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GCSE Chemistry Notes: Explaining using electrolysis to extract aluminium & sodium
3. The extraction of aluminium and sodium metals by electrolysis Doc Brown's GCSE/IGCSE O Level KS4 science–CHEMISTRY Revision Notes Mining of Minerals and Methods of Extracting of Metals How do we extract reactive metals that cannot be obtained using carbon? Why do we need to use electrolysis to obtain certain reactive metals from their ores? e.g. how do we extract aluminium from its bauxite ore containing aluminium oxide? How do we extract sodium from salts like sodium chloride? All the electrolytic process are described complete with the electrolysis electrode equations. Scroll down for revision notes on extraction procedures and theory which should prove useful for school/college assignments/projects on ways of extracting metals from their ores. These revision notes on the electrolytic extraction of aluminium from bauxite ore should prove useful for the new AQA, Edexcel and OCR GCSE (9–1) chemistry science courses.
Metal extraction index
For more on the reactivity series of metals and oxidation-reduction (redox) reaction see
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A summary diagram of important ideas to do with the reactivity series of metals!
3a. Extraction of Aluminium The current method for extracting aluminium is expensive because it involves several stages and uses large amounts of costly electrical energy. It is much more expensive than using carbon reduction to make iron in a blast furnace. Aluminium is very abundant in the Earth's crust but it is always found as very stable compounds in many sources e.g. bauxite (mainly aluminium oxide) and alumino-silicate minerals in many rocks. Bauxite has the highest concentration of aluminium in these sources and is mined extensively around the world. |
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Reminders: Electrolysis (of aluminium oxide) is a way of splitting up (decomposition) of the compound (aluminium oxide) using electrical energy. The electrical energy comes from a d.c. (direct current) power pack supply. A conducting liquid, containing ions, called the electrolyte (molten aluminium oxide), must contain the compound (aluminium oxide) that is being broken down. The electricity must flow through electrodes dipped into the electrolyte to complete the electrical circuit with the battery. Electrolysis can only happen when the circuit is complete, and a d.c. electrical current (electricity) is flowing, then the products of electrolysing molten aluminium oxide are released on the electrode surfaces where they can be collected. Electrolysis always involves a flow of electrons in the external wires and electrodes and a flow of ions in the electrolyte and there is always a reduction at the negative cathode electrode (which attracts positive ions, cations) and an oxidation at the positive anode electrode (which attracts negative ions, anions).
The basic design of the industrial electrolysis cell used in the extraction of aluminium from molten purified aluminium oxide extracted from bauxite ore. |
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Raw materials for the electrolysis process:
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ELECTRODE
EQUATIONS: redox details of the electrode
processes
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Concept diagram for the
electrolysis of molten aluminium oxide |
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The process of electrolysis uses of large amounts of energy in the extraction of a reactive metals like sodium, potassium, magnesium and calcium etc. and makes them expensive to produce. |
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Sodium, like many of the most reactive metals, can be extracted by electrolysis of its molten chloride. This can be done in the 'Down's Cell' shown in the diagram. Electrolysis reminders – the negative electrode (–) is called the cathode and attracts positive ions or cations e.g. Na+, and the positive electrode (+) is called the anode and attracts negative ions or anions e.g. Cl–. The ore–compound containing the sodium (or other metal) must be molten so the ions are free to move to the electrodes. The conducting melt is called the electrolyte. In the molten salt the positive sodium ions migrate to the negative cathode electrode and are reduced by electron gain to form liquid sodium atoms.
Equally mobile in the molten chloride salt are the negative chloride ions, which migrate to the positive anode electrode and get oxidised by electron loss to form green chlorine gas molecules. Initially two chlorine atoms are formed and these rapidly combine to give chlorine molecules. The positive anode attracts negative ions, eg the chloride ion. At the (+) anode half equation overall: 2Cl– ==> Cl2 + 2e– or 2Cl– ==> 2Cl + 2e– and then 2Cl ==> Cl2 This is an oxidation change - electron gain to free the chlorine from the sodium chloride
Overall chemical change: 2NaCl ==> 2Na + Cl2 Other very reactive metals like lithium, potassium and calcium can be extracted in the same way by electrolysing their molten salts. As you can see from the diagram on the right, all these metals are above carbon in the reactivity series and cannot be displaced by carbon. Some general notes on electrolysis AND ELECTROCHEMISTRY INDEX: 1. INTRODUCTION to electrolysis – electrolytes, non–electrolytes, electrode equations 2. Electrolysis of acidified water (dilute sulfuric acid) 3. Electrolysis of sodium chloride solution (brine) 4. Electrolysis of copper(II) sulfate solution and electroplating 5. Electrolysis of molten lead(II) bromide (and other molten compounds) 6. Electrolysis of copper(II) chloride solution 7. Electrolysis of hydrochloric acid 8. Summary of electrode equations and products 9. Summary of electrolysis products from various electrolytes 10. Simple cells (batteries) 11. Fuel Cells 12. The extraction of aluminium from purified molten bauxite ore 13. Anodising aluminium to thicken and strengthen the protective oxide layer 14. The extraction of sodium from molten sodium chloride using the 'Down's Cell' 15. The purification of copper by electrolysis 16. The purification of zinc by electrolysis 17. Electroplating coating conducting surfaces with a metal layer 18. Electrolysis of brine (NaCl) for the production of chlorine, hydrogen & sodium hydroxide 19. Electrolysis calculations
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