CHAPTER 6 ELECTROCHEMISTRY. Example. 2NaCl (l) 2Na (s) + Cl2 (g)

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1 CHAPTER 6 ELECTROCHEMISTRY Electrolytes are substances that can conduct electricity when they are in molten state and aqueous solution. This due to the present of free moving ions in the electrolytes. Electrolysis is a process whereby compounds in molten or aqueous state are broken down into their constituent elements by passing electricity through them. Non-electrolytes are substances that can not conduct electricity when they are in all state. This because non-electrolyte exist as molecule which means contain no ions. Example 2NaCl (l) 2Na (s) + Cl2 (g) Sodium Metal Chlorine Gas 100

2 Electrolytic Cell (molten state) [ Draw the apparatus of electrolysis molten sodium chloride ] a) Electrode attach to positive terminal (battery) = b) Electrode attach to negative terminal (battery) = c) Anion (negative ion) discharged at electrode = Anion will electrons. d) Cation (positive ion) discharged at electrode = Cation will electrons. e) Electron flow from to f) Electrolytic Cell will change the energy to energy. 101

3 Product of Electrolysis Ion discharged at Cathode All ion metal except Copper ion Observation Half-equation Product Copper ion Hydrogen ion Ion discharged at Anode Observation Half-equation Product Oxide ion Chloride ion Bromide ion Iodide ion Hydroxide ion Hydrogen gas Gas Test Oxygen gas Chlorine gas 102

4 Electrolysis Molten Lead(II) Bromide P Q 1. State the ion consists in the electrolyte. 2. Which electrode is 3. Which ion will be discharged at 4. State the observation at 5. State the product formed at 6. Write the half equation at 7. Draw the electron flow on the diagram above. 103

5 Exercise 1. State the ion present in the following electrolyte. Predict the products from the electrolysis of some molten compound and write the ionic equation involved. a) Magnesium oxide b) Copper(II) chloride c) Lead(II) iodide 104

6 2. State the meaning of the following terms. a) Anode b) Cathode c) Electrolysis 3. A molten oxide, R2O3 is electrolysed using carbon electrodes. a) Draw a labeled diagram to show the set-up of apparatus for the electrolysis. b) What ions are present in the electrolyte? Write the formulae for the ions present in the electrolyte. c) State the ions move to each of the electrodes during electrolysis. 105

7 d) Write half equation of the reaction at each of the electrodes. e) Name the substances formed at each of the electrodes. f) Label the flow of electron in the diagram (a). 106

8 107

9 Electrolysis of Aqueous Solution Aqueous solution consists of four types of ions. Two ions from the compound and two ions from the water. Example: Molten sodium chloride Sodium chloride solution Generally, there are 3 factors that may influence the selective of ions during electrolysis of an aqueous solution 1. Position of ions in the electrochemical series 2. Concentration of ions in the electrolytes 3. Types of electrodes used in the electrolysis 108

10 Position of ions in the electrochemical series (ECS) The ions that are lower in the ECS will selected to be discharged. [ Draw the apparatus of electrolysis sodium chloride solution ] 1. State the ion consists in the electrolyte. 2. Which electrode is 3. Which ion will be discharged at 4. State the observation at 5. State the product formed at 6. Write the half equation at 7. Draw the electron flow on the diagram above. 109

11 Exercise 1. Carbon electrode Copper(II) sulphate solution For the electrolysis of copper(ii) sulphate solution, (a) State all the ions that are present in the electrolyte. (b) State the ions in (a) which discharged to the i) anode : ii) cathode : (c) Write a half equation for the reaction at the i) anode : ii) cathode : (d) The blue colour of copper(ii) sulphate solution fades if the electrolysis is carried for a long period of time. Explain why. 110

12 2. Carbon electrode Dilute sulphuric acid For the electrolysis of dilute sulphuric acid, a) State all the ions that are present in the electrolyte b) State the ion in (a) which discharged to i) anode ii) cathode c) Write half equation for the reaction at the i) anode ii) cathode d) Explain why the concentration of dilute sulphuric acid increases gradually during the electrolysis 3. Base on the answer 1(c) and 2(c), name the process that occur at the a) anode b) cathode 111

13 Concentration of ions in the electrolytes If the concentrations of particular ions are high, the ion is selectively discharged [ Draw the apparatus of electrolysis concentrated sodium chloride solution ] 1. State the ion consists in the electrolyte. 2. Which electrode is 3. Which ion will be discharged at 4. State the observation at 5. State the product formed at 6. Write the half equation at 7. Draw the electron flow on the diagram above. 112

14 Exercise 1. Carbon electrode Dilute Hydrochloric acid solution Experiment A Carbon electrode Concentrated Hydrochloric acid Experiment B Diagram above show the apparatus set up for the experiments of electrolysis using two different concentration of hydrochloric acid. a) State all the ions that are present in the electrolyte i) Experiment A : ii) Experiment B : 113

15 b) State the ion in (a) which discharged to anode and cathode in i) Experiment A : ii) Experiment B : c) Write half equation for the reaction at the anode and cathode in i) Experiment A : ii) Experiment B : d) State the observation occur at cathode and anode in i) Experiment A : ii) Experiment B : e) State the product formed at cathode and anode in i) Experiment A : ii) Experiment B : 114

16 Types of electrodes used in the electrolysis If using the active electrode at anode, ions that are present in the electrolytes are not discharge. Instead the active electrode will corrodes and dissolves in the electrolytes. [ Draw the apparatus of electrolysis silver chloride solution using silver electrodes ] 1. State the ion consists in the electrolyte. 2. Which electrode is 3. Which ion will be discharged at 4. State the observation at 5. State the product formed at 6. Write the half equation at 7. Draw the electron flow on the diagram above. 115

