Electrolysis

• Electrolysis

  The process of breaking down an ionic substance into its elements by passing an electric current through it
• The process of electrolysis


  1. Ionic substance is melted or dissolved

  2. Ions are free to move about

  3. Current is passed through the molten or solution

  4. Substance is broken down into elements
• Cathode


  The negative electrode
• Anode

  The positive electrode
• What happens during electrolysis


  1. Positively charged ions move to the cathode

  2. Negatively charged ions move to the anode

  3. Ions are discharged at the electrodes producing elements
• Electrolysis of molten ionic compounds


  1. Metal is produced at the cathode

  2. Non-metal is produced at the anode
• This is because the metal is the positive ions and the non-metal is the negative ions
• Metals extracted by electrolysis

  • Metals more reactive than carbon (e.g. aluminium)
  • Large amounts of energy used to melt compounds and produce electrical current
• Aluminium extraction


  • Electrolysis of molten mixture of aluminium oxide and cryolite
  • Carbon used as positive electrode (anode)
  • Positive electrodes need to be continually replaced
• Metals that react with carbon can also be extracted by electrolysis
• Electrolysis of aqueous solutions


  1. Ions discharged depend on relative reactivity of elements

  2. Hydrogen produced at cathode unless metal less reactive than hydrogen

  3. Halide ion or oxygen produced at anode depending on ions present

  
• This happens because water molecules break down producing H+ and OH- ions that are discharged
• Half equation

  Representation of reactions at electrodes
• Writing half equations


  1. Negative electrode: X+ + e- -> X (positive ions reduced)

  2. Positive electrode: X- -> e- + X (negative ions oxidised)