Electrolysis of Liquids

1. Electrolysis of Liquids In this presentation you will: • explore the electrolysis of liquids Next >

2. e- e- e- P+ n P+ P+ n n n P+ e- e- e- Introduction There is a close relationship between electricity and chemistry, since atoms themselves are made up of electrically charged particles (protons and electrons). The chemical effects of electricity are of considerable economic importance, since electricity is used in the extraction of many elements from their compounds. Next >

3. _ + Source of direct current + + + _ _ _ Cathode Anode Definition Electricity flows through a molten salt or through a solution of an electrolyte, causing it to split up in a chemical process called electrolysis. Electrolysis is the use of an electric current to bring about chemical change. Next >

4. _ + Source of direct current + + + _ _ _ Cathode Anode Reactions at the Electrodes When an electric current is driven through an ionic solution, the current is carried by the movement of both positive and negative ions through the liquid. Next >

5. _ + Source of direct current + + + _ _ _ Cathode (reductionoccurs here) Anode (oxidation occurs here) Reactions at the Electrodes Positive ions (cations) migrate towards the negative electrode (cathode), and negative ions (anions) migrate towards the positive electrode (anode). The passage of electricity through the electrolyte causes chemical reactions to occur at both electrodes. Next >

6. Question 1 What is the name given to negative ions? A) Cathodes B) Anodes C) Cations D) Anions Next >

7. Question 1 What is the name given to negative ions? A) Cathodes B) Anodes C) Cations D) Anions Next >

8. _ + Source of direct current + + + _ _ _ Reactions at the Electrodes A source of direct current (battery) connected to an electrolytic cell pumps electrons round the circuit to one electrode (cathode), giving it a negative charge. Anode (oxidation: A- → A + e-) Cathode (reduction: B+ + e- → B) Next >

9. _ + Source of direct current + + + _ _ _ Reactions at the Electrodes Cations from the solution are reduced (electron gain) at the cathode. The battery simultaneously draws electrons from the other electrode (anode), giving it a positive charge. At the anode, negatively charged ions (anions) are oxidized (electron loss). Anode (oxidation: A- → A + e-) Cathode (reduction: B+ + e- → B) Next >

10. Question 2 Which of the following statements about an electrolytic cell is correct? A) When an electric current is driven through an ionic compound, cations migrate towards the cathode, and anions migrate towards the anode. B) When an electric current is driven through an ionic compound, cations migrate towards the anode, and anions migrate towards the cathode. C) At the anode, anions are reduced. D) In an electrolytic cell, electrical current can be produced by chemical reactions. Next >

11. Question 2 Which of the following statements about an electrolytic cell is correct? A) When an electric current is driven through an ionic compound, cations migrate towards the cathode, and anions migrate towards the anode. B) When an electric current is driven through an ionic compound, cations migrate towards the anode, and anions migrate towards the cathode. C) At the anode, anions are reduced. D) In an electrolytic cell, electrical current can be produced by chemical reactions. Next >

12. Cathode, where metallic sodium is formed _ Anode, where chlorine gas is formed e- + e- 2Cl-(l) → Cl2(g) + 2e- 2Na+(l) + 2e- → 2Na(l) Molten sodium chloride 2Cl- + 2Na+ → 2Na(l) + Cl2(g) Battery CI- Na+ Reactions at the Electrodes For the electrolysis of molten NaCl, the chemical changes occurring at the two electrodes can be summarized by writing the two half equations: Oxidation at the anode: Reduction at the cathode: The net reaction equation is: Next >

13. Question 3 What liquid is formed by reduction at the cathode of a molten NaCl electrolytic cell? A) Chlorine B) Metallic sodium C) Sodium chloride D) Water Next >

14. Question 3 What liquid is formed by reduction at the cathode of a molten NaCl electrolytic cell? A) Chlorine B) Metallic sodium C) Sodium chloride D) Water Next >