17 Exercise 1. Carbon electrode Copper(II) sulphate solution Experiment A Copper plate Copper(II) sulphate solution Experiment B Diagram above show the apparatus set up for the experiments of electrolysis using two different electrodes immersed in copper(ii) sulphate solution. a) State all the ions that are present in the electrolyte i) Experiment A : ii) Experiment B : 116

18 b) State the observation occur at anode and cathode in i) Experiment A : ii) Experiment B : c) Write half equation for the reaction at the anode and cathode in i) Experiment A : ii) Experiment B : d) Explain the observation on the colour of copper(ii) sulphate solution in i) Experiment A : ii) Experiment B : 117

19 Electrolysis in Industry Most common application: i) Extraction of metal 1) Extraction of Metal Extraction of aluminium from aluminium oxide. ii) Purification of metal iii) Electroplating 2) Purification of Metal In purification: The impure metal is made to be the anode The cathode is a thin layer of pure metal Copper nugget 3) Electroplating Electroplating is a process to coat one metal onto another metal. The purposes of electroplating onto metal are:- i) Make it look more attractive ii) more resistant to corrosion In electroplating : object to be electroplated as the cathode anode is the metal used for plating Electrolyte is a solution of the compound of the electroplating 118

20 Extraction of Metal [ Draw the apparatus of electrolysis for extraction of aluminium from aluminium oxide ] 1) Ion present in electrolyte = 2) Ion discharged 3) Observation 4) Half equation 5)Function of cryolite, Na3AlF6 119

21 Purification of Metal [ Draw the apparatus of electrolysis for purification of impure copper ] 1) Ion present in electrolyte = 2) Ion discharged 3) Observation 4) Half equation 5) Colour changes of electrolyte 120

22 Electroplating [ Draw the apparatus of electrolysis to electroplate key by using copper as electrode ] 1) Ion present in electrolyte = 2) Ion discharged 3) Observation 4) Half equation 5) Colour changes of electrolyte 121

23 Voltaic Cells A simple voltaic cell can be made by dipping two different types of metals in an electrolyte Electron flow from one metal to another metal through the connecting wire in the external circuit. More electropositive metal will release electron, thus act as the negative terminal. Less electropositive metal will accept electron and act as the positive terminal. Continuous flow of electron produces an electric current. Simple zinc-copper Voltaic Zinc more reactive than copper Zinc will act as terminal, and copper will act as terminal. Zinc will release electron to form Zn 2+. Half equation: Cu 2+ ions from copper(ii) sulphate solution receive electron to form copper metal. Half equation : Overall equation: The further the distance between the position of two metals is in ECS the bigger the cell voltage. 122

24 Different Types of Voltaic Cells Two types of voltaic cell: 1) Primary cells: non rechargeable cell Example: Daniell cell, dry cells, alkaline cell 2) Secondary cells: rechargeable cells Example: Lead-acid accumulator, Nickel-cadmium Daniell Cell 1 1. Used salt bridge Salt bridge contain inert ions or salt that does not react with electrolyte. Example: 2. Used porous pot Daniell Cell 2 Porous pot has fine pores that allow ions flow through. What is the function of salt bridge and porous pot in Daniell Cell? 123

25 Exercise 1. For the simple voltaic cell that you see at the diagram Mg Cu Magnesium sulphate solution a) State how electricity was produced. b) What are the chemical changes that occur at the magnesium ribbon and the copper plate. c) Write the half equation for the changes that occur at each the electrode. d) What is the direction of electron flow from terminal to another through the external circuit. 124

26 Electrochemical series (ECS) Tendency of metal to release electrons to form ions increases Tendency of cation to receive electrons to form metals increases The electrochemical series (ECS) can be constructed by two method: a) The potential difference (voltage difference) between pairs of metal. b) The ability of metal to displace another metal from its salt solution. 125

27 The potential difference (voltage difference) between pairs of metal. The bigger the voltage value the further apart their position. The metal act as negative terminal is placed at higher position in electrochemical series (ECS). How to determine the positive/negative terminal? Example: The voltaic cells are constructed as shown in the figure. The voltmeter reading of the cell I is 1.1 V while that of cell II is 2.5 V. Q P R P Cell 1 Cell 2 Arrange the metals in descending order in the electrochemical series. 126

28 The ability of metal to displace another metal from its salt solution If the M can displace metal N from an aqueous N salt solution, then: i) Metal M is more electropositive than metal N ii) Metal M is placed at a higher position than metal N in the ECS Example: Zinc and copper(ii) sulphate sulphate solution Observation : Half-equation : The Important of ECS ECS can be used to determine: The terminal of voltaic cell The standard cell voltage The ability of a metal to displace another metal from its salt solution. 127

29 Exercise 1. The diagram shows an electrolytic cell. The left section of the cell (S) is a source of electricity to drive the right section (T) of the cell. Aluminium Zinc Copper Aluminium Sulphate S Zinc Sulphate T Copper(II) Sulphate a) State the change of energy in cell S b) i) For cell S, state the positive terminal of the cell ii) Explain your choice for b(i) c) i) State what has happened at the negative terminal ii) What process has happened in this electrode 128

30 d) Explain why the color of copper(ii) sulphate remain unchanged e) Determine the anode of cell T f) Write down the half equation for the cathode in cell T g) What will happened if the aluminium in cell S is replaced by copper 129