Electrolysis of Ionic Compounds

Objectives

• Explain why ionic compounds conduct electricity when molten or dissolved.
• Describe reactions at the electrodes during electrolysis.
• Identify reactions as reduction or oxidation (Higher tier students).

Background on Ionic Compounds

• Example Used: Lead Bromide (PbBr₂)
  • Formed when lead (Pb) reacts with bromine (Br).
  • Lead atom loses 2 electrons: ( \text{Pb} \rightarrow \text{Pb}^{2+} + 2e^- ).
  • Bromine atoms gain electrons: ( \text{2Br} + 2e^- \rightarrow 2\text{Br}^- ).
  • Results in an ionic compound: PbBr₂, containing ( \text{Pb}^{2+} ) and ( \text{Br}^- ) ions.

Key Facts About Electrolysis

• Solid Ionic Compounds:

  • Cannot conduct electricity.
  • Ions are locked in a regular pattern by strong electrostatic forces.

• Molten/Dissolved Ionic Compounds:

  • Electrostatic forces are broken.
  • Ions are free to move, allowing the compound to conduct electricity.
  • These liquids/solutions are referred to as electrolytes.

Electrolysis Process

• Molten Lead Bromide (PbBr₂):
  • Contains ( \text{Pb}^{2+} ) (positive ions) and ( \text{Br}^- ) (negative ions).

Electrodes

• Cathode (Negative Electrode):

  • Connected to negative terminal of power pack.
  • Covered with electrons.
  • Reduction Reaction:
    • ( \text{Pb}^{2+} ) ions gain electrons to form lead atoms (( \text{Pb}^{2+} + 2e^- \rightarrow \text{Pb} )).

• Anode (Positive Electrode):

  • Connected to positive terminal of power pack.
  • Lacks electrons.
  • Oxidation Reaction:
    • ( \text{Br}^- ) ions lose electrons to form bromine atoms (pairs to form ( \text{Br}2 )).

Next Steps

• Future videos will cover using electrolysis to extract reactive metals like aluminum.
• Practice questions available in the revision workbook.

Conclusion

• Ionic compounds conduct electricity when molten or dissolved because ions are free to move.
• At electrodes:
  • Positive ions undergo reduction at the cathode.
  • Negative ions undergo oxidation at the anode.
• Understanding of reduction and oxidation reactions is crucial for higher tier students.