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2. Electrolysis
Electrolysis is the chemical decomposition of an ionic compound by a direct electrical current.
Electrolysis is carried out in an electrolytic cell.
The compound must be either molten, or in aqueous solution for electrolysis to take place.
DC Supply
A pair of metal or graphite electrodes.
Electrolyte, either a molten ionic solid or an ionic aqueous solution.
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3. Electrolysis
When an ionic solid melts, or dissolves in water, positive and negative ions are formed. In a liquid medium these ions are free to migrate within the body of the liquid. They will migrate under the influence of an electrical field.
Negative ions (anions) migrate to the positive electrode (anode).
Positive ions (cations) migrate to the negative electrode (cathode).
DC Supply
A pair of metal or graphite electrodes.
Electrolyte, either a molten ionic solid or an ionic aqueous solution.
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4. Electrolysis
At the anode, the anions surrender electrons which pass around the circuit driven by the DC supply. The anions become atoms... Y-  Y + e-
At the cathode, the cations take up electrons from the circuit. The cations become atoms... X+ + e-  X
DC Supply
A pair of metal or graphite electrodes.
Electrolyte, either a molten ionic solid or an ionic aqueous solution.
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5. The Electrolysis of Molten Sodium Chloride
Anode
Cathode
Molten sodium chloride electrolyte
Negative chloride ion migrates to anode
Positive sodium ion migrates to cathode
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6. The Electrolysis of Molten Sodium Chloride
When NaCl (s) is melted, the molten sodium chloride becomes Cl- (l) + Na+ (l).
Therefore during electrolysis:
At the anode...
2Cl-  Cl2(g) + 2e-
...chlorine gas is formed,
and at the cathode...
2Na e-  2Na(s)
...sodium metal is formed.
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7. The Electrolysis of Sodium Chloride Solution
A solution of sodium chloride will also act as an electrolyte.
The electrolyte will contain Na+ and Cl- ions.
In addition, the water will also provide hydrogen ions (H+) and hydroxide ions (OH-) to the electrolyte.
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8. The Electrolysis of Sodium Chloride Solution
Oxygen can be produced at the anode...
4OH-  O H2O + 4e-
Chlorine can also be produced at the anode...
2Cl-  Cl e-
The production of oxygen at the anode means that we will get a mixture of chlorine and oxygen gas.
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9. Electroplating EPNS stands for: Electro-Plated Nickel Silver.
This nickel object has been plated with a thin layer of silver.
Metal objects can be plated with gold , silver, copper or other metals using electrolysis.
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10. Copper Plating
Impure copper anode
Metal cathode
Layer of copper forms on cathode
Copper ions migrate to cathode
The anode is a piece of impure copper, which is used to ‘plate’ the metal cathode.
The electrolyte is a solution of a copper salt, e.g. copper sulphate.
The cathode may be a piece of jewellery, or an ornament. The surface of the cathode will become plated with copper from the anode.
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11. A piece of zinc foil is plated with copper.
Copper Plating
A piece of zinc foil is plated with copper.
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12. Copper Plating At the anode, Cu  Cu2+ + 2e-
...copper ions are formed,
and at the cathode...
Cu e-  Cu
...copper metal is formed.
A similar approach is used to plate various objects with a variety of metals...
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13. Chromium Plating
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14. Gold Plating Silver Plating
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15. Dry Cells Dry Cells are the most obvious everyday use of electrolysis.
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16. Dry Cells Insulating plastic case Positive terminal (brass cap)
Carbon rod (positive electrode)
Zinc case (negative electrode)
Powdered carbon and manganese IV oxide
Ammonium chloride paste (electrolyte)
Negative terminal (zinc case bottom)
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17. Dry Cells - Rechargeable
Ni-Cd (Nickel Cadmium) and Ni-MH (Nickel Metal Hydride) batteries are rechargeable, and have a much longer life than disposable dry cells.
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18. Extension Material
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19. The Electrolysis of Water
Pure water is not a good conductor of electricity.
However, the electrolysis of water can be carried out by adding a small amount of dilute sulphuric acid to distilled water.
Oxygen
Hydrogen
Water
+ drops of dilute Sulphuric acid
Graphite rod (Cathode –)
Graphite rod (Anode +)
Electric Current
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20. The Electrolysis of Water
4H2O  4H+ + 4OH-
At the cathode...
4H e- = 2H2
At the anode…
4OH- = O H2O + 4e-
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21. The Electrolysis of Other Salt Solutions
Problems can arise when using aqueous solutions of electrolytes because of the presence of water.
If two types of cation are present in the electrolyte then a competition reaction will take place at the cathode. The relative positions of the metal and hydrogen (from water) in the reactivity series will determine which element is formed at the cathode.
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22. Reactivity Series
1. Potassium 2. Sodium 3. Calcium 4. Magnesium 5. Aluminium 6. Zinc 7. Iron 8. Lead 9. Hydrogen 10. Copper 11. Mercury 12. Silver 13. Gold
The element which is lowest in the table will be produced at the cathode, e.g...
1. In a solution of sodium chloride, sodium and hydrogen ions will be present. Hydrogen is lowest in the table and therefore hydrogen is produced at the cathode.
2. In a solution of silver chloride, silver and hydrogen ions will be present. Silver is lowest in the table and therefore silver is produced at the cathode.
If an anion present in an aqueous solution is nitrate or sulphate, oxygen will also be produced at the anode.
If the anion present in an aqueous solution is a halide ion, the halogen is produced at the anode.
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