15. Hydrogen H2 Oxygen O2 Test tubes + _ Anode + Cathode - Electrolysis of Water By itself, water is a poor conductor of electricity. However, if ions (electrolyte) are added to it, the ions can flow through the solution so that water can be electrolyzed. The electrolyte added must be soluble in water and, most importantly, must be harder to oxidize and reduce than water (needs more energy). Next >

16. O2(g) H2(g) + _ 2H2O(l) + 2e-→H2(g) + 2OH-(aq) 2H2O(l) → O2(g) + 4H+(aq) + 4e- 2H2O →2H2(g) + O2(g) Electrolysis of Water Reduction at the cathode: 2 H2O(l) + 2e- →H2(g) + 2OH-(aq) In the electrolysis of an electrolyte, such as the salt K2SO4 (K+, (SO4)2- ions), neither of the two ions undergo any transformation; they just remain in solution. Oxidation at the anode: 2H2O(l) → O2(g) + 4H+(aq) + 4e- The only reactions are those involving water: K2SO4 solution Reduction at the cathode: Overall net reaction: Oxidation at the anode: Next >

17. e- e- _ + _ + Cl- H+ OH- H2O Na+ Anode Aqueous Solutions The electrolysis of an aqueous solution is not as simple as that of water or molten salt, since both water molecules and the solute ions can be oxidized and reduced. Water molecules, as well as ions from the solute, can be reduced at the cathode and oxidized at the anode. Cathode Sodium chlorate solution Next >

18. e- e- _ + _ + Cl- H+ OH- H2O Na+ Anode Aqueous Solutions When different reactions are possible, it is considered that only the more reactive ions (stronger oxidizing or reducing agents) would react at the electrodes. Cathode Sodium chlorate solution Next >

19. e- e- 2Na+(aq) + 2e- → 2Na(l) 2H2O(l) + 2e-→H2(g) + 2OH-(aq) _ + _ + Cl- H+ 2Cl-(aq) → Cl2(g) + 2e- 2H2O(l) → O2(g) + 4H+(aq) + 4e- OH- Na+ Anode 2Cl- + 2Na+ + 2H2O →2Na(l) + Cl2(g) + 2H2(g) + O2(g) Aqueous Solutions The reactions occurring in an aqueous solution of NaCl are: Reduction at the cathode: Oxidation at the anode: Cathode Sodium chlorate solution Overall net reaction: Next >

20. Question 4 In the electrolysis of an aqueous solution of an ionic compound, what ions would be formed in addition to those from the solute? A) H+ ions B) (OH)- ions C) H+ and (OH)- ions D) K+, and (SO4)2- ions Next >

21. Question 4 In the electrolysis of an aqueous solution of an ionic compound, what ions would be formed in addition to those from the solute? A) H+ ions B) (OH)- ions C) H+ and (OH)- ions D) K+, and (SO4)2- ions Next >

22. Faraday’s First Law In 1833, Michael Faraday discovered a relationship between the amount of electrical charge passed through an electrolytic cell and the extent of the chemical change during electrolysis. Next >

23. Impure copper anode e- e- Dissolution of copper from anode _ + _ + Deposition of copper on the cathode Anode mud (Ag, Au, Pt) Faraday’s First Law This relationship is known as Faraday’s First Law. Pure copper cathode It states that the amount of a substance deposited on each electrode of an electrolytic cell is directly proportional to the quantity of electricity passed through the cell. Next >

24. NO3- Ag+ Ag+ e- e- _ + Ag+(aq) + e-→ Ag(s) Battery Spoon cathode Silver anode Faraday’s First Law The diagram shows a cell for the electrolysis of silver nitrate. The reaction occurring at the cathode is: Next >

25. NO3- Ag+ Ag+ e- e- _ + Battery Spoon cathode Silver anode Faraday’s First Law The mass of silver deposited can be measured by weighing the cathode before and after the electrolysis. The results of experimentation on this cell with different currents and times can be plotted as a straight line graph. This shows Faraday’s First Law to be true. Next >

