Electrolysis

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Electrolysis

  1. 1. Electrolysis<br />Notes<br />Electrolytic Cells with inert electrodes<br />Some ionic solution eg. HCl (aq)<br /><ul><li>Electrons flow from the negative terminal of the battery to the cathode
  2. 2. Cations (positively charged ions) are attracted to the cathode
  3. 3. Cations take in electrons to become atoms/molecules (Reduction)
  4. 4. Anions (negatively charged ions) move to the anode
  5. 5. Anions give away electrons to become atoms/molecules (Oxidation)
  6. 6. The electrons flow back to the positive terminal of the battery</li></ul>At the cathode:Refer to the cations in the reactivity series. The less reactive ions are discharged first.<br />H2+ (aq) + 2e- --> H2 (g)<br />Observation: Effervescence is observed. The gas liberated extinguishes a lighted splint with a ‘pop’ sound.<br />At the anode:Most easily discharged: OH-In concentrated solutions: Cl-, Br-, I-SO42- and NO3- are never discharged.<br />4OH- (aq) - 4e- --> O2 (g) + 2H2O (l)<br />Observation: Effervescence is observed. The gas liberated relights a glowing splint.<br /> Overall equation:<br />2H2+ (aq) + 4OH- (aq) --> H2 (g) + O2 (g) + 2H2O (l)<br />Reactive (non-inert) electrodes<br />Copper barCoin (to be plated)<br />CuSO4 (aq)<br />At the cathode,<br />Cu2+ (aq) + 2e- --> Cu (s)<br />Observation: The cathode (coin) is coated with a reddish brown deposit. It becomes thicker.<br />At the anode,<br />Cu (s) - 2e- --> Cu2+ (aq)<br />Observation: The anode (copper bar) becomes thinner.<br />Note: This is an example of electroplating.<br />Simple Cells<br />e-<br />Cu: Less reactive metalZn: More reactive metal<br />dilute HCl (aq)<br /><ul><li>The electrode made of the more reactive metal displaces the cation in the solution
  7. 7. The more reactive metal gives out electrons to become ions (Oxidation)
  8. 8. Electrons flow through the wire to reach the less reactive metal
  9. 9. There is a current flowing through the wire now
  10. 10. Cations in the solution move to the less reactive metal
  11. 11. The cations take in electrons to become atoms/molecules (Reduction)</li></ul>The more reactive metal:Zn (s) - 2e- --> Zn2+ (aq)<br />Observation: The electrode becomes thinner.<br />The less reactive metal:H+ (aq) + 2e- --> H2 (g)<br />Observation: Effervescence is observed. The gas liberated extinguishes a lighted splint with a ‘pop’ sound.<br />Notes:<br />The further away the two metals are from each other in the reactivity series, the larger the current generated.<br />Workout questions<br /><ul><li>An electric current is passed through aqueous Potassium Nitrate with inert electrodes. What is formed at the anode?
  12. 12. Hydrogen gas
  13. 13. Potassium deposits
  14. 14. Oxygen gas
  15. 15. Nitrogen dioxide gas
  16. 16. In the electrolysis of Copper(II) Sulphate with Copper electrodes, how do the masses of the electrodes change?
  17. 17. Mass of cathodeMass of anodea.increasesIncreasesb.increasesDecreasesc.decreasesIncreasesd.decreasesdecreases
  18. 18. In the electrolysis of liquid X using inert electrodes, the volume of the gas formed at the cathode is twice that of the gas formed at the anode. What could liquid X be?
  19. 19. Aqueous copper (II) sulphate
  20. 20. Concentrated HCl
  21. 21. Molten NaCl
  22. 22. Dilute H2SO4
  23. 23. In a simple cell using rods made of material X and material Y and dilute HCl as the electrolyte, bubbles are seen at the rod made of material Y. Which of the following must be true?
  24. 24. X is more reactive than Y
  25. 25. Y is more reactive than X
  26. 26. There is no way to tell which material is more reactive
  27. 27. X and Y are equally reactive
  28. 28. Molten NaCl is electrolysed using platinum electrodes. What is discharged at the cathode?
  29. 29. Chlorine
  30. 30. Hydrogen
  31. 31. Oxygen
  32. 32. Sodium
  33. 33. Concentrated potassium iodide is electrolysed using carbon electrodes. What is observed?
  34. 34. cathodeanodea.Bubbles are seenSolution turns brownb.Bubbles are seenBrown ppt depositedc.Solution turns brownSolution turns dark purpled.Solution turns dark purpleBrown gas is liberated
  35. 35. Aqueous zinc chloride is electrolysed using platinum electrodes. What happens at the cathode?
  36. 36. Zinc ions are oxidized
  37. 37. Zinc ions are reduced
  38. 38. Hydrogen ions are oxidized
  39. 39. Hydrogen ions are reduced
  40. 40. A simple cell is set up with dilute HCl as the electrolyte and electrodes made of aluminium and zinc. Which correctly describes the flow of electrons?
