Chapter 29. Simple chemical cells

Transcription

1 Chapter 29 Simple chemical cells 29.1 Simple chemical cells consisting of two metal electrodes and an electrolyte 29.2 The Electrochemical Series of metals 29.3 Simple chemical cells consisting of metal-metal ion half cells and salt bridge/porous device Key terms Progress check Summary Concept map P. 1 / 55

2 29.1 Simple chemical cells consisting of two metal electrodes and an electrolyte Building a simple chemical cell How can we build a simple chemical cell? P. 2 / 55

3 magnesium digital voltmeter (as voltmeter) copper The reading also indicates electrons are flowing through the external circuit. copper(ii) sulphate solution Figure 29.1 The voltmeter shows a reading, indicating that electricity is being produced Simple chemical cells consisting of two metal electrodes and an electrolyte Think about P. 3 / 55

4 digital voltmeter (as voltmeter) copper copper copper(ii) sulphate solution Figure 29.2 The voltmeter does not show a reading when two copper strips are used instead Simple chemical cells consisting of two metal electrodes and an electrolyte P. 4 / 55

5 Two more methods for building simple chemical cells are shown in Figures 29.3 and Metal couples (i.e. two strips of different metals) other than Mg and Cu can also be used. voltmeter two different metal strips filter paper (soaked in sodium chloride solution) Figure 29.3 A simple chemical cell consisting of two different metal strips and a piece of filter paper soaked in sodium chloride solution Simple chemical cells consisting of two metal electrodes and an electrolyte P. 5 / 55

6 digital multimeter (as voltmeter) Cu Mg Figure 29.4 A simple chemical cell can be built by using a lemon with two different metal strips inserted into it Simple chemical cells consisting of two metal electrodes and an electrolyte P. 6 / 55

7 Key point A simple chemical cell can be set up by dipping two different metals in an. electrolyte The two metals are connected by an external wire Simple chemical cells consisting of two metal electrodes and an electrolyte P. 7 / 55

8 Chemical changes in simple chemical cells A chemical cell produces electricity by chemical reactions. What are the reactions in the chemical cell? Where do the electrons flowing through the external circuit come from? 29.1 Simple chemical cells consisting of two metal electrodes and an electrolyte P. 8 / 55

9 A chemical cell using magnesium and copper as a metal couple, represented by Mg/Cu. light bulb magnesium copper 29.1 Simple chemical cells consisting of two metal electrodes and an electrolyte copper(ii) sulphate solution (electrolyte) P. 9 / 55

10 Mg is more reactive than Cu. Mg loses electrons more readily than Cu. Mg atoms lose electrons, and enter the electrolyte (i.e. copper(ii) sulphate solution) as Mg 2+ ions. Electrons flow from Mg to Cu in the external circuit. electron flow light bulb magnesium copper copper(ii) sulphate solution (electrolyte) 29.1 Simple chemical cells consisting of two metal electrodes and an electrolyte P. 10 / 55

11 Mg gives out electrons; it is the negative electrode. Cu receives electrons; it is the positive electrode. Mg is the negative pole or negative terminal of the chemical cell. Cu is the positive pole or positive terminal of the chemical cell. electron flow light bulb electron flow magnesium (negative electrode or negative pole) copper (positive electrode or positive pole) Figure 29.5 Electrons flow from the negative pole to the positive pole in the external circuit of a chemical cell Simple chemical cells consisting of two metal electrodes and an electrolyte copper(ii) sulphate solution (electrolyte) P. 11 / 55

12 At the magnesium electrode Mg atoms lose electrons to form Mg ions. The change taking place at the Mg electrode can be represented by the half equation: At the copper electrode Mg(s) Mg 2+ (aq) + 2e Copper(II) ions in the solution gain electrons and form Cu atoms. The change taking place at the Cu electrode can be represented by the half equation: Cu 2+ (aq) + 2e Cu(s) 29.1 Simple chemical cells consisting of two metal electrodes and an electrolyte P. 12 / 55

13 Overall equation Mg(s) + Cu 2+ (aq) + 2e Mg 2+ (aq) + Cu(s) + 2e i.e. Mg(s) + Cu 2+ (aq) Mg 2+ (aq) + Cu(s) Skill corner 29.1 Class practice Simple chemical cells consisting of two metal electrodes and an electrolyte P. 13 / 55

14 29.2 The Electrochemical Series of metals Measuring the voltage produced by a chemical cell A chemical cell acts like an electron pump. The voltage (or electromotive force (e.m.f.)) of the cell is a measure of how strongly electrons are pushed through the circuit. The voltage is expressed in volts (V). P. 14 / 55

15 Simple chemical cells made from different metal couples The voltage of a chemical cell depends on which couple of metals it has. Copper can be used as the reference electrode to set up various cells. Each of the cells consists of copper coupled with another metal The Electrochemical Series of metals P. 15 / 55

