Aluminium and its Compounds — Revision Notes

⚡ 30-Second Revision

Electronic Config: — $[Ne]3s^23p^1$
Oxidation State: — $+3$ (common)
Amphoteric: — $ext{Al}$, $ext{Al}_2\text{O}_3$, $ext{Al}(\text{OH})_3$ react with acids & bases.

* $ext{Al} + \text{Acid} \rightarrow \text{Al}^{3+} + \text{H}_2$ * $ext{Al} + \text{Base} \rightarrow [\text{Al}(\text{OH})_4]^- + \text{H}_2$

Passivation: — Protective $ext{Al}_2\text{O}_3$ layer prevents corrosion.
$ ext{AlCl}_3$: — Anhydrous is a strong Lewis acid; exists as dimer $ext{Al}_2\text{Cl}_6$ (bridging Cl atoms).
Bayer Process: — Bauxite $ightarrow$ Pure $ext{Al}_2\text{O}_3$.
Hall-Héroult Process: — Electrolysis of $ext{Al}_2\text{O}_3$ in molten $ext{Na}_3\text{AlF}_6$ (cryolite) at $950-1000^circ\text{C}$.

* Cathode: $ext{Al}^{3+} + 3e^- \rightarrow \text{Al}(l)$ * Anode: $ext{C} + \text{O}^{2-} \rightarrow \text{CO}_2 + 2e^-$ (anodes consumed)

Alums: — Double sulfates, e.g., $ext{KAl}(\text{SO}_4)_2 cdot 12\text{H}_2\text{O}$. Used for water purification (coagulant).

2-Minute Revision

Aluminium, a Group 13 element, is characterized by its $[Ne]3s^23p^1$ electronic configuration and a dominant $+3$ oxidation state. A key property is its amphoteric nature, meaning both the metal and its oxide/hydroxide ($ext{Al}_2\text{O}_3$, $ext{Al}(\text{OH})_3$) react with acids and strong bases.

Despite being reactive, Aluminium exhibits passivation due to a protective, self-forming $ext{Al}_2\text{O}_3$ layer. Anhydrous aluminium chloride ($ext{AlCl}_3$) is a crucial compound; it's a powerful Lewis acid due to its electron-deficient aluminium center and exists as a dimer, $ext{Al}_2\text{Cl}_6$, in the vapor phase and non-polar solvents to achieve octet stability.

The industrial extraction of Aluminium involves the Bayer process to purify bauxite into alumina, followed by the Hall-Héroult process, an electrolytic reduction of alumina dissolved in molten cryolite ($ext{Na}_3\text{AlF}_6$).

Cryolite lowers the melting point and enhances conductivity. Alums, such as potassium alum, are double sulfates primarily used as coagulants in water purification. Remember these core concepts for NEET success.

5-Minute Revision

Aluminium (Al), atomic number 13, is a p-block element with valence shell configuration $3s^23p^1$. It primarily forms compounds in the $+3$ oxidation state by losing these three valence electrons. A critical aspect of Aluminium chemistry is its amphoteric nature.

This means $ext{Al}$ metal, $ext{Al}_2\text{O}_3$, and $ext{Al}(\text{OH})_3$ react with both acids and strong bases. For example, $ext{Al}$ reacts with $ext{HCl}$ to give $ext{AlCl}_3$ and $ext{H}_2$, and with $ext{NaOH}$ to form $ext{Na}[\text{Al}(\text{OH})_4]$ and $ext{H}_2$.

This dual reactivity is a common NEET question point.

Another vital property is passivation. Despite being a reactive metal, Aluminium appears unreactive due to the rapid formation of a thin, dense, and protective layer of aluminium oxide ($ext{Al}_2\text{O}_3$) on its surface, preventing further corrosion. This makes it valuable in many applications.

**Anhydrous Aluminium Chloride ($ext{AlCl}_3$) is a potent Lewis acid** because the central Aluminium atom has an incomplete octet (6 electrons). To achieve octet stability, it dimerizes to $ext{Al}_2\text{Cl}_6$ in the vapor phase and non-polar solvents, forming two bridging chlorine atoms. This Lewis acidity makes it a key catalyst in organic reactions like Friedel-Crafts alkylation.

Industrial Extraction: Aluminium is extracted from bauxite ore. The Bayer process purifies bauxite to pure alumina ($ext{Al}_2\text{O}_3$). Then, the Hall-Héroult process electrolytically reduces alumina.

Alumina is dissolved in molten **cryolite ($ext{Na}_3\text{AlF}_6$)** and fluorspar ($ext{CaF}_2$) at $950-1000^circ\text{C}$. Cryolite lowers the melting point and increases conductivity. At the carbon cathode, $ext{Al}^{3+}$ ions are reduced to molten $ext{Al}$.

