Chapter 32. Electrolysis

Transcription

1 Chapter 32 Electrolysis 32.1 Electrolysis a type of redox reactions 32.2 Predicting preferential discharge of ions 32.3 Electrolysis of dilute sulphuric acid 32.4 Electrolysis of sodium chloride solution 32.5 Electrolysis of copper(ii) sulphate solution 32.6 Predicting products of electrolysis of some aqueous solutions and molten substances P. 1 / 95

2 32.7 Industrial applications of electrolysis 32.8 Environmental impact of the electroplating industry Key terms Progress check Summary Concept map P. 2 / 95

3 32.1 Electrolysis a type of redox reactions What is electrolysis? Many redox reactions are spontaneous, meaning that they take place by themselves. Figure 32.1 Corrosion of metals is a spontaneous process. There are redox reactions that do not occur spontaneously, e.g. electrolysis reactions. P. 3 / 95

4 Electrolysis means decomposition by electricity. Electrical energy forces oxidation (at the anode) and reduction (at the cathode) to take place. During electrolysis, electrical energy is converted into chemical energy Electrolysis a type of redox reactions P. 4 / 95

5 A basic set-up for electrolysis d.c. power supply in the laboratory, a battery is often used. This acts as an electron pump. ammeter used to measure the electric current passing through the circuit. The unit of current is ampere, A. anode variable resistor used to vary the resistance of the circuit, hence to regulate the current. cathode anion electrolyte (in molten state or aqueous solution) cation Figure 32.2 A basic set-up for electrolysis. Concept check 32.1 Electrolysis a type of redox reactions P. 5 / 95

6 Terms commonly used in electrolysis Electrolytic cell An electrolytic cell is a container in which electrolysis occurs. Electrolyte An electrolyte is a compound which, in molten state or in aqueous solution, conducts electricity, and is decomposed at the same time. Electrode Electrodes are conductors immersed in the electrolyte and connected to a d.c. power supply. They are usually solid metals or graphite Electrolysis a type of redox reactions P. 6 / 95

7 Anode An anode of an electrolytic cell is the electrode connected to the positive terminal of the d.c. power supply. It is the positive electrode. Oxidation takes place at the anode. Cathode A cathode of an electrolytic cell is the electrode connected to the negative terminal of the d.c. power supply. It is the negative electrode. Reduction takes place at the cathode Electrolysis a type of redox reactions P. 7 / 95

8 Anion During electrolysis, anions (negative ions) are attracted towards the anode (+). Cation During electrolysis, cations (positive ions) are attracted towards the cathode ( ). Key point Electrolysis means decomposition by electricity. It is the process in which a redox reaction is forced to take place when electricity is supplied to an electrolyte (molten or in aqueous solution) Electrolysis a type of redox reactions P. 8 / 95

9 Comparing a chemical cell and an electrolytic cell In a chemical cell, chemical changes cause electron flow outside the cell, generating electricity. In an electrolytic cell, electrical energy produces chemical changes. CHEMICAL CELL positive electrode (cathode) negative electrode (anode) electron flow anode Figure 32.3 A chemical cell connected to an electrolytic cell. cathode electrolyte ELECTROLYTIC CELL 32.1 Electrolysis a type of redox reactions P. 9 / 95

10 Chemical cells Chemical changes Electrical energy Electrolytic cells (electrolysis) Figure 32.4 A chemical cell and an electrolytic cell have different functions Electrolysis a type of redox reactions P. 10 / 95

11 Function Type of reaction involved Polarity of electrode (and chemical change) Chemical cell A device for generating electricity by chemical changes (Figure 32.3) A spontaneous redox reaction Electrolytic cell A device for bringing about chemical changes by electrical energy (Figure 32.3) A redox reaction is forced to take place cathode + (reduction) (reduction) anode (oxidation) + (oxidation) Table 32.1 Comparison of a chemical cell and an electrolytic cell Electrolysis a type of redox reactions P. 11 / 95

12 32.2 Predicting preferential discharge of ions Electrolysis of a molten salt electron flow anode (graphite) cathode (graphite) Figure 32.5 A simple set-up for the electrolysis of molten sodium chloride. crucible molten sodium chloride Solid sodium chloride consists of Na + and Cl ions. They cannot move freely. When molten, these ions become mobile. P. 12 / 95

13 A d.c. power supply acts as an electron pump. It forces electrons from its negative terminal to the cathode of the electrolytic cell. This makes the cathode negatively charged. Na + ions are attracted to the cathode. When a Na + ion touches the cathode, it gains one electron and becomes discharged: Na + (l) + e Na(l) (reduction) +1 (from 0 cathode) 32.2 Predicting preferential discharge of ions P. 13 / 95

