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ChemistryMediumClass 12

Oxoacids — All major oxoacids of N, P, S, Cl

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

10

JEE Qs

8%

Hard

90

min

Focus on correctly drawing structures and identifying oxidation states to logically deduce properties like acidity, basicity, and redox behavior rather than rote memorization.

🧮 Key Formulas

Oxidation State Calculation: Sum of O.S. in neutral molecule = 0; Sum of O.S. in ion = charge
Acid Strength ∝ Oxidation State of central atom
Acid Strength ∝ Electronegativity of central atom
Basicity = Number of acidic -OH groups (hydrogens attached to oxygen)
Reducing nature often due to P-H or S-H bonds (not acidic)

✅ Key Points for JEE

  • 1Master drawing correct Lewis structures and predicting geometries (using VSEPR) for all major oxoacids of N, P, S, Cl, focusing on the bonding of acidic hydrogens and non-acidic hydrogens (e.g., P-H bonds).
  • 2The acidity of an oxoacid is directly related to the oxidation state of the central atom and the number of terminal oxygen atoms (which stabilize the conjugate base via resonance and inductive effect). Stronger acid = higher oxidation state.
  • 3Oxoacids with central atoms in intermediate oxidation states (e.g., H3PO3, H3PO2, H2SO3, HClO, HClO2) can act as reducing agents (due to P-H, S-H bonds or ability to be further oxidized) and may undergo disproportionation reactions upon heating.
  • 4Distinguish between basicity (number of acidic protons, i.e., -OH groups) and the presence of reducing P-H or S-H bonds. For example, H3PO2 is monobasic but a strong reducing agent due to two P-H bonds.
  • 5Understand the formation and structural features of polyoxoacids (e.g., H4P2O7, H2S2O7) which involve X-O-X linkages.

⚠️ Common Mistakes

  • ✕Assuming all hydrogen atoms in an oxoacid are acidic; only hydrogens attached to oxygen (as -OH groups) are acidic. P-H bonds, for instance, are not acidic.
  • ✕Incorrectly calculating the oxidation state of the central atom, leading to wrong predictions about acid strength, oxidizing, or reducing properties.
  • ✕Confusing the reducing nature of an oxoacid (due to P-H or S-H bonds) with its acidity. For example, H3PO2 is a strong reducing agent but only monobasic.
  • ✕Errors in drawing accurate structures, especially for more complex oxoacids like polyphosphates, hypophosphorous acid, or dithionic acids, which impacts understanding their properties.

NCERT Chapters

  • Class 12 Chemistry Part 1 Ch 7: The p-Block Elements