At the graphite anodes, $ext{O}^{2-}$ ions are oxidized, consuming the anodes to form $ext{CO}$ and $ext{CO}_2$.

Alums are double sulfates, typically $ext{M}_2'\text{SO}_4 cdot \text{M}_2'''(\text{SO}_4)_3 cdot 24\text{H}_2\text{O}$, where $ext{M}'$ is monovalent and $ext{M}'''$ is trivalent ($ext{Al}^{3+}$). Potassium alum ($ext{KAl}(\text{SO}_4)_2 cdot 12\text{H}_2\text{O}$) is a common example. They are widely used in water purification as coagulants to settle suspended impurities by forming gelatinous $ext{Al}(\text{OH})_3$ precipitate. Other uses include mordants in dyeing and styptic agents.

Prelims Revision Notes

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Electronic Configuration & Oxidation State: — Aluminium (Al), atomic number 13, is a p-block element. Its electronic configuration is $[Ne]3s^23p^1$. It predominantly exhibits a $+3$ oxidation state by losing its three valence electrons. The $+1$ oxidation state is unstable for Al.
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Metallic Character: — Aluminium is a silvery-white, lightweight metal. It is highly electropositive.
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Amphoteric Nature: — Aluminium metal, aluminium oxide ($ext{Al}_2\text{O}_3$), and aluminium hydroxide ($ext{Al}(\text{OH})_3$) are amphoteric. They react with both acids and strong bases.

* Al with Acid: $2\text{Al}(s) + 6\text{HCl}(aq) \rightarrow 2\text{AlCl}_3(aq) + 3\text{H}_2(g)$ * Al with Base: $2\text{Al}(s) + 2\text{NaOH}(aq) + 6\text{H}_2\text{O}(l) \rightarrow 2\text{Na}[\text{Al}(\text{OH})_4](aq) + 3\text{H}_2(g)$ * **$ext{Al}_2\text{O}_3$ with Acid:** $ext{Al}_2\text{O}_3(s) + 6\text{H}^+(aq) \rightarrow 2\text{Al}^{3+}(aq) + 3\text{H}_2\text{O}(l)$ * **$ext{Al}_2\text{O}_3$ with Base:** $ext{Al}_2\text{O}_3(s) + 2\text{OH}^-(aq) + 3\text{H}_2\text{O}(l) \rightarrow 2[\text{Al}(\text{OH})_4]^-(aq)$

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Passivation: — Aluminium is reactive but forms a thin, dense, protective layer of $ext{Al}_2\text{O}_3$ on its surface, preventing further corrosion. This is called passivation.
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**Aluminium Chloride ($ext{AlCl}_3$):**

* Anhydrous $ext{AlCl}_3$ is a covalent compound and a strong Lewis acid (electron-deficient, incomplete octet). * It exists as a dimer, $ext{Al}_2\text{Cl}_6$, in the vapor phase and non-polar solvents, with two bridging chlorine atoms to complete the octet of Al. * It's used as a catalyst in Friedel-Crafts reactions.

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Extraction of Aluminium (Hall-Héroult Process):

* Ore: Bauxite ($ext{Al}_2\text{O}_3 cdot x\text{H}_2\text{O}$). * Bayer Process: Purification of bauxite to pure alumina ($ext{Al}_2\text{O}_3$). * Electrolysis: Pure $ext{Al}_2\text{O}_3$ is dissolved in molten cryolite ($ext{Na}_3\text{AlF}_6$) and fluorspar ($ext{CaF}_2$) at $950-1000^circ\text{C}$.

* Role of Cryolite: Lowers melting point of $ext{Al}_2\text{O}_3$ and increases electrical conductivity. * Cathode: Carbon lining; $ext{Al}^{3+}(melt) + 3e^- \rightarrow \text{Al}(l)$ * Anode: Graphite rods; $ext{O}^{2-}(melt) \rightarrow \frac{1}{2}\text{O}_2(g) + 2e^-$.

Oxygen reacts with graphite: $ext{C}(s) + \text{O}_2(g) \rightarrow \text{CO}_2(g)$ (anodes are consumed).

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Alums: — Double sulfates of general formula $ext{M}_2'\text{SO}_4 cdot \text{M}_2'''(\text{SO}_4)_3 cdot 24\text{H}_2\text{O}$.

* Potassium Alum: $ext{KAl}(\text{SO}_4)_2 cdot 12\text{H}_2\text{O}$. * Uses: Water purification (as a coagulant for suspended impurities), mordant in dyeing, styptic agent.

Vyyuha Quick Recall

ALUM is Amphoteric, Lewis acid, Undergoes Hall-Héroult, Makes Alums. (Remember: Amphoteric, Lewis acid $ext{AlCl}_3$, Undergoes Hall-Héroult extraction, Makes Alums for water purification.)