloride (brine) is electrolysed.

Method
1. Clamp the electrolytic cell to the stand and half-fill with concentrated sodium chloride solution.
2. Fill the two small test tubes with concentrated sodium chloride solution and invert them over the electrodes as shown in the diagram.
3. Connect the electrodes to the power supply using the wires and clips.
4. Allow the gases to collect in the test tubes.
5. Disconnect the circuit when one of the test tubes has filled with gas.
6. Carry out the following tests on the gases and the solution remaining in the cell.
a. Test the solution pH using Universal Indicator.
b. Test the gas produced at the negative electrode with a lighted splint.
c. Test the gas produced at the positive electrode with a moist piece of Universal Indicator paper. Do not breathe this gas – it is poisonous.
7. Record your test results in an appropriate table.
Questions
1. Comment on the reactions that have taken place at the electrodes and the solution remaining in the cell.
2. Explain the change in colour of the Universal indicator.
3. Name the 3 main products formed by this process and give one use of each.
4. Explain why this process is economically important.
5. What difference would you expect in the products formed if a very dilute solution had been used in this electrolysis?

Answers
  • Show Solutions
    1. The solution remaining in the cell is sodium hydroxide solution. The H+ ions are discharged at the negative electrode (cathode) and the Cl- ions are discharged at the positive electrode (anode).
    2H+ + 2e- → H2
    2Cl- → Cl2 + 2e-
    The ions H+ and Cl- are removed. Those that remain are Na+ and OH- and they form NaOH.
    2. Sodium Hydroxide is alkaline and changes the Universal Indicator from green to purple.
    3. Hydrogen as a fuel, Chlorine as a disinfectant, Sodium Hydroxide to make bleach.
    4. Salt is cheap and abundant and can be changed by a chemical process into economically important chemicals with large-scale uses.
    5. When dilute NaCl solution is used, oxygen is produced at the anode from the discharge of the OH- ions.
    At the anode: 4OH-(aq) → O2(g) + 2H2O(l) + 4e-
Electrolysis of dilute sodium chloride



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