Electrolysis

• Electrolysis

  The use of an electrical current to break down compounds containing ions into their constituent elements
• Electrolyte

  The substance being broken down
• Electrodes

  Solids that conduct electricity
• During electrolysis

  1. Negatively charged ions move to the anode (positive electrode)

  2. Positively charged ions move to the cathode (negative electrode)
• Electrolysis

  • Can be used to separate ionic compounds into elements
• Example of electrolysis

  • Lead bromide can be split into lead and bromine
• Steps in electrolysis of lead bromide

  1. Lead bromide is heated until it melts

  2. Positively charged lead ions move to the negative electrode (cathode) and gain electrons to form lead atoms - pure lead is produced

  3. Negatively charged bromide ions move to the positive electrode (anode) and lose electrons to form bromine atoms, which join together to form bromine molecules - bromine is released
• Ionic substances can only conduct electricity when they are molten or dissolved in water
• Key Point

  For the electrolysis of molten ionic compounds, the electrodes used must be inert so that they do not react with the electrolyte or the products
• Oxidation and Reduction

  . Reduction occurs when positively charged ions gain electrons at the negative electrode

  . Oxidation occurs when negatively charged ions lose electrons at the positive elelctrode

  . In a redox reaction both reduction and oxidation occur

  . Reactions that take place at the electrodes during electrolysis can be represented by half-equations
• Key Point

  You can remember this by thinking of the word OILRIG:

  . Oxidation Is Loss of electrons (OIL)

  . Reduction Is Gain of electrons (RIG)
• Extraction of Metals

  1. Metals more reactive than carbon can be extracted from their ores using electrolysis

  2. Electrolysis requires lots of heat and electrical energy, making it an expensive process
• Aluminium extraction

  • Obtained by the electrolysis of aluminium oxide mixed with cryolite (a compound of aluminium)
  • Cryolite lowers the melting point of the aluminium oxide, meaning less energy is needed (cheaper energy costs)
  • Aluminium forms at the negative electrode
  • Oxygen gas forms at the positive carbon electrode and reacts with the carbon forming carbon dioxide
  • This wears away the positive electrode, which is replaced regularly
• Key Point

  In the exam you could be asked to suggest a hypothesis to explain given data
• Electrolysis of Aqueous Solutions

  1. When ionic compounds are dissolved in water to form aqueous solutions, it is slightly harder to predict the products of electrolysis

  2. Water molecules break down to form hydroxide ions, OH-, and hydrogen ions, H+
• At the negative electrode

  1. Hydrogen is produced if the metal is more reactive than hydrogen

  2. The metal is produced if the metal is less reactive than hydrogen
• At the positive electrode

  1. Oxygen is produced unless the solution contains halide ions

  2. If halide ions are present, then the halogen is produced
• Electrolysis of sodium chloride solution

  • Hydrogen is released at the negative electrode
  • Chlorine gas is released at the positive electrode
• Key Point

  When electrolysis is used to extract metal, the positive electrode is made of carbon
• RQP - Investigate what happens when aqueous solutions are electrolysed using inert electrodes

  Sample Method:

  1. Set up the equipment

  2. Pass an electric current through the aqueous solution

  3. Observe the products formed at each inert electrode

  Hazards and Risks:

  . A low voltage must be used to prevent an electric shock

  . The room must be well ventilated, and the experiment must only be carried out for a short period of time, to prevent exposure to dangerous levels of chlorine gas