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ChemistryMediumClass 12

Phosphorus Compounds — PCl₃, PCl₅, oxoacids

p-block Elements (Class 12 — Groups 15, 16, 17, 18)

10

JEE Qs

8%

Hard

90

min

Focus on drawing and understanding the structures of phosphorus halides and oxoacids to accurately deduce their hybridization, geometry, acidity, basicity, and redox properties.

🧮 Key Formulas

P₄ + 6Cl₂ → 4PCl₃
P₄ + 10Cl₂ → 4PCl₅
PCl₃ + 3H₂O → H₃PO₃ + 3HCl
PCl₅ + H₂O (limited) → POCl₃ + 2HCl
PCl₅ + 4H₂O (excess) → H₃PO₄ + 5HCl
PCl₅(g) ⇌ PCl₃(g) + Cl₂(g)
H₃PO₃ (phosphorous acid): Dibasic (2 P-OH bonds), Reducing (1 P-H bond)
H₃PO₂ (hypophosphorous acid): Monobasic (1 P-OH bond), Strong Reducing (2 P-H bonds)
H₃PO₄ (orthophosphoric acid): Tribasic (3 P-OH bonds)

✅ Key Points for JEE

  • 1PCl₃ has a trigonal pyramidal geometry (sp³ hybridization). PCl₅ has a trigonal bipyramidal geometry (sp³d hybridization) in gaseous/liquid phase, but exists as ionic [PCl₄]⁺[PCl₆]⁻ in the solid state.
  • 2Hydrolysis of phosphorus halides is crucial: PCl₃ yields phosphorous acid (H₃PO₃), while PCl₅ yields phosphoryl chloride (POCl₃) with limited water, and orthophosphoric acid (H₃PO₄) with excess water.
  • 3Oxoacids of phosphorus always contain at least one P=O bond and one P-OH bond. The number of P-OH bonds determines their basicity (acidity), while P-H bonds are responsible for their reducing properties.
  • 4Oxidation states of phosphorus in its compounds and oxoacids vary from +1 to +5; correctly identifying them helps predict redox behavior.
  • 5Be aware of the thermal decomposition of PCl₅ and disproportionation reactions of oxoacids like H₃PO₃ (on heating, forms H₃PO₄ and PH₃).

⚠️ Common Mistakes

  • ✕Assuming all hydrogen atoms in phosphorus oxoacids are acidic; only hydrogen atoms attached to oxygen (P-O-H) are acidic, not those directly bonded to phosphorus (P-H).
  • ✕Incorrectly predicting the products of PCl₅ hydrolysis, especially confusing the outcomes with limited vs. excess water.
  • ✕Misidentifying the basicity of oxoacids (e.g., H₃PO₃ as tribasic due to 3H atoms) without considering their structures.
  • ✕Confusing the geometry and hybridization of PCl₅ in its gaseous/liquid state vs. solid state.

NCERT Chapters

  • Class 12 Chemistry Ch 7: The p-Block Elements