Electrochemistry

• What is electrolysis 


  Electrolysis is the decomposition of a liquid electrolyte using a direct current of electricity
• Electrolyte 


  This is a liquid or solution that conducts electricity and is decomposed by it
• Electrolysis 


  Two graphite rods, placed in a liquid and connected externally to a power supply such as battery or power pack, can be used to test if a liquid conducts electricity. If a liquid conducts and is decomposed by it, then electrolysis is taking place

  • The graphite rods used in electrolysis are called electrodes. They are inhert electrodes as they don’t take part in reaction
  • Graphite is the main material used to make electrodes because it is a good conductor of electricity and unreactive
  • Negative electrode is a cathode, positive electrode is an anode
• How electrolysis works 


  All electrolytes conduct electricity because they have free ions that can move and carry charge. When positive and negative ions are free to move, the cations (+) move to the negative electrode which is a cathode. Anions (-) move to the positive electrode which is an anode
• Electrolysis works 2

  • Positive ions at negative electrode gain electrons to become atoms
  • Gain of electrons is reduction - happens at cathode
  • Negative ions at positive electrode lose electrons to become atoms which may combine to form diatomic molecules for elements such as Cl2 and Br2
  • Loss of electrons is oxidation - happens at anode
  • Molten ionic compounds and aqueous ionic compounds are the most common electrolytes
• Electrolysis of molten ionic compounds

  A molten ionic compound contains ions that are free to move and carry charge. Apparatus:

  • battery
  • cathode and anode
  • pipe clay triangle
  • heatproof mat
  • tripod
  • crucible
• Electrolysis of molten ionic compounds - observations and half equations

  • First identify positive metal ions and negative non-metal ions
  • Positive metal ions in molten compound are attracted to negative cathode, where ions gain electrons to form atoms
  • Negative non-metal ions are attracted to positive anode where ions lose electrons to form atoms. For diatomic elements, 2 atoms combine to form a molecule
• Molten lead (II) bromide, PbBr2

  PbBr2 contains the following ions:

  • Cation - Pb2+
  • Anion - Br-
  • Cathode: positive ion, Pb2+ attracted to cathode
  • Half equation: Pb2+ + 2e- -> Pb
  • Observation: Silvery grey liquid formed, which sinks to bottom
  • Anode: negative ion, Br- attracted to anode
  • Half equation: 2Br- -> Br2+ + 2e-
  • Observation: red-brown pungent gas evolved
• Electrolysis of Molten Lithium Chloride, LiCl

  LiCl contains ions:

  • Cation - Li+
  • Anion - Cl-
  • Cathode: positive ion, Li+, attracted to cathode
  • Half equation: Li+ + e- -> Li
  • Observation: Silvery grey liquid formed
  • Anode: negative ion, Cl-, attracted to anode
  • Half equation: 2Cl- -> Cl2 + 2e-
  • Observations: yellow-green pungent gas evolved
• Electrolysis of molten sodium iodide, NaI


  Sodium iodide, NaI, contains ions

  • Cation - Na+
  • Anion - I-
  • Cathode: Na+ attracted to cathode
  • Half equation: Na+ + e- -> Na
  • Observations: silvery grey liquid formed
  • Anode: I- attracted to anode
  • Half equation: 2I -> I2 + 2e-
  • Observation: purple, pungent gas evolved
• Extraction of aluminium form its ore 


  Aluminium metal is extracted from its ore using electrolysis - aluminium ore is called bauxite

  Bauxite is purified to form aluminium oxide (alumina). The alumina is dissolved in molten cryolite to reduce operating temperature and increase its conductivity
• Extraction of aluminium

  • A crust of aluminium oxide keeps heat in. The operating temperature is between 900 and 1000°C
  • Cathode + anode are both made of carbon
  • Reaction of cathode is Al3+ + 3e- -> Al
  • Reaction of anode is 2O2- -> O2 + 4e-
  • Carbon anode has to be replaced periodically because it wears away as it reacts with oxygen : C + O2 -> CO2
  • Reaction at cathode is reduction because Al ions are gaining electrons
  • Reaction at anode is oxidation because oxide ions are losing electrons
• Extraction costs

  • Extraction of aluminium is expensive because because I’ll electricity is high and high temperatures are needed to keep the aluminium oxide molten. Use of crytolite Increases the conductivity and reduces operating temperature, saving money.
  • Expanse of recycling aluminium is only a fraction of the cost of recycle materials such as Al. It saves resources, energy, prevents waste going to landfill and costs less