Noesis
3.2.2p-Block

Aluminium — chemistry, alloys; alumina, alums

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🇮🇳 Yeh note Hinglish mein padho →

80/20 core: Al is a Group 13 metal that is amphoteric (reacts with both acids and bases), forms a protective oxide layer (passivation), and gives a covalent-character chloride Al2Cl6Al_2Cl_6. Master these three ideas and the rest follows.

1. Why is Aluminium so special?

2. Reactions of Aluminium — derived, not memorised

2a. With air / oxygen (passivation)

WHAT: Al reacts with O2O_2 to form a thin, tough, impervious oxide film. 4Al+3O2→2Al2O34Al + 3O_2 \rightarrow 2Al_2O_3 WHY it stops: the Al2O3Al_2O_3 layer is dense and adheres tightly, sealing the metal underneath → this passivation is why Al "doesn't rust away" despite being reactive.

2b. With acids

With dilute HCl / H2SO4H_2SO_4: 2Al+6HCl→2AlCl3+3H2↑2Al + 6HCl \rightarrow 2AlCl_3 + 3H_2\uparrow

2c. With alkali (the amphoteric proof)

2Al+2NaOH+6H2O→2Na[Al(OH)4]+3H2↑2Al + 2NaOH + 6H_2O \rightarrow 2Na[Al(OH)_4] + 3H_2\uparrow WHY: OH−OH^- dissolves the protective oxide as aluminate, exposing fresh metal → reaction continues. This reaction with base is the signature of a metal being amphoteric.

2d. Thermite reaction (high affinity for oxygen)

2Al+Fe2O3→Al2O3+2Fe(ΔH≪0)2Al + Fe_2O_3 \rightarrow Al_2O_3 + 2Fe \quad (\Delta H \ll 0) WHY: Al2O3Al_2O_3 is more stable than Fe2O3Fe_2O_3; Al reduces the metal oxide, releasing huge heat (molten Fe) → used for welding rails. This is Al acting as a reducing agent.

3. AlCl3AlCl_3 — the dimer Al2Cl6Al_2Cl_6

Figure — Aluminium — chemistry, alloys; alumina, alums

4. Alumina (Al2O3Al_2O_3)

Amphoteric proof: Al2O3+6HCl→2AlCl3+3H2O(base-like)Al_2O_3 + 6HCl \rightarrow 2AlCl_3 + 3H_2O \quad (\text{base-like}) Al2O3+2NaOH+3H2O→2Na[Al(OH)4](acid-like)Al_2O_3 + 2NaOH + 3H_2O \rightarrow 2Na[Al(OH)_4] \quad (\text{acid-like})

5. Alums

Common potash alum: K2SO4⋅Al2(SO4)3⋅24H2OK_2SO_4\cdot Al_2(SO_4)_3\cdot 24H_2O, i.e. KAl(SO4)2⋅12H2OKAl(SO_4)_2\cdot 12H_2O.

6. Alloys (WHY mix?)

Alloy Composition (key) Use
Duralumin Al + Cu + Mg + Mn aircraft, light strong parts
Magnalium Al + Mg balance beams, light frames
Alnico Al + Ni + Co (+Fe) strong permanent magnets
Recall Feynman: explain to a 12-year-old

Aluminium is a shy-but-tough metal. The moment it meets air it grows an invisible "armour" (oxide skin) that protects it — that's why your soda can doesn't fall apart. It's a "two-faced" friend: it can fight acids and fight strong soaps (bases) — that's "amphoteric." Mix it with a pinch of other metals and it becomes super strong but stays feather-light, perfect for planes. Drop a special salt called alum in dirty water and it grabs all the floating dirt into clumps so they sink — clean water!

