Electrolysis

• Electrolysis uses an electrical current to cause a reaction
• Electrolysis means 'splitting up with electricity'
• During electrolysis, an electric current is passed through an electrolyte (a molten or dissolved ionic compound). The ions move towards the electrodes, where they react, and the compound decomposes
• An electrolyte is just a liquid or solution that can conduct electricity. An electrode is a solid that conducts electricity and is submerged in the electrolyte
• The positive ions in the electrolyte will move towards the cathode (-ve electrode) and gain electrons (they are reduced)
• The negative ions in the electrolyte will move towards the anode (+ve electrode) and lose electrons (they are oxidised)
• +ve and -ve ions moving towards corresponding electrodes creates a flow of charge through the electrolyte
• As ions gain or lose electrons, they form the uncharged element and are discharged from the electrolyte
• Electrolysis of molten ionic solids forms elements
• The electrodes should be inert (not chemically reactive) so they don't react with the electrolyte
• An ionic solid can't be electrolysed because the ions are fixed in positions and can't move
• Molten ionic compounds can be electrolysed because the ions can more freely and conduct electricity
• Molten ionic liquids, e.g. lead bromide, are always broken up into their elements.

  • Positive metal ions are reduced to the element at the cathode:

  Pb2+ + 2e- -> Pb

  • Negative non-metal ions are oxidised to the element at the anode:

  2Br- -> Br2 + 2e-
• Metals can be extracted from their ores using electrolysis
• If a metal is too reactive to be reduced with carbon or reacts with carbon, then electrolysis can be used to extract it
• Extracting metals via electrolysis is very expensive as lots of energy is required to melt the ore and produce the required current
• 1)Aluminium is extracted from the ore bauxite by electrolysis. Bauxite contains aluminium oxide, Al2O3

  
• 2) Aluminium oxide has a very high melting temperature so it's mixed with cryolite to lower the melting point

  
• 3) The molten mixture contains free ions - so it'll conduct electricity

  
• 4) The positive Al3+ ions are attracted to the negative electrode where they each pick up 3 electrons and turn into neutral aluminium atoms. These then sink to the bottom of the electrolysis tank

  
• 5) The negative O2- ions are attracted to the positive electrode where they each lose 2 electrons. The neutral oxygen atoms will then combine to form O2 molecules

  
• At the negative electrode: Reduction - a gain of electrons

  • Al3+ + 3e- -> Al
  • Metals form positive ions, so they're attracted to the negative electrode (cathode)
  • Aluminium is produced at the negative electrode

  
• At the positive electrode: oxidation - a loss of electrons

  • 2O2- -> O2 + 4e-
  • Non-metals form negative ions, so they're attracted to the positive electrode (anode)
  • Oxygen is produced at the positive electrode

  
• The anode is made of carbon and needs to be replaced regularly as it reacts with oxygen to produce carbon dioxide

  
• The overall equation for the electrolysis of aluminium oxide is:

  Aluminium oxide -> aluminium + oxygen

  2Al2O3(l) -> 4Al(l) + 3O2(g)

  
• Cryolite is an aluminium based compound with a lower melting point than aluminium oxide