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Graphite rods are frequently replaced i...

Graphite rods are frequently replaced in Hall-Heroult's process of extracting aluminium metal. Why ?

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Fluoride ions move towards graphite anode and are dischared to give active fluorine. This fluorine displaces oxygen of alumin. Oxygen reacts with graphite. Thus graphite gets corroded and is to be replaced frequently in the Hall-Heroult.s process of extracting aluminium.
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Aluminium is stable in air and water inspite of the fact that it is reactive metal. The reason is that a thin film of its oxide, if formed on its surface which makes it passive for further attack. The layer is so useful that in industry, it is purposely deposited by an electrolytric process called anodizing. Reaction of aluminium with oxygen is highyl exothermic and is called thermite reaction 2Al(s)+(3)/(2)O_(2)(g)rarr Al_(2)O_(3)(s), Delta H=-1670 kJ Thermite reaction finds applications in the metallurgical extraction of many metals from their oxides and for welding of metals. The drawback is that to start the reaction, high temperature is required for which an ignition mixture is used. Anodised aluminium is

Aluminium is stable in air and water inspite of the fact that it is reactive metal. The reason is that a thin film of its oxide, if formed on its surface which makes it passive for further attack. The layer is so useful that in industry, it is purposely deposited by an electrolytric process called anodizing. Reaction of aluminium with oxygen is highyl exothermic and is called thermite reaction 2Al(s)+(3)/(2)O_(2)(g)rarr Al_(2)O_(3)(s), Delta H=-1670 kJ Thermite reaction finds applications in the metallurgical extraction of many metals from their oxides and for welding of metals. The drawback is that to start the reaction, high temperature is required for which an ignition mixture is used. Thermite mixture used for welding is