Chemical changes (electrolysis)

• Anode


  • Positive electrode that attracts negative ions
• Cathode


  • Negative electrode that attracts positive ions
• Electrolysis


  1. Anode: Halogens (group 7) first, if none then Oxygen

  2. Cathode: Least reactive in reactivity series (usually Hydrogen)

  3. Positive ions go to Cathode, gain electron to become atom (reduced)

  4. Negative ions go to Anode, lose electron to become atom (oxidised)
• Reactivity Series (most to least)

  • Potassium
  • Sodium
  • Lithium
  • Calcium
  • Magnesium
  • Aluminium
  • Carbon
  • Zinc
  • Iron
  • Hydrogen
  • Copper
  • Silver
  • Gold
• Metals can be produced at the cathode during electrolysis
• Electrolysis of aqueous solutions

  1. Use a measuring cylinder to add 40ml of a solution into a beaker

  2. Place two graphite rods into the solution - attaching one electrode to the negative terminal of a dc supply, and the other to the positive terminal

  3. Place two small test tubes over each electrode to collect any gases produced

  4. Turn on the power supply and observe what happens at each electrode

  5. Test any gas produced by holding a piece of blue litmus paper next to the electrode or by holding a lit splint next to it
• Wear goggles, gloves, switch off apparatus
• Hydrogen gas at Cathode

  2H⁺(aq) + 2e⁻ → H₂(g)
• Oxygen gas at Anode

  4OH⁻(aq) → 2H₂O + O₂(g) + 4e⁻
• OILRIG


  Oxidation Is Loss (of electrons), Reduction Is Gain
• Anode


  Positive terminal
• Cathode


  Negative terminal
• Electrons


  Flow from anode to cathode
• positive ions go to the cathode, gain an electron to become an atom, they are reduced
• Negative ions go to the anode, loose an electron to become an atom, they are oxidised
• can be produced at the cathode

  Hydrogen

  copper

  silver

  gold
• Electrolyte 


  The solution used in electrolysis
• If the electrolyte is aq

  The solution has to have

  h+

  o-

  oh-
• Anode


  Halogens (group 7) first, if none then oxygen
• Cathode


  Least reactive in reactivity series (usually hydrogen)