14 The positive terminal of the d.c. power supply draws electrons away from the anode of the electrolytic cell. This makes the anode positively charged. Cl ions are attracted to the anode. When a Cl ion touches the anode, it loses one electron and becomes discharged: Cl (l) Cl(g) + e 1 0 (to anode) The chlorine atoms formed join up in pairs to form chlorine molecules. 2Cl (l) Cl 2 (g) + 2e (oxidation) Predicting preferential discharge of ions P. 14 / 95

15 The overall reaction is: 2NaCl(l) 2Na(l) + Cl 2 (g) (redox) During electrolysis, the number of electrons gained by cations must be equal to that lost by anions. Discharge of ions takes place only on the surface of electrodes Predicting preferential discharge of ions P. 15 / 95

16 electron pump draws electrons away from the anode d.c. power supply as an electron pump electron pump forces electrons away into the cathode Cl Cl anode (graphite) Cl Na + Na cathode (graphite) crucible Cl Cl Cl + Na Na Na + Na + molten sodium chloride Figure 32.6 Changes at the surface of electrodes during the electrolysis of molten sodium chloride Predicting preferential discharge of ions P. 16 / 95

17 Key point During electrolysis, electrons move between the electrodes through connecting wires, and mobile ions move in the electrolyte. Class practice Predicting preferential discharge of ions P. 17 / 95

18 Preferential discharge of ions in aqueous solution of electrolytes When molten sodium chloride is electrolysed, the products formed at cathode and anode are sodium and chorine respectively. What are the products at each electrode when an aqueous solution of sodium chloride is electrolysed? 32.2 Predicting preferential discharge of ions P. 18 / 95

19 Basic concepts related to aqueous solution of electrolyte Dissolving electrolytes in water Ionic compounds are common electrolytes. When sodium chloride dissolves in water, it dissociates to form mobile ions. water NaCl(s) Na + (aq) + Cl (aq) dissociation mobile ions 32.2 Predicting preferential discharge of ions P. 19 / 95

20 Acids are electrolytes. When dissolved in water, molecules of acids ionize (i.e. molecules become ions). Examples water HCl(g) H + (aq) + Cl (aq) molecules mobile ions ionization water H 2 SO 4 (l) 2H + (aq) + SO 2 4 (aq) molecules mobile ions ionization 32.2 Predicting preferential discharge of ions P. 20 / 95

21 Ionization of water Water itself ionizes very slightly to give H + (aq) and OH (aq) ions. H 2 O(l) H + (aq) + OH (aq) molecules mobile ions ionization In an aqueous solution of any electrolyte, there are H + (aq) ions and OH (aq) ions, and the ions of that electrolyte Predicting preferential discharge of ions P. 21 / 95

22 Factors affecting preferential discharge of ions The aqueous solution of sodium chloride contains two kinds of cations and two kinds of anions: Cation Anion From sodium chloride (NaCl) Na + (aq) Cl (aq) From water (H 2 O) H + (aq) OH (aq) During electrolysis, both Na + (aq) and H + (aq) ions move towards the cathode both Cl (aq) and OH (aq) ions move towards the anode Predicting preferential discharge of ions P. 22 / 95

23 One kind of cation and one kind of anion are discharged before the others. These ions are said to be preferentially discharged. What are the factors that affect the preferential discharge of ions? 32.2 Predicting preferential discharge of ions P. 23 / 95

24 1. Position of ions in the Electrochemical Series, E.C.S. CATIONS K + Ca 2+ Na + Mg 2+ Al 3+ Zn 2+ Fe 2+ Pb 2+ H + Cu 2+ Ag + increasing ease of discharge at the cathode (increasing readiness to gain electrons) OH I Br Cl NO 3 SO 4 2 increasing ease of discharge at the anode (increasing readiness to lose electrons) ANIONS Figure 32.7 The order of discharge of some ions according to their positions in the E.C.S Predicting preferential discharge of ions P. 24 / 95

25 Learning tip NO 3 (aq) and SO 2 4 (aq) are such weak reducing agents that they are never discharged in aqueous solutions. Cations lower in the E.C.S. are more readily discharged because they are stronger oxidizing agents (i.e. they gain electrons more readily). Anions higher in the E.C.S. are more readily discharged because they are stronger reducing agents (i.e. they lose electrons more readily) Predicting preferential discharge of ions P. 25 / 95

26 2. Concentration of ions in solution The order of discharge of ions may change if the concentration of a type of ions is much higher than the other, and the positions of the two types of ions are not too far apart in the E.C.S Predicting preferential discharge of ions P. 26 / 95