26. Question 5 "Faraday’s First Law explains that the amount of a substance deposited on each electrode of an electrolytic cell is indirectly proportional to the quantity of electricity passed through the cell." Is this statement true or false? Answer True or False. Next >

27. Question 5 "Faraday’s First Law explains that the amount of a substance deposited on each electrode of an electrolytic cell is indirectly proportional to the quantity of electricity passed through the cell." Is this statement true or false? Answer True or False. False Next >

28. Impure copper anode Pure copper cathode e- e- _ + _ + Battery Anode mud (Ag, Au, Pt) Faraday’s Second Law Faraday’s Second Law states that the mass of different substances produced by the same quantity of electricity are directly proportional to the molar masses of the substances concerned. Next >

29. Impure copper anode Pure copper cathode e- e- _ + _ + Battery Anode mud (Ag, Au, Pt) Faraday’s Second Law This means that z moles of electrons are needed to discharge an ion Xz+ or Xz-. Therefore, the quantity of electricity required to produce a mole of a substance from its ions is proportional to the charge on the ions. Next >

30. Ammeter e- A e- C A e- _ C + C e- A Battery A = Anode C = Cathode Faraday’s Second Law The arrangement shown in the figure illustrates Faraday’s Second Law. The current flowing through each solution is the same, so the same quantity of charge flows through each solution. Cr2(SO4)3(aq) AgNO3(aq) CuSO4(aq) Next >

31. Ammeter e- A e- C A e- _ C + C e- A Battery A = Anode C = Cathode Quantity of charge in coulombs Current in amps Time in seconds × = Faraday’s Second Law The quantity of charge (C) is the product of the current (A) and the time (s). Cr2(SO4)3(aq) AgNO3(aq) CuSO4(aq) Next >

32. Ammeter e- A e- C A e- C + _ C e- A Battery A = Anode C = Cathode Faraday’s Second Law A current of 0.1 A flowing for 9,600 s through the three solutions produced 1.08 g (1/100 mol) of silver, 0.32 g (1/200 mol) of copper, and 0.173 g (1/300 mol) of chromium. Cr2(SO4)3(aq) CuSO4(aq) AgNO3(aq) Next >

33. Ammeter e- A e- C A e- C + _ C e- A Battery A = Anode C = Cathode Faraday’s Second Law The quantity of charge is 0.1 A × 9,600 s = 960 C. Therefore, to produce 1 mol of silver from Ag+ requires 96,000 C. To produce 1 mol of copper from Cu2+ requires 190,000 C. To produce 1 mol of chromium from Cr3+ requires 288,000 C. Cr2(SO4)3(aq) CuSO4(aq) AgNO3(aq) Next >

34. Question 6 How is the quantity of electricity calculated? A) Quantity of electricity in amps = Current in coulombs × time in seconds B) Quantity of electricity in coulombs = Current in amps × time in seconds C) Quantity of electricity in amps = Current in coulombs - time in seconds D) Quantity of electricity in coulombs = Current in amps / time in seconds Next >

35. Question 6 How is the quantity of electricity calculated? A) Quantity of electricity in amps = Current in coulombs × time in seconds B) Quantity of electricity in coulombs = Current in amps × time in seconds C) Quantity of electricity in amps = Current in coulombs - time in seconds D) Quantity of electricity in coulombs = Current in amps / time in seconds Next >

36. Electrolysis Applications Various substances are prepared by electrolysis. For example; chlorine by the electrolysis of a solution of common salt; hydrogen by the electrolysis of water. The reduction of some reactive metals for their extraction from their ore is done by electrolysis too. Two metals extracted by electrolysis are aluminum and sodium. Next >

37. Electrolysis Applications Aluminum is extracted from raw bauxite, which contains aluminum oxide (alumina) with impurities such as iron and silica, by the Hall-Héroult process. Graphite anodes Pure powdered bauxite is dissolved in molten cryolite, filtering the insoluble impurities and precipitating the alumina. Steel cell Molten aluminum Molten aluminum out Graphite cathode (cell lining) Next >

