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
• Electrolyte


  The substance being broken down during electrolysis
• What happens during electrolysis

  1. Positively charged ions move to the negative electrode (cathode)

  2. Negatively charged ions move to the positive electrode (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
• Extracting metals by electrolysis

  • Metals more reactive than carbon are extracted by electrolysis of molten compounds
  • Large amounts of energy are used to melt the compounds and produce the electrical current
• Aluminium extraction by electrolysis

  • Molten mixture of aluminium oxide and cryolite is electrolysed
  • Carbon is used as the positive electrode (anode)
  • Oxygen reacts with the carbon electrodes, forming carbon dioxide, so they 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 the relative reactivity of the elements

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

  3. One of the halide ions (Cl-, Br-, I-) is produced at the anode if present, otherwise oxygen is formed
• This happens because in the aqueous solution, water molecules break down producing H+ and OH- ions that are discharged
• Half equation


  Representation of the reactions at the electrodes, where the small number is the same as the 2 larger numbers, and electrons are represented by 'e-'
• Extracting Metals

  Aluminium is manufactured by the electrolysis of a molten mixture of aluminium oxide and cryolite using carbon as the positive electrode (anode).
• Reason why cryolite is mixed with aluminium oxide ?

  
  

  Aluminium is extracted from its ore (bauxite) by electrolysis and bauxite contains aluminium oxide . Aluminium oxide has a very high melting point and so it is mixed with cryolite to lower its melting point