Electrolysis

• Electrolysis - the splitting of a compound using electricity (DC)
• For an aqueous solution at the positive electrode, oxygen is always produced, except when there is a group 7 element present
• For an aqueous solution, at the negative electrode hydrogen is always produced, except when there is a precious metal present
• Precious metals:

  • Gold
  • Silver
  • Platinum
  • Palladium
• Anode - positive electrode
• Cathode - negative electrode
• For electrolysis to happen the compound that is being split needs to be molten (for an insoluble compound) or aqueous. This is so ions can move around and travel to the electrodes.
• Electrolyte - a solution that has ions dissolved in it
• What happens during electrolysis - Molten ionic compounds :

  1. Negative non-metal ions move towards the anode, and ion is oxidised (loss of electrons) to become an atom (e.g. bromide ions, 2Br- → Br2 + 2e-).
  2. Electrons lost during oxidation are transfered to cathode, so metal ions can use them for reduction.
  3. Positive metal ions move towards the cathode, and the ion is reduced (gain of electrons) to become an atom (e.g. lead ions, Pb2+ + 2e- → Pb).
• Electrolysis example - Lead Bromide, PbBr2:

  1. 2Br- ions move towards the anode
  2. 2Br- are oxidised, so loss of electrons to become Br (bromine atom)
  3. Lost electrons from oxidation of bromide are carried to the cathode so the lead can use them to reduce
  4. Pb2+ ions move towards the cathode
  5. Pb2+ are reduced, so electrons gained to become Pb (lead atom)
  6. Molten lead metal sinks to the bottom
  7. Bromine gas is given off
• Rules for electrolysis of an aqueous ionic compound:

  • If metal at cathode is less reactive than hydrogen then it will be discharged if not hydrogen will be discharged, (basically less reactive element is discharged)
  • Oxygen and water are produced from OH- at the anode if no group 7 elements are present
  • As compound will be dissolved in water there will be 4 ions (metal, non-metal, hydrogen H+, hydroxide OH-)
• Rules for electrolysis of a molten compound;

  • Metal is produced at the cathode.
  • Non-metal is produced at the anode.
• Test for chlorine gas is using litmus paper. If chlorine is present then it will bleach.
• Test for hydrogen gas is lighting a splint and placing it near the test tube/beaker. If hydrogen is present then there will be a squeaky pop.
• Test for oxygen gas is to place a glowing splint (blown out) near the test tube/beaker. If oxygen is present then it will relight.
• Explain why is oxygen gas produced in electrolysis; (exam q)

  • Water molecules will break down/dissociate to produce OH- ions.
  • OH- ions will be attracted to the anode.
  • Where the OH- ions will be oxidised (loss of electrons).
• Explain why hydrogen gas is produced in electrolysis; (exam q)

  • Water molecules are broken down to produce hydrogen ions, H+.
  • H+ ions are discharged at cathode, as the metal is more reactive then hydrogen.
  • So H+ ions are reduced (gain of electrons).
  • To produce hydrogen molecule.
• Graphite is usually used for electrodes as it is inert. This is helpful as they won't react with the electrolyte.
• Reasons that can affect the actual mass created to be different from predicted mass during electrolysis;

  1. Concentration / Volume of solution was different.
  2. There were impurities in solution.
  3. Voltage / Current was different.
  4. Time circuit was switched on.
  5. Some of the product fell off.
• Extraction of Aluminium from its ore;

  1. First the aluminium ore is purified to aluminium oxide.
  2. Then, as aluminium oxide needs to be melted to allow ions to move freely, but as it has a very high melting point it is very expensive. So instead it is dissolved in cryolite. This lowers the boiling point.
  3. Finally, aluminium can be extracted using electrolysis.
  4. Throughout this process oxygen is constantly produced, and reacts with carbon, in anode, to produce carbon dioxide, causing electrode to burn away. Therefore, the graphite anode needs to be constantly replenished.