Electrolysis

• Ionic compounds


  Can conduct electricity when molten or dissolved in water
• Electrolysis


  1. Reactions at positive electrode (anode)

  2. Reactions at negative electrode (cathode)
• Reduction


  Ions gaining electrons
• Oxidation


  Ions losing electrons
• Solid ionic compounds cannot conduct electricity because the ions are locked in place and not free to move
• When an ionic compound is melted or dissolved in water, the forces of attraction are broken and the ions are free to move
• Electrolyte


  Liquids or solutions containing free-moving ions that can conduct electricity
• Cathode


  • Negative electrode, covered with electrons from power supply
• Anode


  • Positive electrode, lacking electrons
• Reactions at cathode (negative electrode)

  Positive ions (e.g. lead ions) gain electrons to form atoms (e.g. lead atoms) - reduction
• Reactions at anode (positive electrode)


  Negative ions (e.g. bromide ions) lose electrons to form atoms (e.g. bromine atoms) - oxidation
• Bromine atoms pair to form bromine molecules
• Electrolysis can be used to extract reactive metals like aluminium
• Electrolysis


  Used to extract some metals
• Extracting aluminium by electrolysis

  1. Mix aluminium oxide with compound cryolite to lower melting point

  2. Apply electric current to molten aluminium oxide

  3. Aluminium ions attracted to cathode and gain 3 electrons to form aluminium atoms

  4. Oxide ions attracted to anode and lose 2 electrons to form oxygen atoms
• Half equation for cathode reaction

  Al3+ + 3e- → Al
• Half equation for anode reaction

  2O2- → O2 + 4e-
• Oxygen atoms formed at anode pair up to form oxygen molecules O2
• Anode must be replaced regularly as the oxygen reacts with the graphite to form carbon dioxide
• Extracting metals by electrolysis

  • Requires a lot of energy to melt the compounds
  • Requires a lot of energy to produce the electric current