Electrolysis

• Electrolysis


  Using electricity to break down a substance
• Ionic substance when melted or dissolved in water

  The ions become free to move around
• Positive electrode


  The anode
• Negative electrode


  The cathode
• Positive ions move to


  The cathode
• Negative ions move to

  The anode
• Zn²⁺(aq) turning into Zn(s)

  Occurs at the cathode (needs to gain two electrons)
• Cl⁻(aq) turning into Cl₂(g)

  Occurs at the anode (needs to lose one electron)
• Balancing the equation: Al³⁺ + e⁻ → Al


  Al³⁺ + 3e⁻ → Al
• Balancing the equation: Cl⁻ → Cl₂ + e⁻

  2Cl⁻ → Cl₂ + 2e⁻
• Balancing the equation: O²⁻ → O₂ + e⁻

  2O²⁻ → O₂ + 4e⁻
• Electrolysis of molten iron bromide

  • Iron and bromine
• Electrolysis of molten zinc oxide

  • Zinc and oxygen
• Metals extracted using electrolysis

  Ones more reactive than carbon, e.g. aluminium
• Disadvantages of using electrolysis to extract metals

  • Requires a large amount of energy to melt the compounds and to produce the necessary electricity
• Why aluminium oxide is mixed with cryolite


  To lower the melting point
• Products in the electrolysis of aluminium oxide


  Aluminium at the cathode and oxygen at the anode
• Why the anode needs to be replaced in the electrolysis of aluminium oxide

  The oxygen reacts with the carbon electrode to produce carbon dioxide
• Electrolysis of sodium chloride solution

  • Chlorine gas, hydrogen gas and sodium hydroxide solution
• Hydrogen gas used for

  Used to produce margarine
• Chlorine gas used for

  Used to sterilise water as it kills microbes; producing bleach
• Sodium hydroxide used for


  Used to make soap
• Why sodium is not produced in the electrolysis of sodium chloride solution
• What is produced at the anode in electrolysis of solutions

  Either a halogen or oxygen (when there is no halogen present). The oxygen is from the hydroxide (OH-) ion
• Chemical cell


  A unit which contains chemicals which produce electricity through a reaction
• Main components of a chemical cell

  • An anode, a cathode and an electrolyte
• Electrolyte


  A solution containing ions which allows current to flow
• Potential difference of a cell


  Depends on the type of electrode and the electrolyte
• Battery


  Two or more cells connected in series
• What happens to the electrodes in a chemical cell

  The more reactive metal depletes and the less reactive one increases in size
• Why chemical reactions stop over time in non-rechargeable cells

  One of the reactants becomes used up
• How certain cells can be recharged


  Applying an external electric current
• How reactivity of metal electrodes affects potential difference

  The greater the difference in reactivity, the greater the potential difference
• How to order reactivity of metals using voltage

  The higher the voltage the more reactive the metal, by comparing the voltages of different metals with the same metal as one electrode their reactivity can be ordered
• How to predict voltage of a cell from other cells of known voltage

  By calculating the difference in voltage
• Voltage for tin/lead cell if iron/tin cell = -0.30V and iron/lead cell = -0.31V


  • 0.01V or -0.01 depending on which way round the electrodes are connected
• How to predict voltage of a battery from voltages of different cells

  Add all the voltages for the different cells together
• Fuel cell


  A cell which uses a fuel and oxygen (or air) to generate electricity
• Advantages of hydrogen fuel cells

  • Do not need to be recharged
  • No pollutants are produced
  • Can be different sizes for different uses
• Disadvantages of hydrogen fuel cells

  • Hydrogen is highly flammable
  • Hydrogen is sometimes produced through non-renewable means
  • Hydrogen is difficult to store