1. Electrolysis

2. Terms used in electrolysis
Electrolysis is the decomposition of an electrolyte in molten state or aqueous solution by electricity.
An electrolyte is a substance which conducts an electric current in molten state or aqueous solution, and is decomposed by electricity.
The anode is the electrode where oxidation occurs. It is the electrode connected to the positive terminal of the d.c. supply.
The cathode is the electrode where reduction occurs. It is the electrode connected to the negative terminal of the d.c. supply.

3. Terms used in electrolysis
An anion is a negative ion and is attracted to the anode.
A cation is a positive ion and is attracted to the cathode.
An ammeter is an instrument used to measure the electric current passing through a circuit. Electric current is measured in ampere (A).
A variable resistor (or rheostat) is used to vary the resistance and then regulate the current.

4. Electrolysis

5. Factors affecting electrolysis
The position of ions in the electrochemical series.
The concentration of ions in the solution.
The nature of the electrodes.

6. Position of cations in the e.c.s.

7. Position of anions in the e.c.s.

8. Case 1: Electrolysis of molten lead(II) bromide
Nichrome wire
Electrode (-)
Electrode (+)
Molten lead(II)
bromide
Electron flow

9. Case 1: Electrolysis of molten lead(II) bromide
Solid lead(II) bromide does not conduct electricity because the ions are not mobile.
Molten lead(II) bromide contains mobile ions.
Cation
Anion
Pb2+()
Br-()

10. Case 1: Electrolysis of molten lead(II) bromide
Nichrome wire
Electrode (+)
Electrode (-)
Molten lead(II)
bromide
Electron flow

11. Case 1: Electrolysis of molten lead(II) bromide
At cathode, lead(II) cations receive electrons, they undergo reduction and discharge to form lead atoms.
Pb2+() + 2e–  Pb()

12. Case 1: Electrolysis of molten lead(II) bromide
At anode, bromide anions give up electrons, they undergo oxidation and discharge to form bromine atoms.
2Br-()  Br2() + 2e–

13. Case 1: Electrolysis of molten lead(II) bromide
Bromine atoms then join in pair to form bromine molecules.

14. Case 2: Electrolysis of acidified water using platinum electrodes
Although water is known to be poor electrical non-conductor, it actually ionizes slightly to give hydrogen ions and hydroxide ions. H2O() H+(aq) + OH–(aq)
Pure acids are covalent compounds. However they ionize in water.
HCl(g) + water  HCl(aq)
HCl(aq)  H+(aq) + Cl–(aq)

15. Case 2: Electrolysis of acidified water using platinum electrodes
Cation
Anion
H+(aq)
OH-(aq)

16. Case 2: Electrolysis of acidified water using platinum electrodes
At cathode, hydrogen ions receive electrons, they undergo reduction and discharge to form hydrogen gas.
2H+(aq) + 2e–  H2(g)

17. Case 2: Electrolysis of acidified water using platinum electrodes
At anode, hydroxide ions give up electrons, they undergo oxidation and discharge to form oxygen gas.
4OH-(aq)  O2(g) + 2H2O() + 4e–

18. Case 2: Electrolysis of acidified water using platinum electrodes
2H+(aq) + 2e–  H2(g) (1)
4OH-(aq)  O2(g) + 2H2O() + 4e– (2)
(1)x2: 4H+(aq) + 4e–  2H2(g) (3)
(2)+(3): 4OH-(aq) + 4H+(aq)  O2(g) + 2H2O() + 2H2(g)
4H2O()  O2(g) + 2H2O() + 2H2(g)
Overall equation: 2H2O()  O2(g) + 2H2(g)

19. Case 2: Electrolysis of acidified water using platinum electrodes
Dilute acid is added to provide more mobile ions so as to increase the conductivity of the water.
The concentration of dilute acid increases at the end as water is consumed in the electrolysis.

20. Case 3: Electrolysis of dilute sodium chloride solution using carbon electrodes
Cation
Anion
H+(aq)
OH-(aq)
Na+(aq)
Cl-(aq)

21. Case 3: Electrolysis of dilute sodium chloride solution using carbon electrodes
The sodium ions and hydrogen ions move towards the cathode.
At the cathode, the position of hydrogen ions in the electrochemical series is lower than that of sodium ions. Hydrogen ions are preferentially discharged (reduced) to form colourless hydrogen gas.
2H+(aq) + 2e–  H2(g)

22. Case 3: Electrolysis of dilute sodium chloride solution using carbon electrodes
The chloride ions and hydroxide ions move towards the anode.
At the anode: the position of hydroxide ions in the electrochemical series is higher than that of chloride ions. Hydroxide ions are preferentially discharged (oxidized) to form colourless oxygen gas.
4OH-(aq)  O2(g) + 2H2O() + 4e–

23. Case 3: Electrolysis of dilute sodium chloride solution using carbon electrodes
Overall reaction: 2H2O()  O2(g) + 2H2(g)
Water ionizes continuously to replace the hydrogen ions discharged at the cathode. Thus there is an excess of hydroxide ions near the cathode and the solution there becomes alkaline.
Water ionizes continuously to replace the hydroxide ions discharged at the anode. Thus there is an excess of hydrogen ions near the anode. The solution there becomes acidic.

