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Although aluminium is above hydrogen in ...

Although aluminium is above hydrogen in the electrochemical series, it is stable in air and water Why?

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The thin protective film of oxide, `Al_2O_3` is formed on the surface of aluminium.
This protects the metal from further attack of air and water. Thus the metal is stable in air and water.
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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