27 Example Suppose a solution containing the same concentration of OH (aq) ions and Cl (aq) ions is electrolysed. OH (aq) ions are preferentially discharged at the anode, as predicted from the relative positions of the ions in the E.C.S. However, if the concentration of Cl (aq) ions is much higher than that of OH (aq) ions, Cl (aq) ions are preferentially discharged instead Predicting preferential discharge of ions P. 27 / 95

28 3. Nature of electrodes Inert electrodes (graphite or platinum) are often used in electrolysis. They do not take part in any reaction inside the electrolytic cell. They do not affect the order of discharge of ions. However, other electrodes may affect the preferential discharge of ions and hence the products formed Predicting preferential discharge of ions P. 28 / 95

29 Learning tip Strictly speaking, graphite and platinum are not completely inert. When oxygen is liberated at a graphite anode, the product is contaminated with traces of carbon dioxide due to oxidation of carbon. When chlorine is liberated at a platinum anode, it will attack platinum. Class practice Predicting preferential discharge of ions P. 29 / 95

30 32.3 Electrolysis of dilute sulphuric acid A Hofmann voltameter (or simply called voltameter) is used for the electrolysis of dilute sulphuric acid using platinum electrodes. dilute sulphuric acid stopcock hydrogen (2 volumes) oxygen (1 volume) platinum foil platinum foil (cathode) platinum foil (anode) Figure 32.8 The electrolysis of dilute sulphuric acid using a Hofmann voltameter. P. 30 / 95

31 Cation Anion From sulphuric acid (H 2 SO 4 ) H + (aq) SO 4 2 (aq) From water (H 2 O) H + (aq) OH (aq) At the cathode: H + (aq) ions are discharged (reduced) to form hydrogen gas. 2H + (aq) + 2e H 2 (g)...(1) At the anode: Two types of anions (SO 2 4 (aq) and OH (aq) ions). OH (aq) ions are preferentially discharged (oxidized) to form oxygen gas and water. 4OH (aq) O 2 (g) + 2H 2 O(l) + 4e...(2) 32.3 Electrolysis of dilute sulphuric acid P. 31 / 95

32 Overall reaction: Combine the above two half equations [(1) 2 + (2)] to get the overall equation: 4H + (aq) + 4OH (aq) 2H 2 (g) + O 2 (g) + 2H 2 O(l) 4H 2 O(l) i.e. 2H 2 O(l) 2H 2 (g) + O 2 (g) Volume ratio 2 : 1 The ratio of volume of H 2 (g) formed at the cathode to the volume of O 2 (g) formed at the anode is about 2 : 1. The electrolysis of dilute sulphuric acid is actually the electrolysis of water Electrolysis of dilute sulphuric acid P. 32 / 95

33 Changes in the solution: As electrolysis goes on, water molecules ionize continuously to replace the H + (aq) and OH (aq) ions discharged. H 2 O(l) H + (aq) + OH (aq) The amount of water molecules decreases. The number of H + (aq) and SO 4 2 (aq) ions from sulphuric acid remains unchanged. H 2 SO 4 2H + SO 4 2 Figure 32.9 Ions preferentially discharged in the H + OH electrolysis of dilute sulphuric acid using platinum H 2 O electrodes. discharged platinum cathode platinum anode 32.3 Electrolysis of dilute sulphuric acid e P. 33 / 95 e

34 The concentration of sulphuric acid slowly increases. Experiment 32.1 Experiment 32.1 Example 32.1 Class practice Electrolysis of dilute sulphuric acid P. 34 / 95

35 32.4 Electrolysis of sodium chloride solution Electrolysis of very dilute sodium chloride solution hydrogen oxygen electrolytic cell graphite very dilute sodium chloride solution Figure The electrolysis of very dilute sodium chloride solution using graphite electrodes. e e Cation Anion From sodium chloride (NaCl) Na + (aq) Cl (aq) From water (H 2 O) H + (aq) OH (aq) P. 35 / 95

36 At the cathode: H + (aq) ions are preferentially discharged (reduced) to form hydrogen gas. 2H + (aq) + 2e H 2 (g) (reduction) +1 0 Learning tip H + (aq) is lower than Na + (aq) in the E.C.S. It is a stronger oxidizing agent than Na + (aq). Water ionizes continuously to replace the H + (aq) ions discharged. discharged H 2 O(l) H + (aq) + OH (aq) 32.4 Electrolysis of sodium chloride solution P. 36 / 95

37 The reaction at the cathode can be written as: 2H 2 O(l) + 2e H 2 (g) + 2OH (aq) (reduction) The excess OH (aq) ions around the cathode make the solution there alkaline Electrolysis of sodium chloride solution P. 37 / 95

