Electrolysis

• During electrolysis of a solution:

  • Positive ions are called cations and go to the cathode
  • Negative ions are called anions and go to the anode
  • The electrolyte must be molten or in solution for the ions to be able to move
  • When molten, the first element goes to the cathode and the second element goes to the anode
• Products at the cathode:

  • If the metal is above hydrogen in the reactivity series, hydrogen ions (H+) are attracted to the cathode, forming hydrogen gas
  • If the metal is below hydrogen in the reactivity series, the metal ions are attracted to the cathode and form the metal
• Products at the anode:

  • If a halogen is present, it will form at the anode
  • If no halogen is present, oxygen will form at the anode
  • This is only applicable if in solution
• A halogen is any non-metal from group 7 of the periodic table
• Required practical steps for electrolysis:

  • Put 15cm cubed of solution into a beaker
  • Connect the power block (use 4 volts)
  • Turn on power and leave for 5 minutes
  • Hold blue litmus paper just above the anode
  • Check for copper forming at the cathode
• Half equations in electrolysis:

  • 2Cl - 2e -> Cl
  • Cu + 2e -> Cu
  • 2H + 2e -> H
  • 2O - 4e -> O
  • A half equation represents the reaction at an electrode during electrolysis
  • It shows what happens when ions gain or lose electrons
  • Electrons are shown as e-
  • The numbers of atoms of each element must be the same on both sides
  • The total charge on each side must be the same (usually zero)
• During the extraction of aluminium:

  • Aluminium ore is bauxite which becomes alumina during electrolysis
  • Cryolite is used to keep the temperature constant to speed up the reaction
  • The cathode and anode are made of graphite which is made out of carbon atoms
• Electrodes in aluminium extraction:

  • Negative electrode: Each Al ions gain 3 electrons, reducing into aluminium ions
  • Positive electrode: Each oxide ion loses 2 electrons, oxidising into oxygen atoms which pair up to form O molecules
• Aluminium is extracted from its ore using electrolysis because it is much more reactive than carbon

  • Bauxite must be purified before electrolysis to prevent impurities from being oxidised
  • Aluminium is joined with other elements, rock, and clays in the Earth's crust, making extraction difficult
• Testing for gases:

  • Hydrogen: Lit splint goes 'pop'
  • Chlorine: Damp blue litmus paper goes red and then gets bleached (turns white)
  • Oxygen: Blown out lit splint will relight in the presence of oxygen
• During electrolysis in brine (sodium chloride solution):

  • Hydrogen goes to the cathode
  • Chlorine goes to the anode
  • Sodium and water react to form sodium hydroxide
• Uses of products from electrolysis of brine:

  • Hydrogen: Rocket fuel, making sunflower oil more solid
  • Chlorine: Cleaning products, bleach, cleaning swimming pools
  • Hydrochloric acid: Purifying water in taps
  • Sodium hydroxide: Used to make paper, soap, production of ceramics
• In the electrolysis of brine:

  • Negative ions (Cl- and OH-) are attracted to the positive anode
  • Only chloride ions are oxidised and lose electrons, forming chlorine gas
  • Positive ions (H+ and Na+) are attracted to the negative cathode
  • Only hydrogen ions are reduced and gain electrons to form hydrogen gas
  • Sodium remains as ions in the solution, forming NaOH (sodium hydroxide solution)
  • The diaphragm is a porous membrane that keeps the sodium chloride solution separate from the sodium hydroxide solution