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ChemistryMediumClass 11

Aluminium — Reactions, alum

p-block Elements (Class 11 — Groups 13 & 14)

10

JEE Qs

8%

Hard

75

min

Focus on the amphoteric nature of Aluminium and its compounds, the unique properties and reactions of anhydrous AlCl3, and the general formula and applications of alums, as these are high-yield concepts.

🧮 Key Formulas

2Al + 3/2 O2 → Al2O3
2Al + 6HCl → 2AlCl3 + 3H2
2Al + 2NaOH + 6H2O → 2Na[Al(OH)4] + 3H2
Al2O3 + 6HCl → 2AlCl3 + 3H2O
Al2O3 + 2NaOH + 3H2O → 2Na[Al(OH)4]
Al(OH)3 + 3HCl → AlCl3 + 3H2O
Al(OH)3 + NaOH → Na[Al(OH)4]
AlCl3 + 3H2O → Al(OH)3(s) + 3HCl(aq) (Hydrolysis of anhydrous AlCl3)
2AlCl3(g) <=> Al2Cl6(g) (Dimerization of anhydrous AlCl3)
M^1+ M^3+(SO4)2 . 12H2O (General formula of alum, where M^1+ is a monovalent cation like K+, NH4+ and M^3+ is a trivalent cation like Al3+, Cr3+, Fe3+)

✅ Key Points for JEE

  • 1Aluminium is amphoteric: it reacts with both acids (e.g., HCl) and strong bases (e.g., NaOH) to produce hydrogen gas.
  • 2Aluminium metal becomes passive towards concentrated nitric acid due to the formation of a thin, tenacious, and non-porous protective layer of aluminium oxide (Al2O3).
  • 3Anhydrous AlCl3 is a covalent compound, exists as a dimer (Al2Cl6) in the vapor phase and non-polar solvents via chlorine bridges, and acts as a strong Lewis acid.
  • 4Anhydrous AlCl3 undergoes extensive hydrolysis in water, producing Al(OH)3 and HCl, making its aqueous solution acidic and fuming in moist air.
  • 5Alums are double salts (M^1+ M^3+(SO4)2 . 12H2O) that crystallize as octahedral crystals and are widely used for water purification (coagulant) due to the acidic nature of the trivalent ion upon hydrolysis.

⚠️ Common Mistakes

  • ✕Failing to recognize aluminium and its oxide/hydroxide as amphoteric, or incorrectly predicting products with acids/bases.
  • ✕Confusing the properties of anhydrous AlCl3 (covalent, dimeric, Lewis acid) with hydrated AlCl3 (ionic, hexahydrate, [Al(H2O)6]3+).
  • ✕Incorrectly recalling the general formula for alum, specifically omitting the 12 molecules of water of crystallization or incorrect valencies of the cations.
  • ✕Overlooking the passivity of Aluminium with concentrated nitric acid due to oxide layer formation.

NCERT Chapters

  • Class 11 Chemistry Ch 11: The p-Block Elements