c6 - electrolysis

• discharged


  when an ion turns into a neutral atom
• ionic equation


  only shows ions that are changing and anything that changes state
• the easier it is for a metal to form its positive ion, the more reactive the metal is
• the electrolysis of aqueous solutions

  A) halogen

  B) oxygen

  C) hydrogen

  D) metal

  
• ionic compound

  • dissolved in water to form an aqueous solution
  • in an ionic compound there are 4 types of ions present: 2 ions (from the ionic solution), hydrogen and hydroxide
• half equation

  • shows ions and the movement of electrons
  • electrons are added to balance the charge
  • the number of atoms has to be balanced
  • the small number is always the same as the 2 larger numbers within the equation
  • electrons are represented by the symbol e-
• electrolysis


  • breaking down an ionic compound, using an electric current
  • electrolyte: an ionic compound that dissolves in water (making it delocalised) to make a solution that conducts electricity
  • electrode: where ions lose of gain electrons, depending on their charge (anions lose electrons and cations gain electrons to become neutral)
  • expensive since lots of energy is needed to melt the solid ionic compound to allow the ions to flow and to produce the electrical current
• • electrolyte: formed by dissolving an ionic compound and contains hydrogen and hydroxide ions (from the water) and positive and negative ions from the compound
  • the ions at each electrode gain or lose electrons
• • when an ionic compound is melted or dissolved in water, the ions are delocalised within the liquid or solution
  • passing an electric current through electrolytes causes the ions to move to the electrodes
  • cations (metals) move to the cathode
  • anions (non metals) move to the anode
• electrolysis of aluminium


  • drawbacks: expensive to extract aluminium from its ore (bauxite)
  • bauxite is purified to aluminium oxide
  • aluminium oxide must be melted so that electricity can pass through it and aluminium can be extracted
  • however aluminium oxide has a high melting point and would be expensive to melt, so instead its dissolved in molten cryolite to lower its melting point
  • this reduces some of the energy costs involved in extracting aluminium
  • aluminium forms at the cathode and oxygen is produced at the anode
• electrodes


  • cathode: negative electrode (attracts cations), where reduction happens and if the metal produced at the cathode is less reactive than hydrogen the metal produced
  • anode: positive electrode (attracts anions), where oxidation happens and oxygen is produced (apart from chloride, bromide or iodide) unless chloride, bromide or iodide are present (in that case they lose electrons and form the corresponding halogen)
  • anodes (abundant, good conductor and are cheap) are replaced regularly as they burn away as the oxygen reacts with the carbon to form carbon dioxide
• • oxidation: the loss of electrons and gain of oxygen
  • reduction: the gain of electrons and loss of oxygen
• • water dissociates to form hydrogen and hydroxide ions in the electrolyte solution
  • hydrogen ion: H+
  • hydroxide ion: OH-
• the more reactive element stays in solution

  the ion of the least reactive element will be discharged