Chemistry_Grade_10-12 (1).pdf

The reactions that take place are as follows at the

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cathode, where reduction takes place to produce pure copper metal. The reactions that take place are as follows: At the anode : Cu ( s ) Cu 2+ ( aq ) + 2 e At the cathode : Cu +2 ( aq ) + 2 e Cu ( s ) ( > 99%purity) 5. The other metal impurities (Zn, Au, Ag, Fe and Pb) do not dissolve and form a solid sludge at the bottom of the tank or remain in solution in the electrolyte. + + positive anode negative cathode impure copper electrode pure copper electrode Cu 2+ Figure 17.6: A simplified diagram to illustrate what happens during the electrowinning of copper 17.6.2 The production of chlorine Electrolysis can also be used to produce chlorine gas from brine/seawater (NaCl). This is some- times referred to as the ’Chlor-alkali’ process. The reactions that take place are as follows: At the anode the reaction is: 2 Cl Cl 2 ( g ) + 2 e whereas at the cathode , the following happens: 2 Na + + 2 H 2 O + 2 e 2 Na + + 2 OH + H 2 The overall reaction is: 2 Na + + 2 H 2 O + 2 Cl 2 Na + + 2 OH + H 2 + Cl 2 Chlorine is a very important chemical. It is used as a bleaching agent, a disinfectant, in solvents, pharmaceuticals, dyes and even plastics such as polyvinlychloride (PVC). 348
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CHAPTER 17. ELECTROCHEMICAL REACTIONS - GRADE 12 17.7 + + positive anode negative cathode electrode electrode NaCl solution Cl Na + Figure 17.7: The electrolysis of sodium chloride 17.6.3 Extraction of aluminium Aluminum metal is a commonly used metal in industry where its properties of being both light and strong can be utilized. It is also used in the manufacture of products such as aeroplanes and motor cars. The metal is present in deposits of bauxite which is a mixture of silicas, iron oxides and hydrated alumina ( Al 2 O 3 x H 2 O ). Electrolysis can be used to extract aluminum from bauxite. The process described below produces 99% pure aluminum: 1. Aluminum is melted along with cryolite ( Na 3 AlF 6 ) which acts as the electrolyte. Cryolite helps to lower the melting point and dissolve the ore. 2. The anode carbon rods provide sites for the oxidation of O 2 and F ions. Oxygen and flourine gas are given off at the anodes and also lead to anode consumption. 3. At the cathode cell lining, the Al 3+ ions are reduced and metal aluminum deposits on the lining. 4. The AlF 3 6 electrolyte is stable and remains in its molten state. The basic electrolytic reactions involved are as follows: At the cathode : Al +3 + 3 e Al ( s ) (99%purity) At the anode : 2 O 2 O 2 ( g ) + 4 e The overall reaction is as follows: 2 Al 2 O 3 4 Al + 3 O 2 The only problem with this process is that the reaction is endothermic and large amounts of electricity are needed to drive the reaction. The process is therefore very expensive. 17.7 Summary An electrochemical reaction is one where either a chemical reaction produces an external voltage, or where an external voltage causes a chemical reaction to take place.
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