24. Case 3: Electrolysis of dilute sodium chloride solution using carbon electrodes
If a few drops of universal indicator are added to the sodium chloride solution, the solution near the cathode will turn blue while that near the anode will turn red.
The sodium chloride becomes more concentrated as water is consumed in the electrolysis.

25. Case 4: Electrolysis of dilute copper(II) sulphate solution using carbon electrodes
Cation
Anion
H+(aq)
OH-(aq)
Cu2+(aq)
SO42-(aq)

26. Case 4: Electrolysis of dilute copper(II) sulphate solution using carbon electrodes
The copper(II) ions and hydrogen ions move towards the cathode.
At the cathode, the position of copper(II) ions in the electrochemical series is lower than that of hydrogen ions. Copper(II) ions are preferentially discharged (reduced) to form brown copper metal.
Cu2+(aq) + 2e–  Cu(s)

27. Case 4: Electrolysis of dilute copper(II) sulphate solution using carbon electrodes
The sulphate ions and hydroxide ions move towards the anode.
At the anode: the position of hydroxide ions in the electrochemical series is higher than that of sulphate ions. Hydroxide ions are preferentially discharged (oxidized) to form colourless oxygen gas.
4OH-(aq)  O2(g) + 2H2O() + 4e–

28. Case 4: Electrolysis of dilute copper(II) sulphate solution using carbon electrodes
Overall reaction:
2Cu2+(aq) + 4OH–(aq)  2Cu(s) + O2(g) + 2H2O()
Water ionizes continuously to replace the hydroxide ions discharged at the anode. Thus there is an excess of hydrogen ions near the anode. The solution there becomes acidic.
If a few drops of universal indicator is added into the solution, red colour appears around anode.

29. Case 4: Electrolysis of dilute copper(II) sulphate solution using carbon electrodes
The blue colour of the solution fades out because the concentration of copper(II) ions decreases.
Copper(II) ions and hydroxide ions are consumed in the electrolysis. Hydrogen ions and sulphate ions remain in the solution. Thus the solution eventually becomes sulphuric acid.

30. Case 4: Electrolysis of dilute copper(II) sulphate solution using carbon electrodes
After a few minutes, cathode is coated with copper.
If the polarities of cells are then reversed, anode is coated with copper. The factor of electrode should be considered as in case 8.

31. Case 5: Electrolysis of dilute sodium iodide solution using carbon electrodes
Cation
Anion
H+(aq)
OH-(aq)
Na+(aq)
I-(aq)

32. Case 5: Electrolysis of dilute sodium iodide solution using carbon electrodes
The sodium ions and hydrogen ions move towards the cathode.
At the cathode, the position of hydrogen ions in the electrochemical series is lower than that of sodium ions. Hydrogen ions are preferentially discharged (reduced) to form colourless hydrogen gas.
2H+(aq) + 2e–  H2(g)

33. Case 5: Electrolysis of dilute sodium iodide solution using carbon electrodes
The iodide ions and hydroxide ions move towards the anode.
At the anode: the position of hydroxide ions in the electrochemical series is higher than that of chloride ions. However, the concentration of iodide ions is much greater than that of hydroxide ions. Iodide ions are preferentially discharged (oxidized) to form iodine.
2I-(aq)  I2(aq) + 2e–

34. Case 5: Electrolysis of dilute sodium iodide solution using carbon electrodes
Overall reaction:
2H+(aq) + 2I–(aq)  H2(g) + I2(aq)
The solution near the cathode becomes alkaline.
The iodine produced at the anode dissolves in the solution. Therefore a brown colour develops around the anode.

35. Case 5: Electrolysis of dilute sodium iodide solution using carbon electrodes
Hydrogen ions and iodide ions are consumed in the electrolysis. Sodium ions and hydroxide ions remain in the solution. The solution eventually becomes sodium hydroxide solution.

36. Case 6: Electrolysis of conc
Case 6: Electrolysis of conc. sodium chloride solution using carbon electrodes
Cation
Anion
H+(aq)
OH-(aq)
Na+(aq)
Cl-(aq)
Conc.

