Aluminium Francisco Javier Cervigon Ruckauer

Aluminium

OBTAINING AND ISOLATION OF ALUMINIUM

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REACTIVITY IN AQUEOUS SOLUTION

Aluminium presents an interesting behavior in aqueous solution. It is considered an amphoteric metal, since this metal reacts with both acid and base:

2Al(s)+6H3O+(aq)⇄2[Al(OH2)6]3+(aq)+3H2(g)

2Al(s)+2OH−(aq)+6H2O(l)⇄2[Al(OH)4]−+3H2(g)

As we anticipated before, the aluminium ion exists in aqueous solution in the form of hexaaquaaluminum ion, [Al(OH2)6]3+, but it suffers a hydrolysis reaction to give a solution of the pentaaquahydroxoaluminum ion, [Al(OH2)5(OH)]2+, and the hydronium ion, H3O+. Subsequently, it also produces the tetraaquadihydroxoaluminum ion in a further step. For this reason, solutions of aluminium salts are acidic, and present acid ionization constant, Kaaround 10−5, similar to ethanoic (acetic) acid.

[Al(OH2)6]3+(aq)+H2O(l)⇄[(Al(OH2)5(OH))]2+(aq)+H3O+(aq)↔..............

Moreover, the aluminium ion shows different solubility depending on the pH conditions in the aqueous medium. Thus, the addition of hydroxide ion to aluminium ion first gives a gelatinous precipitate of aluminium hydroxide, but this product redissolves in excess hydroxide ion to give the aluminate ion (more precisely named the tetrahydroxoaluminate ion):

[Al(OH2)6]3+(aq)→OH−Al(OH)3(s)→OH−[Al(OH)4]−(aq)

In conclusion, aluminium 3+ ion is soluble at low and high pH solutions, but insoluble under neutral conditions (Figure 4.2).

Fig. 4.2 Aluminium 3+ ion solubility as a function of pH.

CHEMISTRY OF ALUMINIUM IN THE LAB

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REACTIVITY IN THE SOLID STATE

The reason for the stability of aluminium despite its negative reduction potential is found in the rapid reaction of any uncovered surface of aluminium metal with oxygen gas to form aluminium oxide, Al2O3 , which acts as a thin (~0.01 µm) impermeable oxide layer that protects the aluminium atoms underneath. This reaction is possible because of a perfect accomplishment of the corresponding resulting ions with the metallic aluminium atoms, since the oxide ion (O2− ) has an ionic radius (124 pm) comparable to the metallic radius of the aluminium atom (143 pm), and as a result, the surface packing is almost unchanged given that the small aluminium ions (Al3+ , 68 pm) fit into interstices in the oxide surface structure (Figure 10).

In fact, to prevent more efficiently their corrosion, manufactures anodize all aluminium products by using them as the anode in an electrochemical cell, increasing in this way the outermost aluminium oxide coating in 103 folds (up to ~0.01 mm), with the additional useful feature that colored surface can be produced by absorbing dyes.

4Al(s)+3O2(g)⇄2Al2O3(s)

Fig. 3. Formation of a single oxide layer on the surface of aluminium metal. The small aluminium 3+ ions are indicated by the solid circles.

USES OF ALUMINIUM

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Francisco Javier Cervigon Ruckauer