Electrolysis of Aqueous Solutions (Edexcel GCSE Chemistry): Revision Note

Electrolysis of Aqueous Solutions

Rules:

• Aqueous solutions will always have water present

• Some water molecules split up into hydrogen and hydroxide ions, H⁺ and OH^–, which participate in the electrolysis reactions

Positive electrode (anode)

• Negatively charged OH^– ions and non-metal ions are attracted to the positive electrode (anode)

• If halide ions (Cl^-, Br^-, I^-) and OH^- are present:

  • The halide ion is discharged at the anode

  • It loses electrons and forms the halogen (chlorine, bromine or iodine)

• If no halide ions are present:

  • OH^- is discharged at the anode

  • It loses electrons and forms oxygen gas

• In both cases, the other negative ion remains in solution

Negative electrode (cathode)

• H⁺ ions and metal ions are attracted to the negative electrode (cathode)

  • But only one will gain electrons

  • So, either hydrogen or a metal will be produced

• If the metal is above hydrogen in reactivity series, hydrogen will be produced

  • This is seen as bubbling at the cathode

The reactivity series of metals enables chemists to predict the products at the cathode in the electrolysis of aqueous solutions

Electrolysing aqueous solution where there are gaseous products

• The apparatus can be modified for the collection of gases by using inverted test tubes over the electrodes

• The electrodes are made from graphite

  • Graphite is used because it is inert and does not interfere with the electrolysis reactions

The electrolysis of aqueous solutions using inverted test tubes to collect gases at the electrodes

Using named electrolytes

• The products at the electrodes from solutions of copper chloride, sodium chloride, sodium sulfate and water acidified with sulfuric acid are:

Explaining the products

• Sodium chloride:

  • Sodium is above hydrogen in the reactivity series

    • This means that hydrogen ions are preferentially discharged at the negative electrode (cathode)

  • Chlorine is a halogen

    • This means that it is preferentially discharged at the positive electrode (anode)

• Copper chloride:

  • Copper is below hydrogen in the reactivity series

    • This means that copper(II) ions are preferentially discharged at the negative electrode (cathode)

  • Chlorine is a halogen

    • This means that it is preferentially discharged at the positive electrode (anode)

• Sodium sulfate:

  • Sodium is above hydrogen in the reactivity series

    • This means that hydrogen ions are preferentially discharged at the negative electrode (cathode)

  • Hydroxide ions are preferentially discharged over sulfate ions

    • This means that oxygen is produced at the positive electrode (anode)

• Dilute sulfuric acid (acidified water):

  • Hydrogen ions are discharged at the negative electrode (cathode)

  • Oxygen from water molecules is preferentially discharged at the positive electrode (anode)