38. Electrolysis Applications The alumina is then dissolved and electrolyzed using a carbon cathode. Molten aluminum forms at the cathode. Electrolytic processes are used in the production and purification of Lithium, Potassium, Sodium, Magnesium, and Copper. Next >

39. Question 7 How is electrolysis used in the extraction of aluminum from bauxite? A) Raw solid bauxite is electrolyzed, melting the aluminum. B) Pure bauxite is dissolved to produce alumina, which is then electrolyzed, precipitating the aluminum. Next >

40. Question 7 How is electrolysis used in the extraction of aluminum from bauxite? A) Raw solid bauxite is electrolyzed, melting the aluminum. B) Pure bauxite is dissolved to produce alumina, which is then electrolyzed, precipitating the aluminum. Next >

41. Impure copper anode Pure copper cathode e- e- _ + _ + Battery Anode mud (Ag, Au, Pt) Electrolysis Applications In the purification of copper, a sheet of pure copper is the cathode of an electrolysis cell. An electrolyte containing copper ions is used and, at the appropriate voltage, the impure copper is oxidized. Next >

42. Impure copper anode Pure copper cathode e- e- _ + _ + Cu(s) → Cu2+(aq) + 2e- Battery Anode mud (Ag, Au, Pt) Cu2+(aq) + 2e- → Cu(s) Electrolysis Applications The copper anode itself dissolves, the impurities falling to the base of the cell forming the anode mud: While at the cathode, copper is deposited, obtaining about 99.96% purity. Next >

43. Question 8 Which substance is used as the cathode in the purification of copper? A) Magnesium B) Zinc C) Copper D) Carbon Next >

44. Question 8 Which substance is used as the cathode in the purification of copper? A) Magnesium B) Zinc C) Copper D) Carbon Next >

45. Question 9 Which of the following half-cell formulas represents the reaction occurring on the anode of an electrolytic cell for purification of copper? A) Cu(s) → Cu2+ (aq) + 2e- B) Cu2 + (aq) + 2e- → Cu(s) C) Cu2 + (aq) → Cu(s) + 2e- D) Cu(s) + 2e- → Cu2 + (aq) Next >

46. Question 9 Which of the following half-cell formulas represents the reaction occurring on the anode of an electrolytic cell for purification of copper? A) Cu(s) → Cu2+ (aq) + 2e- B) Cu2 + (aq) + 2e- → Cu(s) C) Cu2 + (aq) → Cu(s) + 2e- D) Cu(s) + 2e- → Cu2 + (aq) Next >

47. Electroplating Some metallic parts can be plated to improve functionality (corrosion and wear resistance) and/or decorative purposes. Electroplating is the coating of an electrically conductive item with a layer of metal using electrical current. The process used in electroplating is called electrodeposition. The item to be coated is placed into a solution of metal salts. Next >

48. NO3- Ag+ Ag+ e- e- _ + Battery Spoon cathode Silver anode Electroplating In electroplating, the item is connected to form the cathode of an electric circuit; an electrode, typically of the same metal to be plated, forms the anode. When an electrical current is passed through the circuit, metal ions in the solution are attracted to the cathode, depositing on it. Next >

49. NO3- Ag+ Ag+ e- e- _ + Battery Spoon cathode Silver anode Electroplating The anode replenishes the supply of positive metal ions, thus gradually being dissolved. The result is a thin, smooth, even coat of metal on the object. Next >

50. Question 10 What happens when an electrical current is passed through an electrolysis plating circuit? A) Metal ions in the solution are attracted to the anode, replacing its molecular composition. B) Metal ions in the solution are attracted to the cathode, replacing its molecular composition. C) Metal ions in the solution are attracted to the anode, depositing on it. D) Metal ions in the solution are attracted to the cathode, depositing on it. Next >