Aluminium (Al)

Fresh aluminium reacts instantly with oxygen to form a thin, adherent film of aluminium oxide (Al₂O₃) a few nanometres thick. This passive layer seals the surface and prevents further oxidation in neutral environments. Chloride-rich or strongly acidic/basic media can disrupt this film, increasing corrosion.

Yes. Aluminium oxide/hydroxide dissolves in both acids and bases.

• Acidic: \(\mathrm{Al_2O_3 + 6\,H^+ \rightarrow 2\,Al^{3+} + 3\,H_2O}\)
• Basic: \(\mathrm{Al_2O_3 + 2\,OH^- + 3\,H_2O \rightarrow 2\,[Al(OH)_4]^-}\) or \(\mathrm{Al(OH)_3 + OH^- \rightarrow [Al(OH)_4]^-}\)

Metallic Al also reacts with hot, concentrated bases to release hydrogen because the oxide film is dissolved.

Bayer: Bauxite (AlOOH/Al(OH)₃ with impurities) is digested in hot NaOH; alumina dissolves as aluminate and is reprecipitated as pure Al(OH)₃, then calcined to Al₂O₃.

Hall–Héroult: Alumina is dissolved in molten cryolite (Na₃AlF₆) and electrolyzed. Overall (with carbon anodes):

\(\mathrm{2\,Al_2O_3 + 3\,C \;\rightarrow\; 4\,Al + 3\,CO_2}\)

The cryolite lowers the melting point and improves conductivity.

Finely divided Al burns with a bright white flame to Al₂O₃. In the thermite reaction, aluminium reduces iron(III) oxide to molten iron:

\(\mathrm{Fe_2O_3 + 2\,Al \;\rightarrow\; 2\,Fe + Al_2O_3\;(+\,heat)}\)

This highly exothermic process is used for welding rails and emergency metal repairs.

Al has only six valence electrons in AlCl₃ (electron-deficient). It readily accepts a lone pair to complete an octet, forming adducts such as \(\mathrm{AlCl_3 + Cl^- \rightarrow AlCl_4^-}\) or complexes with bases (e.g., THF). This behaviour underpins its use as a catalyst in Friedel–Crafts reactions.

Anodizing is an electrochemical oxidation that thickens and orders the surface Al₂O₃ into a porous, hard layer that can be sealed or dyed. Benefits include enhanced corrosion resistance, wear hardness, and decorative colouring without adding significant weight.

Per unit cross-section, copper conducts better (higher conductivity). However, aluminium is much lighter (\(\rho_{Al}\approx 2.7\,\mathrm{g\,cm^{-3}}\) vs \(\rho_{Cu}\approx 8.9\,\mathrm{g\,cm^{-3}}\)), so for weight-limited applications (power lines), a larger-area Al conductor can deliver required current at lower mass, giving superior specific conductivity.

The native oxide film blocks reaction with cold water. In hot, concentrated alkali the film dissolves, exposing metal that reacts to produce hydrogen and aluminate:

\(\mathrm{2\,Al + 2\,NaOH + 6\,H_2O \rightarrow 2\,Na[Al(OH)_4] + 3\,H_2}\)

Alloys with Mg, Si, Cu, Zn, and Mn tailor strength, corrosion resistance, and formability. Examples:

• 6xxx (Al–Mg–Si, e.g., 6061): structural extrusions, frames.
• 7xxx (Al–Zn–Mg–Cu, e.g., 7075): high-strength aerospace parts.
• 5xxx (Al–Mg): marine applications for corrosion resistance.

Recycling saves ~95% of the energy compared with primary smelting and dramatically cuts CO₂ emissions. Aluminium can be recycled repeatedly with minimal property loss, making it a cornerstone of circular manufacturing (e.g., cans to new cans within weeks).