16 Copper is connected to the positive terminal of the voltmeter. The other metal is connected to the negative terminal. digital multimeter (as voltmeter) metal X filter paper soaked in sodium chloride solution copper Figure 29.6 Experimental set-up for measuring the voltages of chemical cells with different metal couples The Electrochemical Series of metals P. 16 / 55

17 Metal couple in cell Voltage of cell (V) Direction of electron flow in the external circuit Magnesium/copper (Mg/Cu) Zinc/copper (Zn/Cu) Iron/copper (Fe/Cu) Copper/copper (Cu/Cu) Silver/copper (Ag/Cu) From Mg to Cu From Zn to Cu From Fe to Cu From Cu to Ag Table Results of the experiment that measures the voltages of chemical cells with different metal couples The Electrochemical Series of metals P. 17 / 55

18 The sign of the voltage indicates the polarity of the reference copper electrode. In the Mg/Cu cell, the voltage has a positive sign. It indicates that Cu is the positive electrode. Mg loses electrons more readily than Cu. Electrons flow from the magnesium electrode to the copper electrode in the external circuit. digital multimeter (as voltmeter) magnesium filter paper soaked electron flow in sodium chloride solution copper Figure 29.7 Electrons flow from the magnesium electrode to the copper electrode in the external circuit The Electrochemical Series of metals P. 18 / 55

19 The voltages for the Zn/Cu cell and the Fe/Cu cell have a positive sign. Zinc and iron lose electrons more readily than copper. The higher the voltage, the greater is the difference in the electron losing tendencies between copper and the other metal The Electrochemical Series of metals P. 19 / 55

20 In the Ag/Cu cell, the voltage has a negative sign. It indicates that copper is the negative electrode. Cu has a greater tendency to lose electrons than Ag. Electrons flow from the copper electrode to the silver electrode in the external circuit. digital multimeter (as voltmeter) silver filter paper soaked in sodium chloride solution copper electron flow Figure 29.8 Electrons flow from the copper electrode to the silver electrode in the external circuit The Electrochemical Series of metals P. 20 / 55

21 Relative positions of metals in the Electrochemical Series An arrangement of the tendencies of the metals to lose electrons in an aqueous solution is called the Electrochemical Series (E.C.S.) of metals. Learning tip In fact, the Electrochemical Series is drawn up using hydrogen electrode as the reference electrode. So hydrogen is included in Figure 29.9 for reference The Electrochemical Series of metals 29.2 The Electrochemical Series of metals P. 21 / 55

22 metals lose electrons most readily metals lose electrons least readily K(s) K + (aq) + e Ca(s) Ca 2+ (aq) + 2e Na(s) Na + (aq) + e Mg(s) Mg 2+ (aq) + 2e Al(s) Al 3+ (aq) + 3e Zn(s) Zn 2+ (aq) + 2e Fe(s) Fe 2+ (aq) + 2e Pb(s) Pb 2+ (aq) + 2e H 2 (s) 2H + (aq) + 2e Cu(s) Cu 2+ (aq) + 2e Ag(s) Ag + (aq) + e Au(s) Au + (aq) + e Figure 29.9 The Electrochemical Series (E.C.S.) of some metals The Electrochemical Series of metals P. 22 / 55

23 Electrochemical Series Metal reactivity series Potassium Calcium Sodium Magnesium Aluminium Zinc Iron Lead Copper Silver Potassium Sodium Calcium Magnesium Aluminium Zinc Iron Lead Copper Silver Table 29.2 Comparison of relative positions of some metals in the Electrochemical Series and in the metal reactivity series The Electrochemical Series of metals P. 23 / 55

24 The relative positions of metals in the two series are very similar. The more readily a metal loses electrons, the higher its position is in the E.C.S. The more readily a metal loses electrons, the more reactive it is, and the higher its position is in the metal reactivity series. Example 29.1 Key point For a simple chemical cell consisting of two metals and an electrolyte, the further apart the two metals are in the E.C.S., the higher is the voltage of the cell. Experiment 29.1 Experiment 29.1 Example 29.2 Class practice The Electrochemical Series of metals P. 24 / 55

25 29.3 Simple chemical cells consisting of metal-metal ion half cells and salt bridge/porous device Problems associated with a simple chemical cell consisting of two metal electrodes and an electrolyte The simple chemical cells discussed in Figure 29.5 has some drawbacks during operation. The magnesium reacts directly with the copper(ii) sulphate solution. Mg(s) + Cu 2+ (aq) Mg 2+ (aq) + Cu(s) P. 25 / 55

26 digital multimeter (as voltmeter) digital multimeter (as voltmeter) magnesium copper copper(ii) sulphate solution after some time copper Figure Magnesium reacts directly with copper(ii) sulphate solution in the above simple chemical cell Simple chemical cells consisting of metal-metal ion half cells and salt bridge/porous device P. 26 / 55 copper

