Detailed Discussions On Aluminium

ChemistryThe p-Block Elements: Group 13 & 14For NEET aspirants

Aluminium although is a reactive metal according to the electrochemical series, it is revered unreactive due to the formation of a oxide film on its surface.

Occurrence

It does not occur in native form. The important ores of aluminium are

(i) Bauxite (Al₂O₃.2H₂O)

(ii) Cryolite (Na₃AlF₆)

(iii) Feldspar (KAlSi₃O₈)

(iv) Mica KAlSiO₁₀(OH)₂

Extraction

Aluminium is extracted from bauxite in a two stage process.

(i) Stage – I

It involves the extraction of alumina (Al₂O₃) from bauxite.

(ii) Stage – II

Involves extraction of pure aluminium from Al₂O₃ by its electrolysis in molten cryolite [Na₃AlF₆].

(a) Purification of Bauxite

Bauxite contains SiO₂, iron oxide and titanium (IV) oxide as impurities. The bauxite ore is digested with a hot concentrated solution of NaOH at about 473 – 523 K and 35 – 36 bar pressure.

Aluminium oxide and silica dissolves to form sodium aluminate and sodium silicate respectively leaving behind iron oxide and TiO₂ which are filtered off.

$A l_{2} O_{3} (s) + 2 N a O H (a q .) + 3 H_{2} O (ℓ) 2 N a [A l (O H)_{2}] (a q)$

$S i O_{2} + 2 N a O H 2 N a_{2} S i O_{3} + H_{2} O$

The filtrate containing sodium aluminate and sodium silicate is diluted and seeded with freshly precipitated aluminium hydroxide which induces the precipitation of aluminium hydroxide leaving behind sodium silicate in solution.

$N a [A l (O H)_{4}] (a q) H_{2} O A l (O H)_{3} (s) + N a O H (a q)$

The aluminium hydroxide is filled dried and calcinated at 1473 K to yield pure alumina (Al₂O₃)

$2 A l (O H)_{3} A l_{2} O_{3} + 3 H_{2} O$

(b) Electrolysis of pure alumina (Hall – Heroult process)

Alumina is obtained from alumina by electrolysis. Pure alumina is dissolved in molten cryolite (Na₃AlF₆) and is electrolysed in an iron tank lined by carbon internally. Carbon lining serves as cathode, while the number of dipping carbon rods in the fused electrolyte serves as anode.

The electrolyte composition ranges are Na₃AlF₆(80 - 85%), CaF₂(5 – 7%), AlF₃(5 – 7%) Al₂O₃

(2 – 8%).

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Cryolite improves the electrical conductivity of the cell as alumina is a poor conductor. Also, cryolite lowers the melting point of the mixture to about 1250K. When electric supply is given aluminium is liberated at cathode and gets collected at the bottom of the tank from where it is removed through outlet. Oxygen liberates at anode and combines with the carbon of the anode to produce carbon monoxide which either burns or escape out.

At cathode: Al³⁺(melt) +3e^- $\to$ Al $(ℓ)$ .

At anode: O^-2 $\to$ O + 2e^-

C(s) + O $\to$ CO(g)

CO + O $\to$ CO (g)

CO + O $\to$ CO₂ (g)

Properties of Aluminium

Physical properties

Aluminium is a soft light silvery – white metal which soon looses its shine due to formation of a layer of oxide on it. It is malleable and ductile. It m.p. is 932K. Electrical conductivity is twice than copper on weight to weight basis.

Chemical properties

1. Action of water

It does not react with pure water but it is corroded by water containing salts.

2. Action of air

It is not affected by dry air but highly electropositive and redily reacts with moist air and forms a hard protective layer of Al₂O₃, which renders it positive

$4 A l + 3 O_{2} 2 A l_{2} O_{3}$

3. Action of Acids

Aluminium dissolves in dilute mineral acids and produces hydrogen.

$2 A l + 6 H C l 2 A l C l_{3} + 3 H_{2}$

$2 A l + 3 H_{2} S O_{4} \to A l_{2} (S O_{4})_{3} + 3 H$

4. Reaction with strong alkali

Aluminium gives aluminate with liberating hydrogen too.

2Al\left( s \right) + 2NaOH\left( {aq} \right) + 6{H_2}\left( \ell \right) \to \,\begin{array}{*{20}{c}} {2Na\left[ {Al{{\left( {OH} \right)}_4}} \right]\left( {aq} \right) + 3{H_2}\left( g \right)} \\ {sodium\,meta\,alu\min ate} \end{array}

$2 A l (s) + 2 N a O H (a q) + 2 H_{2} O (ℓ) so d i u m m e t a a l u m i n a t e 2 N a A l O_{2} (a q) + 3 H_{2} (g)$

Uses of Aluminium

Aluminium is extensively used in industries as well as in every day life.

1. It is used in making many alloys, utensils, construction aerospace, window angles.

2. It is a good conductor of electricity, thicker cables of aluminium are used for transmission of electricity.

3. Aluminium foil is used in wrapping cigarettes, confectionary etc.

4. Aluminium is also used in the aluminothermic process for production of chromium and manganese from their ores.

$C r_{2} O_{3} \to 2 A l \to A l_{2} O_{3} + 2 C r .$

$3 M n O_{2} + 4 A l \to 2 A l_{2} O_{3} + 3 M n$

Illustration : Name two alloys of aluminium with their constituents.

Solution: Magnalium (Al and Mg), Duralumin (Al, Cu, Mg and Mn)

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