38 At the anode: OH (aq) ions are preferentially discharged (oxidized) to form oxygen gas. 4OH (aq) O 2 (g) + 2H 2 O(l) + 4e (oxidation) 2 0 Learning tip OH (aq) is higher than Cl (aq) in the E.C.S. It is a stronger reducing agent than Cl (aq). Water ionizes continuously to replace the OH (aq) ions discharged. H 2 O(l) H + (aq) + OH (aq) discharged The excess H + (aq) ions around the anode make the solution there acidic Electrolysis of sodium chloride solution P. 38 / 95

39 Overall reaction: Combine the two half equations: 4H + (aq) + 4OH (aq) 2H 2 (g) + O 2 (g) + 2H 2 O(l) 4H 2 O(l) i.e. 2H 2 O(l) 2H 2 (g) + O 2 (g) 32.4 Electrolysis of sodium chloride solution P. 39 / 95

40 Changes in the solution: As electrolysis continues, water is decomposed. The concentration of sodium chloride solution increases. NaCl e Na + H + Cl OH e H 2 O discharged graphite cathode graphite anode Figure Ions preferentially discharged in the electrolysis of very dilute sodium chloride solution using graphite electrodes Electrolysis of sodium chloride solution Writing practice 32.1 P. 40 / 95

41 Electrolysis of concentrated sodium chloride solution 1. Using graphite electrodes At the cathode (graphite): H + (aq) ions are preferentially discharged (reduced) to form hydrogen gas. 2H + (aq) + 2e H 2 (g) (reduction) +1 0 At the anode (graphtie): Cl (aq) ions are preferentially discharged (oxidized) to form chlorine gas. 2Cl (aq) Cl 2 (g) + 2e (oxidation) Electrolysis of sodium chloride solution P. 41 / 95

42 The concentration of Cl (aq) ions is much higher than that of OH (aq) ions while the positions of Cl (aq) and OH (aq) in the E.C.S. are not too far apart. Due to the concentration effect, the discharge of Cl (aq) ions becomes more favourable. Chlorine gas is the main product at the anode, but a little amount of OH (aq) ions may also be discharged to form oxygen gas. Concept check 32.4 Electrolysis of sodium chloride solution P. 42 / 95

43 If a few drops of universal indicator are added to the electrolyte solution, the solution near the anode turns red and then colourless very quickly. When chlorine dissolves in water, hydrochloric acid and hypochlorous acid form. Cl 2 (g) + H 2 O(l) HCl(aq) + HOCl(aq) hydrochloric acid hypochlorous acid Hydrochloric acid turns universal indicator red. Hypochlorite ions ionized from hypochlorous acid turn the universal indicator colourless Electrolysis of sodium chloride solution P. 43 / 95

44 Overall reaction: Combine the two half equations of reduction and oxidation: 2H + (aq) + 2Cl (aq) H 2 (g) + Cl 2 (g) However, the actual volume of Cl 2 (g) collected would be smaller than that of H 2 (g). This is because some Cl 2 (g) dissolves in water Electrolysis of sodium chloride solution P. 44 / 95

45 Changes in the solution: As electrolysis goes on, water molecules ionize continuously to replace the H + (aq) ions discharged. discharged H 2 O(l) H + (aq) + OH (aq) OH (aq) ions accumulate. At the same time, Cl (aq) ions are consumed more rapidly than OH (aq) ions. The solution therefore gradually becomes alkaline. Figure Ions preferentially discharged in the electrolysis of concentrated sodium chloride solution using graphite electrodes Electrolysis of sodium chloride solution Na + e H + NaCl Cl OH H 2 O e discharged graphite cathode graphite anode P. 45 / 95 Think about

46 2. Using mercury cathode and graphite anode When concentrated sodium chloride solution is electrolysed using a mercury cathode, sodium rather than hydrogen is produced at the cathode. plastic-covered wire water graphite (as anode) concentrated sodium chloride solution Think about bare platinum wire mercury (as cathode) Figure A simple set-up for the electrolysis of concentrated sodium chloride solution using a mercury cathode and a graphite anode Electrolysis of sodium chloride solution P. 46 / 95

47 At the cathode (mercury): Na + (aq) ions are preferentially discharged (reduced) to form sodium metal, which immediately dissolves in the mercury cathode to give sodium amalgam. Learning tip Sodium amalgam is a liquid alloy of sodium in mercury. It can be considered as a solution of sodium metal in mercury Electrolysis of sodium chloride solution P. 47 / 95

48 As H + (aq) is a much stronger oxidizing agent than Na + (aq), we would expect H + (aq) to be preferentially discharged. However, the use of mercury as the cathode favours the discharge of Na + (aq) ions. Na + (aq) + e Na(s) (reduction) +1 0 Na(s) + Hg(l) Na/Hg(l) sodium amalgam 32.4 Electrolysis of sodium chloride solution P. 48 / 95