27 A layer of copper forms on the surface of magnesium. The voltage of the cell drops quickly and the current stops flowing after a short time. How can we improve the cell? 29.3 Simple chemical cells consisting of metal-metal ion half cells and salt bridge/porous device P. 27 / 55

28 Building simple chemical cells using metal-metal ion half cells and salt bridges Animation (Simple chemical cell with a salt bridge) electron flow voltmeter connecting wire magnesium salt bridge (soaked in KNO 3 (aq)) copper Simulation (A chemical cell with metal/metal ion half cells) magnesium sulphate solution Figure A cell made up by two half cells with a salt bridge connecting them Simple chemical cells consisting of metal-metal ion half cells and salt bridge/porous device P. 28 / 55 copper(ii) sulphate solution

29 The magnesium electrode is in a solution of its own ions magnesium ions, Mg 2+ (aq), from the magnesium sulphate solution. The copper electrode is in a solution of copper(ii) ions, Cu 2+ (aq), from the copper(ii) sulphate solution. A salt bridge joins the two half cells. It is a strip of filter paper soaked in a solution of an electrolyte Simple chemical cells consisting of metal-metal ion half cells and salt bridge/porous device P. 29 / 55

30 The electrolyte can be potassium nitrate, or of any other ionic compound that does not react with the substances of the cell. Learning tip Sodium chloride and sodium sulphate are also electrolytes commonly used for making the salt bridge. Since the magnesium and the copper(ii) sulphate solution are now separated into two half cells, they cannot react directly Simple chemical cells consisting of metal-metal ion half cells and salt bridge/porous device P. 30 / 55

31 At the magnesium electrode: Mg(s) Mg 2+ (aq) + 2e At the copper electrode: Cu 2+ (aq) + 2e Cu(s) Overall equation: Mg(s) + Cu 2+ (aq) Mg 2+ (aq) + Cu(s) 29.3 Simple chemical cells consisting of metal-metal ion half cells and salt bridge/porous device P. 31 / 55

32 In the magnesium-magnesium ion half cell The magnesium electrode dissolves and the concentration of magnesium ions in solution increases. This results in a build-up of excess positive charges. What will happen if there is no salt bridge? 29.3 Simple chemical cells consisting of metal-metal ion half cells and salt bridge/porous device P. 32 / 55

33 If there is no salt bridge, the excess positive charges will prevent more magnesium ions from entering the solution. The reaction will soon stop Simple chemical cells consisting of metal-metal ion half cells and salt bridge/porous device P. 33 / 55

34 In the copper-copper ion half cell Electrons arrive at the copper electrode. The copper(ii) ions in the solution gain electrons, leaving an excess of sulphate ions. This results in a build-up of excess negative charges. What will happen if there is no salt bridge? 29.3 Simple chemical cells consisting of metal-metal ion half cells and salt bridge/porous device P. 34 / 55

35 If there is no salt bridge, the excess negative charges in the solution will prevent more electrons from entering the electrode. The reaction will soon stop Simple chemical cells consisting of metal-metal ion half cells and salt bridge/porous device P. 35 / 55

36 Functions of a salt bridge The salt bridge allows ions to move from one half cell to the other. magnesium electron flow voltmeter salt bridge (soaked in KNO 3 (aq)) connecting wire copper magnesium sulphate solution copper(ii) sulphate solution Figure The charges in the solutions of the two half cells are balanced with the use of a salt bridge Simple chemical cells consisting of metal-metal ion half cells and salt bridge/porous device P. 36 / 55

37 In the left beaker Magnesium ions (Mg 2+ ) move into the salt bridge or nitrate ions (NO 3 ) move out of the salt bridge. Charges in the solution can be balanced. In the right beaker Sulphate ions (SO 4 2 ) move into the salt bridge or potassium ions (K + ) move out of the salt bridge. Charges in the solution can be balanced Simple chemical cells consisting of metal-metal ion half cells and salt bridge/porous device P. 37 / 55

38 The salt bridge serves two main functions: 1. It completes the circuit by allowing ions to move from one half cell into the other. 2. It provides ions to balance the charges in the solutions of the two half cells Simple chemical cells consisting of metal-metal ion half cells and salt bridge/porous device P. 38 / 55

39 Simple chemical cells with metal-metal ion half cells and a porous device Daniell cell consists of a copper/copper(ii) sulphate half cell and a zinc/zinc sulphate half cell. The half cells are separated by a porous device. electron flow zinc strip (negative electrode) digital multimeter (as voltmeter) porous pot Figure The Daniell cell a chemical cell with a porous device. copper container (positive electrode) copper(ii) sulphate solution zinc sulphate solution 29.3 Simple chemical cells consisting of metal-metal ion half cells and salt bridge/porous device P. 39 / 55

