CUET UG Chemistry: Inorganic Chemistry - p-Block Elements, Groups 15, 16, 17, 18, Oxoacids, Anomalous Properties

Must-Know (15–20 detailed bullets)

• Nitrogen exhibits anomalous behavior due to absence of d-orbitals, small size, high electronegativity, and high ionization enthalpy; for example, it forms p?-p? multiple bonds (N?N), while P forms P? tetrahedral structure.
• Nitrogen shows maximum covalency of 4 (e.g., NH), whereas other Group 15 elements can expand covalency beyond 4 due to d-orbital availability (e.g., PF).
• HNO? has a planar structure with nitrogen in +5 oxidation state; it acts as a strong oxidizing agent and decomposes on exposure to light: 4HNO?-4NO? + O? + 2H?O.
• Phosphorus forms two common oxoacids: H?PO? (phosphorous acid, diprotic, +3 oxidation state) and H?PO? (orthophosphoric acid, triprotic, +5 oxidation state).
• H?PO? is diprotic because only two –OH groups are ionizable; the H directly bonded to P is not acidic.
• In Group 16, oxygen differs from others due to absence of d-orbitals, high electronegativity (3.5), and ability to form hydrogen bonds; e.g., H?O is liquid, H?S is gas at room temperature.
• Ozone (O?) is an allotrope of oxygen, bent structure (116.8°), resonance hybrid, and acts as a powerful oxidizing agent; it absorbs UV radiation in stratosphere.
• SO? is a bent molecule (O–S–O angle-119°), acidic oxide, reducing agent; turns acidified K?Cr?O? paper green.
• Sulphuric acid (H?SO?) is manufactured via Contact Process: 2SO? + O?-2SO? (V?O? catalyst, 2 bar, 720 K); SO? absorbed in H?SO? to form oleum.
• H?SO? is dibasic, highly corrosive, and acts as dehydrating agent (e.g., converts HCOOH to CO); concentrated H?SO? has low volatility due to hydrogen bonding.
• In Group 17, fluorine shows anomalous behavior: highest electronegativity (4.0), low electron gain enthalpy (due to small size, interelectronic repulsion), and absence of d-orbitals (no higher oxidation states).
• HF is a weak acid (pKa = 3.2) due to strong H–F bond and extensive hydrogen bonding; other HX are strong acids.
• Interhalogen compounds like ClF? (T-shaped, sp³d hybridization) and IF? (pentagonal bipyramidal, sp³d³d²) are more reactive than halogens except F?.
• Bleaching action of Cl? is due to oxidation: Cl? + H?O-HCl + HOCl; HOCl-HCl + [O]; colored substance + [O]-colorless.
• Chlorine oxoacids: HClO (hypochlorous acid, +1), HClO? (chlorous acid, +3), HClO? (chloric acid, +5), HClO? (perchloric acid, +7); acidity increases with oxidation state.
• HClO? is the strongest oxoacid of chlorine due to maximum oxidation state (+7) and greater delocalization of negative charge in conjugate base.
• Noble gases have very high ionization enthalpies; He has highest (2372 kJ/mol), making it chemically inert under normal conditions.
• Xenon forms compounds with highly electronegative elements: XeF? (linear, sp³d), XeF? (square planar, sp³d²), XeF? (distorted octahedral, sp³d³).
• XeF? hydrolyzes: XeF? + 2H?O-2HF + Xe + O? (partial hydrolysis); complete hydrolysis gives XeO?, which is explosive.
• verify from NCERT: The order of thermal stability of hydrides of Group 16: H?O > H?S > H?Se > H?Te > H?Po.

Difficulty Level

Intermediate — requires understanding of periodic trends, molecular geometry, and acid strength trends, but most facts are directly from NCERT.

Common CUET Traps

• Trap: Assuming H?PO? is triprotic like H?PO?.
  Avoid: Remember only –OH groups are acidic; H?PO? has two –OH and one P–H bond-diprotic.

• Trap: Thinking bleaching action of SO? and Cl? are the same.
  Avoid: SO? bleaches by reduction (temporary), Cl? by oxidation (permanent); SO? decolorizes acidified KMnO?, Cl? does not.

• Trap: Believing noble gases cannot form compounds.
  Avoid: Xenon forms stable compounds with F and O (e.g., XeF?, XeO?); He, Ne, Ar do not form compounds under normal conditions.

Practice MCQs

1. Which of the following oxoacids of phosphorus is diprotic?
   A. H?PO?
   B. H?P?O?
   C. H?PO?
   D. H?PO?
   Answer: C
   Explanation: H?PO? has two ionizable –OH groups and one P–H bond, making it diprotic.
   Why others fail: H?PO? is triprotic, a common confusion due to similar naming.

2. The bleaching action of chlorine is permanent because:
   A. It reduces the colored substance
   B. It forms a colorless complex
   C. It oxidizes the colored substance
   D. It absorbs light
   Answer: C
   Explanation: Cl? produces nascent oxygen via HOCl, which oxidizes the colored material.
   Why others fail: SO? bleaches by reduction (temporary), leading to confusion with Cl?.

3. Which interhalogen compound has a T-shaped geometry?
   A. ClF
   B. ClF?
   C. BrF?
   D. IF?
   Answer: B
   Explanation: ClF? has sp³d hybridization and T-shaped geometry due to two lone pairs on Cl.
   Why others fail: BrF? is square pyramidal; students often confuse geometries of interhalogens.

4. Among the following, which acid is the strongest?
   A. HClO
   B. HClO?
   C. HClO?
   D. HClO?
   Answer: D
   Explanation: Acid strength increases with oxidation state of Cl; HClO? (+7) is strongest.
   Why others fail: HClO? is less acidic than HClO? and HClO?, but students misremember the order.

5. Which noble gas compound is isostructural with ICl?
   A. XeF?
   B. XeF?
   C. XeO?
   D. XeF?
   Answer: A
   Explanation: ICl has linear geometry (sp³d, two lone pairs), same as XeF?.
   Why others fail: XeF? is square planar; students often misapply hybridization rules.

Last?Minute Revision

• H?PO? is diprotic; H?PO? is monoprotic — count –OH groups only.
• Ozone layer absorbs UV-B (280–320 nm) in stratosphere.
• H?S has bent shape, ~92° bond angle — no hydrogen bonding, weak dipole.
• Nitrogen cannot form pentahalides (no d-orbitals); PCl? exists.
• HF stored in wax bottles — attacks glass: SiO? + 4HF-SiF? + 2H?O.
• XeF? has two lone pairs, square planar (sp³d²).
• H?SO?: dibasic, dehydrating, oxidizing agent when hot/concentrated.
• O? molecule is paramagnetic — two unpaired electrons in ?* orbitals.
• Cl? + 2NaOH (cold, dil.)-NaCl + NaClO + H?O.
• Cl? + 6NaOH (hot, conc.)-5NaCl + NaClO? + 3H?O.
• Bleaching powder: Ca(OCl)Cl — active ingredient OCl?.
• H?O has higher boiling point than H?S due to H-bonding.
• NH? has higher boiling point than PH? — hydrogen bonding in NH?.
• OF? is oxygen difluoride; oxygen has +2 oxidation state.
• XeO? is explosive, trigonal pyramidal, sp³ hybridized.
• Mnemonic: "I See 6 F's" — XeF? has 6 F atoms, distorted octahedral.