49 At the anode (graphite): Due to the concentration effect, Cl (aq) ions are preferentially discharged (oxidized) to give chlorine gas as the main product. 2Cl (aq) Cl 2 (g) + 2e 1 0 (oxidation) Overall reaction: Combine the two half equations: 2Hg(l) + 2Na + (aq) + 2Cl (aq) 2Na/Hg(l) + Cl 2 (g) 32.4 Electrolysis of sodium chloride solution P. 49 / 95

50 Changes in the solution: As electrolysis continues, the number of Na + (aq) ions and Cl (aq) ions decreases. The sodium chloride solution thus becomes more and more dilute. discharged e Na + NaCl Cl e Figure Ions preferentially discharged H + in the electrolysis of concentrated sodium chloride solution using a mercury cathode H 2 O and a graphite anode. mercury cathode OH graphite anode Experiment 32.2 Experiment 32.2 Class practice Electrolysis of sodium chloride solution P. 50 / 95

51 32.5 Electrolysis of copper(ii) sulphate solution Electrolysis of copper(ii) sulphatesolution using graphite electrodes graphite anode graphite cathode Figure A set-up for the electrolysis of copper(ii) sulphate solution using graphite electrodes. copper(ii) sulphate solution Cation Anion From copper(ii) sulphate (CuSO 4 ) Cu 2+ (aq) SO 2 4 (aq) From water (H 2 O) H + (aq) OH (aq) P. 51 / 95

52 At the cathode (graphite): Cu 2+ (aq) ions are preferentially discharged (reduced) to form copper. A layer of copper is deposited on the cathode. Cu 2+ (aq) + 2e Cu(s) (reduction) +2 0 The graphite cathode gets bigger and heavier as copper is deposited on it. Think about 32.5 Electrolysis of copper(ii) sulphate solution P. 52 / 95

53 At the anode (graphite): OH (aq) ions are preferentially discharged (oxidized) to form oxygen gas. 4OH (aq) O 2 (g) + 2H 2 O(l) + 4e (oxidation) 2 0 Overall reaction: Combine the two half equations: 2Cu 2+ (aq) + 4OH (aq) 2Cu(s) + O 2 (g) + 2H 2 O(l) 32.5 Electrolysis of copper(ii) sulphate solution P. 53 / 95

54 Changes in the solution: The number of Cu 2+ (aq) ions decreases, so the blue colour of the solution fades gradually. Cu 2+ (aq) and OH (aq) ions are consumed and H + (aq) and SO 4 2 (aq) ions remain. The solution eventually becomes sulphuric acid. e discharged CuSO 4 Cu 2+ SO 4 2 Figure Ions preferentially discharged in H + the electrolysis of copper(ii) sulphate solution H 2 O using graphite electrodes. graphite cathode OH e graphite anode 32.5 Electrolysis of copper(ii) sulphate solution P. 54 / 95

55 Electrolysis of copper(ii) sulphatesolution using copper electrodes copper anode copper cathode copper(ii) sulphate solution Figure A set-up for the electrolysis of copper(ii) sulphate solution using copper electrodes. Cation Anion From copper(ii) sulphate (CuSO 4 ) Cu 2+ (aq) SO 4 2 (aq) From water (H 2 O) H + (aq) OH (aq) 32.5 Electrolysis of copper(ii) sulphate solution P. 55 / 95

56 At the cathode (copper): Cu 2+ (aq) ions are preferentially discharged (reduced) to form copper. A layer of copper is deposited on the cathode. Cu 2+ (aq) + 2e Cu(s) (reduction) Electrolysis of copper(ii) sulphate solution P. 56 / 95

57 At the anode (copper): Two types of anions (SO 4 2 (aq) and OH (aq) ions). Neither of them is discharged (oxidized). The copper electrode itself dissolves to form Cu 2+ (aq) ions (oxidized). Cu(s) Cu 2+ (aq) + 2e 0 +2 (oxidation) This occurs because Cu(s) is a stronger reducing agent than OH (aq) and SO 4 2 (aq). Cu 2+ (aq) + 2e O 2 (g) + 2H 2 O(l) + 4e S 2 O 2 8 (aq) + 2e 32.5 Electrolysis of copper(ii) sulphate solution Cu(s) 4OH (aq) 2SO 2 4 (aq) P. 57 / 95 stronger reducing agent

58 Changes at the electrodes: The copper cathode gets bigger and heavier as copper is deposited on it. The copper anode becomes smaller and lighter, as it dissolves to form Cu 2+ (aq) ions. increase in mass of cathode = decrease in mass of anode 32.5 Electrolysis of copper(ii) sulphate solution P. 58 / 95