37. Case 6: Electrolysis of conc
Case 6: Electrolysis of conc. sodium chloride solution using carbon electrodes
The sodium ions and hydrogen ions move towards the cathode.
At the cathode, the position of hydrogen ions in the electrochemical series is lower than that of sodium ions. Hydrogen ions are preferentially discharged (reduced) to form colourless hydrogen gas.
2H+(aq) + 2e–  H2(g)

38. Case 6: Electrolysis of conc
Case 6: Electrolysis of conc. sodium chloride solution using carbon electrodes
The chloride ions and hydroxide ions move towards the anode.
At the anode: the position of hydroxide ions in the electrochemical series is higher than that of chloride ions. However, the concentration of chloride ions is much greater than that of hydroxide ions. Chloride ions are preferentially discharged (oxidized) to form chlorine gas.
2Cl-(aq)  Cl2(g) + 2e–

39. Case 6: Electrolysis of conc
Case 6: Electrolysis of conc. sodium chloride solution using carbon electrodes
Overall reaction:
2H+(aq) + 2Cl–(aq)  H2(g) + Cl2(aq)
Water ionizes continuously to replace the hydrogen ions discharged at the cathode. Thus there is an excess of hydroxide ions near the cathode. The solution there becomes alkaline.
The chlorine gas formed at the anode dissolves in the solution. The solution there becomes acidic and has a bleaching effect.

40. Case 6: Electrolysis of conc
Case 6: Electrolysis of conc. sodium chloride solution using carbon electrodes
Hydrogen ions and chloride ions are consumed in the electrolysis. Sodium ions and hydroxide ions remain in the solution. Eventually, the solution becomes sodium hydroxide solution.

41. Case 7: Electrolysis of conc
Case 7: Electrolysis of conc. sodium chloride solution using mercury electrodes
Cation
Anion
H+(aq)
OH-(aq)
Na+(aq)
Cl-(aq)
Hg()

42. Case 7: Electrolysis of conc
Case 7: Electrolysis of conc. sodium chloride solution using mercury electrodes
The sodium ions and hydrogen ions move towards the cathode.
At the cathode, the position of hydrogen ions in the electrochemical series is lower than that of sodium ions. However, sodium ions are preferentially discharged (reduced) to form sodium metal. The sodium metal formed dissolves in the mercury to form a sodium amalgam.
Na+(aq) + e– + Hg(l)  Na/Hg(l) sodium amalgam

43. Case 7: Electrolysis of conc
Case 7: Electrolysis of conc. sodium chloride solution using mercury electrodes
The sodium amalgam then reacts with water to form sodium hydroxide and hydrogen.
2Na/Hg(l) + 2H2O(l)  2NaOH(aq) + H2(g) + 2Hg(l)

44. Case 7: Electrolysis of conc
Case 7: Electrolysis of conc. sodium chloride solution using mercury electrodes
The chloride ions and hydroxide ions move towards the anode.
At the anode: the position of hydroxide ions in the electrochemical series is higher than that of chloride ions. However, the concentration of chloride ions is much greater than that of hydroxide ions. Chloride ions are preferentially discharged (oxidized) to form chlorine gas.
2Cl-(aq)  Cl2(g) + 2e–

45. Case 7: Electrolysis of conc
Case 7: Electrolysis of conc. sodium chloride solution using mercury electrodes
Overall reaction:
2Na+(aq) + 2Cl–(aq) + 2Hg(l)  2Na/Hg(l) + Cl2(g)
Sodium ions and chloride ions are consumed in the electrolysis. Thus the sodium chloride solution becomes more and more dilute.
This reaction is very important in the manufacture of chlorine bleaching solution.

46. Case 8: Electrolysis of dilute copper(II) sulphate solution using copper electrodes
Cation
Anion
H+(aq)
OH-(aq)
Cu2+(aq)
SO42-(aq)
Cu(s)

47. Case 8: Electrolysis of dilute copper(II) sulphate solution using copper electrodes
The copper(II) ions and hydrogen ions move towards the cathode.
At the cathode, the position of copper(II) ions in the electrochemical series is lower than that of hydrogen ions. Copper(II) ions are preferentially discharged (reduced) to form brown copper metal. Cu2+(aq) + 2e–  Cu(s)

48. Case 8: Electrolysis of dilute copper(II) sulphate solution using copper electrodes
The sulphate ions and hydroxide ions move towards the anode.
At the anode: the position of hydroxide ions in the electrochemical series is higher than that of sulphate ions. However, copper is a stronger reducing agent than hydroxide ions and thus more easily oxidized. The copper anode dissolves to form copper(II) ions (oxidized).
Cu(s)  Cu2+(aq) + 2e–

49. Case 8: Electrolysis of dilute copper(II) sulphate solution using copper electrodes
Overall reaction:
Cu(s)  Cu(s)
anode cathode
The net effect is the transfer of copper from the anode to the cathode. The rate at which copper deposits on the cathode is equal to the rate at which the copper anode dissolves.
Increase in mass of cathode = decrease in mass of anode

50. Case 8: Electrolysis of dilute copper(II) sulphate solution using copper electrodes
The concentration of copper(II) ions in the solution remains the same. The blue colour of the solution does not change.