  41. 41. Through the wire, from the Al electrode to the Zn electrode
  42. 42. Through the wire, from the Zn electrode to the Al electrode
  43. 43. Through HCl (aq), from the Al electrode to the Zn electrode
  44. 44. Through HCl (aq), from the Zn electrode to the Al electrode
  45. 45. The highest current is required to discharge one mole of the following
  46. 46. Sodium
  47. 47. Zinc
  48. 48. Aluminium
  49. 49. Magnesium
  50. 50. What happens when concentrated NaCl (aq) is electrolysed using inert electrodes?
  51. 51. The pH of the solution increases
  52. 52. The pH of the solution decreases
  53. 53. There is no way to know how the pH of the solution changes
  54. 54. The pH of the solution is unaffected
  55. 55. Which requires the largest quantity of electricity in order to be discharged?
  56. 56. 1 mole of aluminium ions
  57. 57. 4 moles of zinc ions
  58. 58. 6 moles of calcium ions
  59. 59. 10 moles of potassium ions
  60. 60. To electroplate a necklace with silver, what must be used as the anode?
  61. 61. The necklace
  62. 62. Carbon
  63. 63. Platinum
  64. 64. Silver
  65. 65. Draw an electrolytic cell with platinum electrodes and dilute NaCl (aq) as the electrolyte.
  66. 66. Cathode
  67. 67. Reaction: ___________________________________________________________
  68. 68. Observation: _________________________________________________________
  69. 69. Anode
  70. 70. Reaction: ___________________________________________________________
  71. 71. Observation: _________________________________________________________
  72. 72. Overall reaction: ______________________________________________________
  73. 73. Draw an electrolytic cell with carbon electrodes and concentrated NaCl (aq) as the electrolyte.
  74. 74. Cathode
  75. 75. Reaction: ___________________________________________________________
  76. 76. Observation: _________________________________________________________
  77. 77. Anode
  78. 78. Reaction: ___________________________________________________________
  79. 79. Observation: _________________________________________________________
  80. 80. Overall reaction: ______________________________________________________
  81. 81. Draw an electrolytic cell with graphite electrodes and molten NaCl as the electrolyte.
  82. 82. Cathode
  83. 83. Reaction: ___________________________________________________________
  84. 84. Observation: _________________________________________________________
  85. 85. Anode
  86. 86. Reaction: ___________________________________________________________
  87. 87. Observation: _________________________________________________________
  88. 88. Overall reaction: ______________________________________________________
  89. 89. In the space provided below, draw an electrolytic cell with two copper electrodes and copper (II) sulphate as the electrolyte.
  90. 90. Write chemical equations for the reactions at both electrodes.
  91. 91. ________________________________________________________________________________________________________________________________________________
  92. 92. What can be observed?
  93. 93. ________________________________________________________________________________________________________________________________________________
  94. 94. The Copper(II) Sulphate electrolyte is blue because of the Cu2+ ions. How does the color of the solution change throughout the experiment? Explain.
  95. 95. ________________________________________________________________________________________________________________________________________________
  96. 96. In the space below,
  97. 97. draw a simple cell with
  98. 98. A voltmeter
  99. 99. An electrode made of Zinc
  100. 100. An electrode made of Lead
  101. 101. Dilute HCl as the electrolyte
  102. 102. Write the chemical equations for the reactions that take place at each electrode
  103. 103. ________________________________________________________________________________________________________________________________________________
  104. 104. What can be observed?
  105. 105. ________________________________________________________________________________________________________________________________________________
  106. 106. What can be done to increase the reading on the voltmeter?
  107. 107. ________________________________________________________________________
  108. 108. How would the results of the experiment be different if the same material is used for both electrodes?
  109. 109. ________________________________________________________________________

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