59 Changes in the solution: For every Cu 2+ (aq) ion discharged at the cathode, one Cu 2+ (aq) ion forms at the anode. The concentration of copper(ii) sulphate solution does not change. The intensity of blue colour of the solution also remains unchanged. electron flow Think about copper anode Figure The Cu 2+ (aq) ions discharged at the cathode and formed at the anode are equal in number Electrolysis of copper(ii) sulphate solution Cu 2+ Cu 2+ P. 59 / 95 copper cathode copper deposited copper(ii) sulphate solution

60 Key point Factors affecting preferential discharge of ions: Positions of ions in the E.C.S. Concentration of ions in solution Nature of electrodes Experiment 32.3 Class practice 32.5 Experiment Electrolysis of copper(ii) sulphate solution P. 60 / 95

61 32.6 Predicting products of electrolysis of some aqueous solutions and molten substances Molten substance or aqueous solution Electrodes Main products Change in solution Cathode Anode Cathode Anode Molten lead(ii) bromide graphite graphite Pb(l) Br 2 (g) Molten sodium chloride graphite graphite Na(l) Cl 2 (g) Dilute sulphuric acid platinum platinum H 2 (g) O 2 (g) Sodium nitrate solution graphite graphite H 2 (g) O 2 (g) Very dilute sodium chloride solution graphite graphite H 2 (g) O 2 (g) Table 32.2 Main products of electrolysis of some aqueous solutions and molten substances. becomes concentrated becomes concentrated becomes concentrated P. 61 / 95

62 Molten substance or aqueous solution Conc. sodium chloride solution Conc. sodium chloride solution Dilute copper(ii) chloride solution Copper(II) sulphate solution Copper(II) sulphate solution Electrodes Main products Change in solution Cathode Anode Cathode Anode graphite graphite H 2 (g) Cl 2 (g) mercury graphite Na(s) Cl 2 (g) graphite graphite Cu(s) O 2 (g) graphite graphite Cu(s) O 2 (g) becomes alkaline becomes dilute becomes hydrochloric acid becomes sulphuric acid copper copper Cu(s) Cu 2+ (aq) no change Table 32.2 Main products of electrolysis of some aqueous solutions and molten substances. Example 32.2 Example 32.3 Class practice Predicting products of electrolysis of some aqueous solutions and molten substances P. 62 / 95

63 32.7 Industrial applications of electrolysis Electroplating Electroplating is a process to plate (coat) one metal with a thin layer of another metal by electrolysis. A metal is electroplated to protect against corrosion or to improve appearance. P. 63 / 95

64 In electroplating, the process is carried out in an electrolytic cell (often called a plating bath ). the metal object (X) to be electroplated is made the cathode. the plating metal (Y) is made the anode. an aqueous solution containing ions of the plating metal (Y n+ ions) is the electrolyte solution Industrial applications of electrolysis P. 64 / 95

65 copper anode Cu 2+ key (as cathode) Cu 2+ copper(ii) sulphate solution Figure A set-up used for electroplating copper on a key. At the cathode (the metal key): Cu 2+ (aq) ions are discharged to form copper. The metal key becomes coated with copper. Cu 2+ (aq) + 2e Cu(s) (reduction) Industrial applications of electrolysis P. 65 / 95

66 At the anode (copper): The copper electrode dissolves to form Cu 2+ (aq) ions. Cu(s) Cu 2+ (aq) + 2e 0 +2 (oxidation) Changes in the solution: For every Cu 2+ (aq) ion discharged at the cathode, one Cu 2+ (aq) ion forms at the anode. The concentration of copper(ii) sulphate solution does not change. Its colour intensity also remains unchanged Industrial applications of electrolysis P. 66 / 95

67 Common examples of the plating metals: Copper Tin Chromium Nickel Silver Gold Products electroplated with the plating metals: Figure Food cans plated with tin Industrial applications of electrolysis Figure Water tap plated with chromium. Figure Jewellery plated with silver. P. 67 / 95

68 Objects which are not metals can also be electroplated. They are first sprayed with a layer of powdered graphite or metal, and then electroplated in the usual way. This technique is often used to plate plastics, glass, and other materials to make jewellery. Key point Electroplating is a process used to plate (coat) one metal with a thin layer of another metal by electrolysis. Experiment 32.4 Experiment 32.4 Example Industrial applications of electrolysis P. 68 / 95

69 Purification of copper Copper is the most commonly used metal for making electrical conducting wires. To ensure a high quality transmission of electrical signals, very pure copper is used. Figure The metal part of an audio-video cable is made of 100% pure copper. Metals such as copper and lead are purified by electrolysis. The process is called electrolytic refining Industrial applications of electrolysis P. 69 / 95