Flashcards

What is the electronic configuration & common oxidation state of Al?
[Ne]3s23p1[Ne]3s^2 3p^1; oxidation state +3.
Why does conc. HNO3HNO_3 NOT attack aluminium?
It passivates Al by forming an inert protective oxide layer.
Write Al's reaction with NaOH solution.
2Al+2NaOH+6H2O→2Na[Al(OH)4]+3H22Al + 2NaOH + 6H_2O \rightarrow 2Na[Al(OH)_4] + 3H_2.
Why does AlCl3AlCl_3 exist as a dimer Al2Cl6Al_2Cl_6?
Monomer is electron-deficient (6e); Cl lone pairs bridge to complete Al's octet.
How many bridging vs terminal Cl in Al2Cl6Al_2Cl_6?
2 bridging, 4 terminal; each Al is sp3sp^3.
Why is aqueous AlCl3AlCl_3 acidic?
Al3+Al^{3+} has high charge density → hydrolyses, releasing H+H^+.
What does "amphoteric" mean and which Al species are amphoteric?
Reacts with both acids and bases; Al, Al2O3Al_2O_3, Al(OH)3Al(OH)_3.
Write the thermite reaction and its use.
2Al+Fe2O3→Al2O3+2Fe2Al + Fe_2O_3 \rightarrow Al_2O_3 + 2Fe; rail welding (Al is reducing agent).
General formula of an alum.
M2SO4⋅M2′(SO4)3⋅24H2OM_2SO_4\cdot M'_2(SO_4)_3\cdot24H_2O = M+M′3+(SO4)2⋅12H2OM^+M'^{3+}(SO_4)_2\cdot12H_2O.
Formula of potash alum.
KAl(SO4)2⋅12H2OKAl(SO_4)_2\cdot12H_2O.
Why does alum clarify water?
Forms gelatinous Al(OH)3Al(OH)_3 floc that traps and settles suspended impurities.
Composition & use of Duralumin.
Al+Cu+Mg+Mn; aircraft / strong-light parts.
What is alumina and one notable property?
Al2O3Al_2O_3; amphoteric, very hard, high m.p. (corundum/ruby host).

Connections

  • Fajans Rules — explains covalent character & polarising power of Al3+Al^{3+}
  • Group 13 Elements — periodic trends for B, Al, Ga, In, Tl
  • Lewis Acids and Bases — why AlCl3AlCl_3 is an electron-deficient catalyst
  • Passivation and Corrosion
  • Coordination Compounds — [Al(OH)4]−[Al(OH)_4]^- aluminate ion
  • Thermodynamics of Reduction (Ellingham) — why thermite works

Concept Map

oxidation state

Fajans polarisation

explains

covalent character

hydrolysis

reacts with acid

reacts with base

reacts with O2

amphoteric oxide

Cl lone pair bridge

reduces Fe2O3

conc HNO3

Al 3s2 3p1

Al3+ small high charge

High charge density

Amphoterism

Al2Cl6 dimer

Acidic aqueous Al3+

AlCl3 + H2

Aluminate NaAlOH4

Al2O3 passivation

Two bridging Cl sp3 Al

Thermite welding

Inert passivated

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, Aluminium ek "+3" wala chhota aur strong-charge wala cation banata hai (Al3+Al^{3+}). Yahi ek baat samajh lo to aadha chapter clear ho jata hai. Iski high charge density ki wajah se ye electron clouds ko khinchta hai (Fajans' rule) — isiliye AlCl3AlCl_3 covalent character dikhata hai aur paani me ghulke acidic ho jata hai. Air me Al turant ek patli oxide ki "armour" bana leta hai (passivation) — isiliye soda can kharab nahi hota, aur isiliye conc. HNO3HNO_3 Al ko attack nahi karta (ulta usko inert kar deta hai).

Sabse important word: amphoteric. Matlab Al, Al2O3Al_2O_3 aur Al(OH)3Al(OH)_3 dono — acid se bhi react karte hain aur base (NaOH) se bhi. NaOH ke saath aluminate [Al(OH)4]−[Al(OH)_4]^- banta hai aur H2H_2 gas nikalti hai — ye amphoteric hone ka proof hai. Thermite reaction me Al, Fe2O3Fe_2O_3 ko reduce karke molten iron deta hai (rail welding) — kyunki Al2O3Al_2O_3 zyada stable hai.

AlCl3AlCl_3 dimer (Al2Cl6Al_2Cl_6) kyun banta hai? Kyunki single AlCl3AlCl_3 me Al ke paas sirf 6 electron hain (electron-deficient, Lewis acid). Octet complete karne ke liye doosre molecule ka Cl apna lone pair share karta hai — bridging Cl ban jata hai. 2 bridge + 4 terminal, har Al sp3sp^3.

Aur alum: double sulphate, formula M+M′3+(SO4)2⋅12H2OM^+M'^{3+}(SO_4)_2\cdot12H_2O, jaise potash alum KAl(SO4)2⋅12H2OKAl(SO_4)_2\cdot12H_2O. Ye gande paani me daalo to gelatinous Al(OH)3Al(OH)_3 banta hai jo dirt ko pakad ke neeche baith jata hai — paani saaf. Alloys (Duralumin, Magnalium, Alnico) banane se Al light rehte hue strong ho jata hai — planes ke liye perfect. Exam me ye saari cheezein bar-bar aati hain, to inhe derive karke yaad rakho, ratta mat maro.

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Connections

Fajan's rules — covalent character in ionic compoundsChemistry · 2.3.4Acids and bases — Arrhenius, Brønsted-Lowry, Lewis definitionsChemistry · 2.6.7Werner's theory of coordination compoundsChemistry · 3.4.1