51. Comparing a chemical cell and an electrolytic cell

52. Comparing a chemical cell and an electrolytic cell
Function
A device for generating electricity from chemical reactions.
A device for bringing out chemical changes by electricity.
Direction of electricity
Electrons flow from negative electrode to the positive electrode through the external circuit.
Circuit is completed by the movement of mobile electrons.
Cations discharge and gain electrons at cathode, while anions discharge and give up electrons at anode.
Circuit is completed by the movement of mobile ions.

53. Comparing a chemical cell and an electrolytic cell
Reactions at positive electrode
Reduction (Cathode)
Oxidation (anode)
Reactions at negative electrode

54. Commercial uses of electrolysis
Manufacture of hydrogen, chlorine and sodium hydroxide and bleaching solution
Refining of copper
Electroplating
Extracting reactive metals
Aluminium anodization

55. Manufacture of bleaching solution

56. Manufacture of bleaching solution
At the anode: 2Cl–(aq)  Cl2(g) + 2e–
At the cathode: Na+(aq) + e– + Hg(l)  Na/Hg(l)
sodium amalgam
The sodium amalgam then flows into a second cell and reacts with water to form sodium hydroxide, hydrogen and mercury.
Mercury is then recovered and then pumped back into the reaction chamber.
2Na/Hg(l) + 2H2O(l)  2NaOH(aq) + H2(g) + 2Hg(l)

57. Manufacture of bleaching solution
This process also produces waste which contains poisonous mercury compounds. These waste products will cause serious pollution problems if they are discharged into rivers and seas.

58. Refining of copper

59. Refining of copper
Copper ore contains a few impurities — mostly silver, gold, platinum, iron and zinc — reduce the electrical conductivity of copper significantly.
anode: impure copper
cathode: very pure copper
electrolyte: copper(II) sulphate solution and sulphuric acid

60. Refining of copper
Iron and zinc are more reactive than copper. They form ions more readily than copper.
At anode, iron and zinc give up electrons first. Then copper gives up electrons to form copper(II) ions.
Zn(s)  Zn2+(aq) + 2e–
Fe(s)  Fe2+(aq) + 2e–
Cu(s)  Cu2+(aq) + 2e–

61. Refining of copper
Impurities such as silver, gold and platinum settle at the bottom of the container.
At the cathode, the position of copper(II) ions in the electrochemical series is lower than that of hydrogen ions. Copper(II) ions are preferentially discharged (reduced) to form brown copper metal. Cu2+(aq) + 2e–  Cu(s)

62. Refining of copper Refer to case 8 Overall reaction: Cu(s)  Cu(s)
anode cathode
Refer to case 8

63. Electroplating

64. Electroplating
Electroplating is the coating of an object with a thin layer of a metal by electrolysis.
Cathode: object to be plated
Anode: plating metal
Electrolyte: a solution of a compound of the plating metal

65. Electroplating
Objects may be electroplated with copper, nickel, chromium, gold or silver.
Typical example: electroplating of copper

66. Pollution problems of electroplating
The electroplating industry produces many toxic waste by-products.
acids and alkalis
cumulative poisons of heavy metals and ions (such as nickel, chromium and mercury)
toxic cyanides.

67. Solutions Controlling the pH value of effluents
The pH value of acidic effluents can be controlled by adding sodium carbonate.
The pH value of alkaline effluents can be controlled by adding sulphuric acid.
Treatment of heavy metal compounds
Add sodium hydroxide solution to the effluents to form insoluble metal hydroxides. The solid is then filtered off.

68. Solutions Treatment of poisonous chromium waste
Poisonous chromium(VI) compounds are reduced to non-toxic chromium(III) compounds by sodium sulphite. Sodium hydroxide solution is then added to the chromium(III) compounds to form solid chromium(III) hydroxide. The solid is then filtered off.

69. Extraction of reactive metals
Reactive metals such as K, Na, Ca, Mg and Al are extracted from its ores by electrolysis of molten metal ores.
Metal ions are attracted to the cathode and reduced to form metal.
Mn+(l) + ne-  M(s)

70. Anodization of aluminium
Aluminium oxide is a protective oxide layer. It does not react with acids and alkalis.
However, natural occurring aluminium oxide layers are thin and unevenly distributed.
Anode: Al
Cathode: circular sheet of steel
Electrolyte: dilute sulphuric acid

71. Anodization of aluminium
At anode:
4OH-(aq)  O2(g) + 2H2O() + 4e–
Oxygen is then reacted with aluminium anode to form a thick protection oxide layer.
4Al(s) + 3O2(g)  2Al2O3(s)
Aluminium oxide can be dyed.