Electrolysis

• Electrolysis is splitting compounds using electricity.
• The negative ions are called anions and are attracted to the positive electrode which is called the anode.
• The positive ions are called cations and are attracted to the negative electrode, which is called the cathode.
• Electrodes are inert which means they are unreactive. They are made from graphite because it is unreactive and a good conductor of electricity. Platinum can also be used but it is more expensive.
• An electrolyte is a solution or molten ionic solution and can conduct electricity.
• Positive ions gain electrons.
• Negative ions lose electrons.
• Negative ions move towards the anode electrode and they discharge by losing electrons and become atoms.
• Positive ions move towards the cathode electrode and they discharge by gaining electrons and becoming atoms.
• Electrolysis of molten lead(II) Bromide

  • Lead is a molten grey liquid.
  • PbBr ions gain electrons.
  • Lead is reduction.
  • Pb2+ + 2e- --> Pb
• Electrolysis of Molten Lead(II) bromine.

  • Bromine is a red/brown pungent gas.
  • Br- lose electrons.
  • 2Br- --> Br2 + 2e-
  • Bromine is oxidation.
• The main ore of aluminium is bauxite. when it is purified, aluminium oxide (alumina) is formed. for electrolysis to to occur, the ions must be able to move so the alumina must be melted or dissolved. the melting point of alumina is 2072*C. The cost of the heat to melt it would be far too high.
• To reduce the melting point of alumina, you dissolve the alumina in cryolite. This reduces the melting point to around 900oC so it reduces costs and saves energy. The cryolite also increases the conductivity of the alumina.
• The ore is placed in a steel tank called a cell. The cathode is the graphite lining of the cell and graphite anodes dip into the electrolyte.
• The molten aluminium is tapped out of the cell.
• Aluminium ions are attracted to the negative cathode where they discharge by gaining electrons, so they are reduced, to form aluminium metal. the cell is so hot that the molten aluminium is run off at the bottom.
• Oxide ions are attracted to the positive anodes where they discharge by losing electrons, so are oxidised, to form oxygen molecules.
• This oxygen reacts with the graphite anodes and produces carbon dioxide. This means the anodes need to be replaced regularly.
• A crust of aluminium oxide forms on the top of the electrolyte. This crust acts like a lid and keeps heat in, stops impurities entering and prevents the aluminium produced from reacting with air.
• The overall electrolysis of aluminium is expensive because of the high cost of the electricity needed and the high cost of the heat energy needed to melt the compounds and keep them molten.
• Recycling aluminium is much cheaper that extracting from ore because it uses less energy, saves waste going to landfill and saves natural resources of bauxite.