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Electrolysis

Electrolysis. 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.

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Electrolysis

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

  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.

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

  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

  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

  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

  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

  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. Conc. Case 6: Electrolysis of conc. sodium chloride solution using carbon electrodes

  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– H2(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)  Cl2(g) + 2e–

  39. 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. 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

  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) + 2H2O(l)  2NaOH(aq) + H2(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)  Cl2(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) + 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

  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.

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