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ChemistryMediumClass 11
Boron Compounds — Borax, boric acid, diborane
p-block Elements (Class 11 — Groups 13 & 14)
10
JEE Qs
8%
Hard
75
min
Focus on understanding the unique bonding in diborane and the Lewis acidic nature of boric acid, along with their characteristic reactions and applications like the borax bead test.
🧮 Key Formulas
Na₂[B₄O₅(OH)₄].8H₂O (Borax, structural formula)
Na₂B₄O₇.10H₂O (Borax, empirical formula)
H₃BO₃ or B(OH)₃ (Boric acid)
B₂H₆ (Diborane)
✅ Key Points for JEE
- 1Borax is a sodium tetraborate decahydrate, structurally containing [B₄O₅(OH)₄]²⁻ anion. It hydrolyses in water to give an alkaline solution.
- 2Boric acid is a weak monobasic Lewis acid, not a protic acid. It accepts a hydroxyl ion from water: B(OH)₃ + H₂O ⇌ [B(OH)₄]⁻ + H⁺.
- 3Diborane (B₂H₆) has a unique structure with two 3-center-2-electron (3c-2e) 'banana' bonds involving bridging hydrogen atoms and four 2-center-2-electron (2c-2e) terminal B-H bonds.
- 4The borax bead test is used for detecting colored metallic ions. On heating, colored metaborates (e.g., Co(BO₂)₂, Cr(BO₂)₃) are formed with B₂O₃, which itself is formed from borax.
- 5Key reactions to remember include: preparation methods (especially diborane from NaBH₄ or BF₃), hydrolysis of diborane, reaction of boric acid with polyols to enhance its acidity.
⚠️ Common Mistakes
- ✕Mistaking boric acid as a protic acid that donates H⁺, instead of a Lewis acid that accepts OH⁻.
- ✕Incorrectly drawing the structure of diborane, especially misunderstanding the 3-center-2-electron bonds.
- ✕Confusing the various formulas of borax (anhydrous, decahydrate, and the structural form).
- ✕Forgetting the products of reactions, such as the hydrolysis of diborane producing boric acid and hydrogen.
NCERT Chapters
- Class 11 Chemistry Ch 11: p-Block Elements (Groups 13 & 14)