40 The porous device serves two main functions: 1. It prevents direct mixing of the two electrolytes. 2. It completes the circuit by allowing ions to move from one electrolyte into the other. Learning tip Unlike salt bridge, the porous device does not provide ions to balance the charges in the electrolytes of the two half cells Simple chemical cells consisting of metal-metal ion half cells and salt bridge/porous device P. 40 / 55

41 Zinc is more reactive than copper. When the cell operates, zinc loses electrons and goes into the solution as zinc ions. The electrons flow towards the copper container through the external circuit. The copper container is the positive electrode. At the copper surface, copper(ii) ions gain electrons to form copper metal Simple chemical cells consisting of metal-metal ion half cells and salt bridge/porous device P. 41 / 55

42 At the zinc electrode (negative electrode): Zn(s) Zn 2+ (aq) + 2e At the copper electrode (positive electrode): Cu 2+ (aq) + 2e Cu(s) Overall equation: Zn(s) + Cu 2+ (aq) Zn 2+ (aq) + Cu(s) The e.m.f. of the Daniell cell is about 1.10 V Simple chemical cells consisting of metal-metal ion half cells and salt bridge/porous device P. 42 / 55

43 Inside the porous device The zinc electrode dissolves and the concentration of zinc ions in the solution increases. This results in a build-up of excess positive charges. Outside the porous device Electrons arrive at the copper electrode. The copper(ii) ions in the solution gain electrons, leaving an excess of sulphate ions. This results in a build-up of excess negative charges Simple chemical cells consisting of metal-metal ion half cells and salt bridge/porous device P. 43 / 55

44 Since zinc ions are moving out of the device and sulphate ions are moving into the device, the electrical neutrality of the solutions is always maintained. Example 29.3 Class practice Simple chemical cells consisting of metal-metal ion half cells and salt bridge/porous device P. 44 / 55

45 Key terms 1. Daniell cell 丹聶爾電池 2. Electrochemical Series 電化序 3. half cell 半電池 4. half equation 半方程式 5. metal couple 金屬偶 6. reference electrode 參比電極 7. salt bridge 鹽橋 8. simple chemical cell 簡單化學電池 P. 45 / 55

46 Progress check 1. How can we set up a simple chemical cell? 2. What determines the direction of electron flow in a simple chemical cell? 3. Why do different metal couples give different voltages in simple chemical cells? 4. How can we obtain part of the Electrochemical Series by arranging metals in order of tendency to lose electrons? 5. What are the problems associated with a simple chemical cell consisting of two metal electrodes and an electrolyte? P. 46 / 55

47 6. How can we improve the efficiency of a simple chemical cell by using metal/metal ion half cells and salt bridges/porous devices? 7. What is a Daniell cell? How does it work? Progress check P. 47 / 55

48 Summary 29.1 Simple chemical cells consisting of two metal electrodes and an electrolyte 1. A simple chemical cell can be set up by dipping two different metals in an electrolyte. The two metals are connected by an external wire. 2. In a simple chemical cell, electrons flow from the negative electrode to the positive electrode in the external circuit. The negative electrode is made of a metal more reactive than that of the positive electrode. P. 48 / 55

49 29.2 The Electrochemical Series of metals 3. Metals can be arranged based on the voltages of the cells set up by the combination of each metal with a reference electrode (e.g. copper). The voltages obtained help us arrange the metals in order of their readiness to lose electrons and form cations in aqueous solution. This sequence forms the Electrochemical Series (E.C.S.) of metals. Summary P. 49 / 55

50 4. A metal higher in the Electrochemical Series has a stronger tendency to lose electrons (to form cations) in an aqueous solution than a metal lower in the series. 5. Relative positions of metals in the Electrochemical Series are similar to those in the metal reactivity series. Summary P. 50 / 55

51 29.3 Simple chemical cells consisting of metalmetal ion half cells and salt bridge/porous device 6. To improve the efficiency of the chemical cell, a simple chemical cell can be set up by using two metal/metal ion half cells, connected with a salt bridge. 7. The functions of a salt bridge are to complete the circuit and to provide ions to balance the charges. Summary P. 51 / 55

52 8. A simple chemical cell can also be set up by using two metal/metal ion half cells, connected with a porous pot. Daniell cell is an example of this type. Summary P. 52 / 55

53 Concept map Electrochemical Series (E.C.S) Tendencies of metals to lose electrons SIMPLE CHEMICAL CELLS arranged in decreasing order measured voltages can be used to compare P. 53 / 55

54 SIMPLE CHEMICAL CELLS Two metal/metal ion half cells connected by Salt bridge separated by Porous device Concept map P. 54 / 55

55 SIMPLE CHEMICAL CELLS Metal couple Electrolyte more reactive metal Negative electrode Positive electrode less reactive metal Concept map P. 55 / 55