70 After extraction of copper from its ores, it may still contain impurities such as zinc, iron, silver and gold. In refining copper, an aqueous solution of copper(ii) sulphate is electrolysed; a block of impure copper is used as the anode; a sheet of pure copper is used as the cathode 32.7 Industrial applications of electrolysis P. 70 / 95

71 block of impure copper (anode) copper(ii) sulphate solution pure copper sheet electrolysis impurities from anode thicker sheet of pure copper Figure The refining of copper by electrolysis Industrial applications of electrolysis P. 71 / 95

72 At the anode (block of impure copper): The metal impurities (e.g. zinc and iron) which are more reactive than copper lose electrons first. Zn(s), Fe(s) and Cu(s) lose electrons to form Zn 2+ (aq) ions, Fe 2+ (aq) ions and Cu 2+ (aq) ions respectively. Zn(s) Zn 2+ (aq) + 2e Fe(s) Fe 2+ (aq) + 2e Cu(s) Cu 2+ (aq) + 2e The metal impurities which are less reactive than copper (e.g. silver and gold) sink to the bottom of the electrolytic cell Industrial applications of electrolysis P. 72 / 95

73 At the cathode (pure copper sheet): Cu 2+ (aq) ions are preferentially discharged to form copper. The pure copper sheet becomes thicker. Cu 2+ (aq) + 2e Cu(s) Changes in the solution: Reading to learn STSE connections 32.1 Zinc and iron form ions more readily than copper at the anode. Copper(II) ions are always preferentially discharged at the cathode. The concentration of copper(ii) ions in the copper(ii) sulphate solution drops gradually Industrial applications of electrolysis P. 73 / 95

74 32.8 Environmental impact of the electroplating industry Pollution problems associated with the electroplating industry The effluents from the electroplating industry contain various harmful substances, e.g. acids and alkalis, compounds of heavy metals and other toxic chemicals (e.g. cyanides). If they are discharged to the environment directly without treatment, they cause serious pollution problems. P. 74 / 95

75 Acids and alkalis change the ph of the water and affect the normal activities of the aquatic life. Figure Polluted water kills aquatic life. Heavy metal ions, such as the ions of chromium, nickel and cadmium, are readily absorbed into the bodies of living organisms, e.g. shellfish and plants. These ions accumulate within the organism or along the food chain. Eventually, humans may get poisoned by eating them. Cyanides are highly toxic and may kill the aquatic life Environmental impact of the electroplating industry P. 75 / 95

76 Methods to control pollution from the electroplating industry Reducing harmful waste Chromate (compounds containing CrO 4 2 (aq)) used in chromium plating can be replaced with chromium(iii) compounds which are less harmful. Recovering metal ions for reuse Nickel(II) sulphate is commonly used in nickel plating. Nickel(II) ions can be recovered from the waste solution after treatment Environmental impact of the electroplating industry P. 76 / 95

77 Treating effluents before discharge 1. Controlling the ph of effluents Acidic effluents can be neutralized with sodium carbonate or slaked lime and alkaline effluents can be neutralized with sulphuric acid Environmental impact of the electroplating industry P. 77 / 95

78 2. Precipitation of heavy metal ions Heavy metal ions can be removed by precipitating them as insoluble hydroxides or carbonates by adding sodium hydroxide solution or sodium carbonate solution. Example Nickel(II) hydroxide is precipitated when sodium hydroxide solution is added to nickel(ii) chloride. NiCl 2 (aq) + 2NaOH(aq) Ni(OH) 2 (s) + 2NaCl(aq) The precipitate is filtered off before discharge Environmental impact of the electroplating industry P. 78 / 95

79 3. Treatment of chromium waste The wastewater generated from the electroplating industry usually contains chromate ions, CrO 4 2 (aq), which are highly toxic. To treat chromium waste, chromate ions are first reduced to chromium(iii) ions by adding acidified sodium sulphite solution. 3SO 3 2 (aq) + 2CrO 4 2 (aq) + 10H + (aq) 3SO 4 2 (aq) + 2Cr 3+ (aq) + 5H 2 O(l) The chromium(iii) ions are then precipitated as chromium(iii) hydroxide by adding sodium hydroxide solution. Cr 3+ (aq) + 3OH (aq) Cr(OH) 3 (s) Activity 32.1 Class practice Environmental impact of the electroplating industry P. 79 / 95

80 Key terms 1. brine 鹽水 2. concentration effect 濃度效應 3. electrolysis 電解 4. electrolytic refining 電解提煉 5. electroplating 電鍍 6. Hofmann voltameter 霍夫曼電量計 7. preferential discharge 優先放電 8. sodium amalgam 鈉汞齊 P. 80 / 95

