Electrolysis

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  • Created by: Brooke
  • Created on: 16-07-13 11:35
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  • Electrolysis
    • Ionic compounds do not conduct electricity when solid since the ions are not free to move
      • However, ionic compounds do not conduct electricity when MOLTEN or when in AQUEOUS SOLUTION (i.e. when dissolved in water)
        • When an electric current is passed through a molten ionic compound, or through an aqueous solution of an ionic compound, ELECTROLYSIS takes place
          • Electrolysis is the decomposition (i.e. the CHEMICAL breakdown) of a substance by passing an electric current through it
    • Electrolysis of molten compounds
      • This apparatus can be used to show the electrolysis of a molten ionic compound such as lead(II) bromide
      • The products of electrolysing a molten binary compound(i.e. a compound containing only TWO elements) can easily be predicted by using the following rules:
        • 1) The metal is formed at the negative electrode
        • 2) The non-metal is formed at the positive electrode
        • For example, with molten lead(II) bromide, lead is formed at the negative electrode and bromine is formed at the positive electrode
    • How does electrolysis work?
      • Reaction at the negative electrode (cathode)
        • Positive metal ions are attracted to the negative electrode. When they get to the electrode they GAIN electrons so that they are converted into ATOMS
          • Example: when lead(II) bromide is electrolysed, lead(II) ions gain electrons to form lead atoms
      • Reaction at the positive electrode (anode)
        • Negative non-metal ions are attracted to the positive electrode. When they get to the electrode, they LOSE electrons to form atoms
          • Example: when lead(II) bromide is electrolysed, bromide ions lose an electron to form bromine atoms:
    • Electrolysis of aqueous solutions
      • Any aqueous solution that contains ions can be electrolysed.
        • The table shows the products of electrolysing several aqueous solutions of salts and acids
          • 1. The product at the NEGATIVE ELECTRODE is either HYDROGEN or a METAL
          • 2. The product at the POSITIVE ELECTRODE is either OXYGEN or another NON-METAL
          • If the metal in the salt is ABOVE hydrogen in the reactivity series then HYDROGEN is evolved at the cathode
          • If the metal in the salt is BELOW hydrogen in the reactivity series then the METAL is deposited on the cathode
    • Explaining the electrolysis of aqueous solutions
      • 1. Electrolysis of aqueous sodium chloride using inert electrodes
        • Particles present in solution: 1- Sodium ions (Na+), 2- Chlorine ions (Cl-), 3- Water molecules (H2O)
        • Reaction at the negative electrode: Hydrogen gas is produced as water molecules gain electrons: 2H2O+2e---->H2+2OH-
        • Reaction at the positive elctrode: Chlorine gas is produced as chlorine ions lose elctrons: 2Cl---->Cl2+2e-
        • Overall reaction: 2NaCl(aq)+2H2O(l)--->2NaOH(aq)+H2(g)+Cl2(g)
        • the sodium chloride solution is gradually converted into sodium hydroxide solution
      • 2.Electrolysis of dilute sulfuric acid using inert electrodes
        • Particles present in solution: 1) Hydrogen ions (H+), 2) sulfate ions (SO42-), 3) Water Molecules (H2O)
        • Reaction at negative electrode: Hydrogen gas is produced as hydrogen ions gain electrons: 2H+ + 2e- ---> H2
        • Reaction at positive electrode: Oxygen is produced as water olecules lose electrons: 2H2O ---> O2 + 4H+ + 4e-
        • Overall reaction: 2H2O(l) ---> 2H2(g) + O2(g)   As the water is decomposed, the solution of sulfuric acid gradually becomes more concentrated
      • 3. Electrolysis of aqueous copper(II) sulfate using inert electrodes
        • Particles present in solution: 1. Copper(II) ions (Cu2+), 2. Sulfate ions (SO42-), 3. Water molecules (H2O)
        • Reaction at the negative electrode: Copper is deposited on the negative electrode as copper(II) ions gain electrons: Cu2+ + 2e- ---> Cu
        • Reaction at the positive electrode: Oxygen gas is produced as water molecules lose electrons: 2H2O ---> O2 + 4H+ + 4e-
        • Overall reaction: 2CuSO4(aq) + 2H2O(l) ---> 2H2SO4(aq) + 2Cu(s) + O2(g)

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