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Electrolysis. Terms used in electrolysis Electrolysis is the decomposition of an electrolyte in molten state or aqueous solution by electricity. An electrolyte.

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Presentation on theme: "Electrolysis. Terms used in electrolysis Electrolysis is the decomposition of an electrolyte in molten state or aqueous solution by electricity. An electrolyte."— Presentation transcript:

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 (-) Nichrome wire 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. CationAnion Pb 2+ ( )Br - ( )

10 Case 1: Electrolysis of molten lead(II) bromide Nichrome wire Electrode (+) Nichrome wire 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. Pb 2+ ( ) + 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 - ( )  Br 2 ( ) + 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. H 2 O( ) 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 CationAnion 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 –  H 2 (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)  O 2 (g) + 2H 2 O( ) + 4e –

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

23 Case 3: Electrolysis of dilute sodium chloride solution using carbon electrodes Overall reaction: 2H 2 O( )  O 2 (g) + 2H 2 (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 CationAnion H + (aq)OH - (aq) Cu 2+ (aq)SO 4 2- (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. Cu 2+ (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)  O 2 (g) + 2H 2 O( ) + 4e –

28 Case 4: Electrolysis of dilute copper(II) sulphate solution using carbon electrodes Overall reaction: 2Cu 2+ (aq) + 4OH – (aq)  2Cu(s) + O 2 (g) + 2H 2 O( ) 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 CationAnion 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 –  H 2 (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)  I 2 (aq) + 2e –

34 Case 5: Electrolysis of dilute sodium iodide solution using carbon electrodes Overall reaction: 2H + (aq) + 2I – (aq)  H 2 (g) + I 2 (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. sodium chloride solution using carbon electrodes CationAnion H + (aq)OH - (aq) Na + (aq)Cl - (aq) Conc.

37 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 –  H 2 (g)

38 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)  Cl 2 (g) + 2e –

39 Case 6: Electrolysis of conc. sodium chloride solution using carbon electrodes Overall reaction: 2H + (aq) + 2Cl – (aq)  H 2 (g) + Cl 2 (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. 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. sodium chloride solution using mercury electrodes CationAnion H + (aq)OH - (aq) Na + (aq)Cl - (aq) Hg( )

42 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. sodium chloride solution using mercury electrodes The sodium amalgam then reacts with water to form sodium hydroxide and hydrogen. 2Na/Hg(l) + 2H 2 O(l)  2NaOH(aq) + H 2 (g) + 2Hg(l)

44 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)  Cl 2 (g) + 2e –

45 Case 7: Electrolysis of conc. sodium chloride solution using mercury electrodes Overall reaction: 2Na + (aq) + 2Cl – (aq) + 2Hg(l)  2Na/Hg(l) + Cl 2 (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 CationAnion H + (aq)OH - (aq) Cu 2+ (aq)SO 4 2- (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. Cu 2+ (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)  Cu 2+ (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 Chemical cellElectrolytic cell FunctionA 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 Chemical cellElectrolytic cell Reactions at positive electrode Reduction (Cathode)Oxidation (anode) Reactions at negative electrode Oxidation (anode)Reduction (Cathode)

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 At the anode: 2Cl – (aq)  Cl 2 (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) + 2H 2 O(l)  2NaOH(aq) + H 2 (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 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)  Zn 2+ (aq) + 2e – Fe(s)  Fe 2+ (aq) + 2e – Cu(s)  Cu 2+ (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. Cu 2+ (aq) + 2e –  Cu(s)

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

63 Electroplating

64 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. M n+ (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)  O 2 (g) + 2H 2 O( ) + 4e – Oxygen is then reacted with aluminium anode to form a thick protection oxide layer. 4Al(s) + 3O 2 (g)  2Al 2 O 3 (s) Aluminium oxide can be dyed.


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