81 Progress check 1. What is electrolysis? 2. What are the basic components of a set-up for carrying out electrolysis? 3. What are the main differences between a chemical cell and an electrolytic cell? 4. Which two ions are always present in an aqueous solution? Why? 5. What are the factors that affect preferential discharge of ions? 6. Why are electrolysis of dilute sulphuric acid and very dilute sodium chloride solution commonly known as electrolysis of water? P. 81 / 95

82 7. What are the products of electrolysis of concentrated sodium chloride solution using graphite electrodes? 8. What are the products of electrolysis of concentrated sodium chloride solution using mercury cathode and graphite anode? 9. What are the products of electrolysis of copper(ii) sulphate solution using graphite electrodes? 10.What are the products of electrolysis of copper(ii) sulphate solution using copper electrodes? 11.What is electroplating? How can we carry out electroplating? 12.How can we purify impure copper by electrolysis? Progress check P. 82 / 95

83 13. What are the pollution problems associated with the electroplating industry? 14. How can we control pollution from the electroplating industry? Progress check P. 83 / 95

84 Summary 32.1 Electrolysis a type of redox reactions 1. Electrolysis means decomposition by electricity. It is the process in which a redox reaction is forced to take place when electricity is supplied to an electrolyte (molten or in aqueous solution). P. 84 / 95

85 2. CHEMICAL CELL (a device for generating electricity by chemical changes) positive electrode (cathode) negative electrode (anode) electron flow anode cathode electrolyte ELECTROLYTIC CELL (a device for bringing about chemical changes by electrical energy) Summary P. 85 / 95

86 32.2 Predicting preferential discharge of ions 3. During electrolysis, electrons move between the electrodes through connecting wires, and mobile ions move in the electrolyte. 4. Water itself ionizes very slightly to give H + (aq) and OH (aq): H 2 O(l) H + (aq) + OH (aq) Summary P. 86 / 95

87 5. Factors affecting preferential discharge of ions: (a) Position of ions in the E.C.S. Cations lower in the E.C.S. are more readily discharged. Anions higher in the E.C.S. are more readily discharged. (b) Concentration of ions in solution Due to concentration effect, an ion may be more readily discharged if it is present at a higher concentration. (c) Nature of electrodes Summary P. 87 / 95

88 32.3 Electrolysis of dilute sulphuric acid 6. The products of electrolysis of dilute sulphuric acid with inert electrodes are hydrogen and oxygen. The result is equivalent to electrolysis of water. Summary P. 88 / 95

89 32.4 Electrolysis of sodium chloride solution 7. The products of electrolysis of very dilute sodium chloride solution are hydrogen and oxygen. 8. The products of electrolysis of concentrated sodium chloride solution (brine) with graphite electrodes are hydrogen and chlorine. 9. The products of electrolysis of concentrated sodium chloride solution with mercury cathode and graphite anode are sodium and chlorine. Summary P. 89 / 95

90 32.5 Electrolysis of copper(ii) sulphate solution 10. The products of electrolysis of copper(ii) sulphate solution with graphite electrodes are copper and oxygen. 11. Electrolysis of copper(ii) sulphate solution with copper electrodes causes no net change in the electrolyte. The only change is the transfer of copper from the anode to the cathode. Summary P. 90 / 95

91 32.6 Predicting products of electrolysis of some aqueous solutions and molten substances 12. Products of electrolysis of some aqueous solutions and molten substances are summarized in Table 32.2 on p Industrial applications of electrolysis 13. Electroplating is a process used to plate (coat) one metal with a thin layer of another metal by electrolysis. Summary P. 91 / 95

92 14. In electroplating, the metal object to be electroplated is made the cathode; the anode is often made of the plating metal and the electrolyte is an aqueous solution containing its ions. 15. In electrolytic refining of metals, the metal block to be purified is the anode. Summary P. 92 / 95

93 32.8 Environmental impact of the electroplating industry 16. The effluents from electroplating factories would cause serious pollution problems if discharged without treatment. The wastes often include acids and alkalis; compounds of heavy metals and toxic chemicals (e.g. cyanides). Summary P. 93 / 95

94 Concept map Electrolyte decomposed by ELECTROLYSIS Factors affecting the discharge of ions position of ions in the E.S.C. of Concentration ions of Nature electrodes P. 94 / 95

95 ELECTROLYSIS applications Electroplating effluents contain Acids and alkalis Compounds of heavy metals Toxic chemicals (e.g. ) cyanides of Purification some metals Extraction of reactive metals Concept map P. 95 / 95