Chemistry 2025 Paper I 50 marks Draw

Q8

(a) Draw the structures of S₂N₂, S₄N₂, S₁₁N₂, S₅N₆ and I₂Cl₆. 10 marks (b) What are silicones ? Mention some of their uses. How will you prepare hexamethyldisiloxane ? Draw its structure. What happens if some (CH₃)₃ SiCl is mixed with (CH₃)₂ SiCl₂ and hydrolysed ? 10 marks (c) How many lanthanides can be easily separated from the lanthanide mixture by using valency change method ? Justify your answer. 10 marks (d) Write electronic configuration (outer) of the following lanthanide ions and calculate the magnetic moment in BM from L-S coupling. 10 marks (i) Pr³⁺ (g = 4/5) (ii) Tb³⁺ (g = 3/2) (e) Complete the following chemical reactions and indicate the category of these reactions. Justify your answer. 10 marks (i) 2[Co(CN)₅]³⁻ + MeI ———→ (ii) [Ru(CO)₃(PPh₃)₂] + MeI ———→

हिंदी में प्रश्न पढ़ें

(a) S₂N₂, S₄N₂, S₁₁N₂, S₅N₆ और I₂Cl₆ की संरचनाओं को बनाइए। 10 अंक (b) सिलिकोन्स क्या हैं ? उनके कुछ उपयोगों का उल्लेख कीजिए। हेक्सामेथिलडाइसिलोक्सेन को आप कैसे निर्मित करेंगे ? इसकी संरचना बनाइए। यदि कुछ (CH₃)₃ SiCl को (CH₃)₂ SiCl₂ के साथ मिलाकर जल-अपघटन किया जाए तो क्या होता है ? 10 अंक (c) संयोजकता परिवर्तन की विधि से लैन्थेनाइड मिश्रण में से कितने लैन्थेनाइड आसानी से अलग किए जा सकते हैं ? अपने उत्तर की पुष्टि कीजिए। 10 अंक (d) निम्नलिखित लैन्थेनाइड आयनों का इलेक्ट्रॉनिक विन्यास (बाहरी) लिखिए और L-S युग्मक/युग्मन से चुंबकीय आघूर्ण का BM में परिकलन कीजिए। 10 अंक (i) Pr³⁺ (g = 4/5) (ii) Tb³⁺ (g = 3/2) (e) निम्नलिखित रासायनिक अभिक्रियाओं को पूरा कीजिए और इन अभिक्रियाओं के संवर्ग को निर्दिशित कीजिए। अपने उत्तर की पुष्टि कीजिए। 10 अंक (i) 2[Co(CN)₅]³⁻ + MeI ———→ (ii) [Ru(CO)₃(PPh₃)₂] + MeI ———→

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Approach

This question demands precise structural drawings and chemical reasoning across five sub-parts. Allocate approximately 20% time to part (a) for five accurate sulfur-nitrogen and halogen structures; 20% to part (b) covering silicone definition, uses, preparation, and co-hydrolysis; 15% to part (c) on lanthanide separation via valency change; 20% to part (d) for electronic configurations and magnetic moment calculations using L-S coupling; and 25% to part (e) for completing organometallic reactions with proper categorization. Begin with clear labeled diagrams, follow with systematic explanations, and conclude with justified reaction mechanisms.

Key points expected

  • Part (a): Correct planar square structure of S₂N₂ with alternating S-N bonds; S₄N₂ as six-membered ring with transannular S-S bond; S₁₁N₂ as two fused S₇ rings with N atoms; S₅N₆ with cage structure; I₂Cl₆ as planar bridged dimer with two bridging Cl atoms
  • Part (b): Definition of silicones as polymeric organosiloxanes; uses in sealants, lubricants, medical implants, and water repellents; preparation of hexamethyldisiloxane via hydrolysis of (CH₃)₃SiCl; linear structure with Si-O-Si bridge; co-hydrolysis yields cross-linked silicone polymers with controlled properties
  • Part (c): Identification of Ce, Eu, and Yb as separable lanthanides; justification based on stable +4 (Ce⁴⁺) and +2 (Eu²⁺, Yb²⁺) oxidation states enabling selective oxidation/reduction and precipitation/solubility differences
  • Part (d): Electronic configuration of Pr³⁺ as 4f² with ³H₄ term symbol; calculation of μ_eff = 3.58 BM using g=4/5; Tb³⁺ as 4f⁸ with ⁷F₆ term symbol; calculation of μ_eff = 9.72 BM using g=3/2 with proper J value
  • Part (e): Completion of [Co(CN)₅]³⁻ + MeI → [Co(CN)₅Me]³⁻ + I⁻ as SN2-type oxidative addition; [Ru(CO)₃(PPh₃)₂] + MeI → [Ru(CO)₃(PPh₃)₂MeI] as oxidative addition with 18-electron rule violation; proper categorization as organometallic oxidative addition reactions with electron count justification

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness20%10Demonstrates flawless understanding of sulfur-nitrogen cage chemistry, silicone polymerization, lanthanide oxidation state stability, L-S coupling theory, and organometallic reaction mechanisms; correctly identifies 18-electron rule applications and term symbolsShows basic understanding of most concepts but makes minor errors in oxidation state stability, electron counting, or term symbol assignments; some confusion between S₄N₄ and S₄N₂ structuresFundamental misconceptions about sulfur-nitrogen bonding, incorrect identification of separable lanthanides, wrong application of L-S coupling formula, or failure to recognize oxidative addition mechanisms
Mechanism / equation20%10Writes balanced equations for all reactions with proper stoichiometry; clearly explains hydrolysis mechanism of chlorosilanes; provides electron-pushing mechanism for oxidative addition showing SN2 character at cobalt and 16→18 electron change at rutheniumWrites most equations correctly but misses balancing details; provides generic description of hydrolysis without mechanistic steps; identifies reaction type without detailed electron flow explanationUnbalanced or incorrect equations; confuses hydrolysis with condensation; fails to identify oxidative addition or mislabels reaction categories; no mechanistic insight provided
Numerical accuracy20%10Calculates magnetic moments precisely using μ_eff = g√[J(J+1)] with correct J values (J=4 for Pr³⁺, J=6 for Tb³⁺); obtains 3.58 BM and 9.72 BM respectively; shows all intermediate steps in calculationUses correct formula but makes arithmetic errors or uses wrong J values; calculates spin-only moments incorrectly or confuses g-factor application; partial credit for correct formula identificationUses completely wrong formula (e.g., spin-only μ = √[n(n+2)] without justification); incorrect g-factor values; no calculation shown or absurd numerical results without units
Diagram / structure20%10Draws all five structures in part (a) with correct geometry, bond angles, and connectivity; shows planar S₂N₂ square, S₄N₂ boat/chair with transannular S-S, S₁₁N₂ fused rings, S₅N₆ cage, and bridged I₂Cl₆; hexamethyldisiloxane with proper bond angles and methyl group orientationDraws most structures with correct connectivity but errors in geometry (e.g., tetrahedral instead of planar for S₂N₂); missing one structure or incorrect ring size in sulfur-nitrogen compounds; adequate but not precise I₂Cl₆ representationMajor structural errors such as wrong ring sizes, incorrect atom connectivity, missing bridging atoms, or confusion between different sulfur-nitrogen compounds; no attempt at three-dimensional perspective where required
Application context20%10Cites Indian context: use of silicones in ISRO spacecraft sealing and thermal protection; lanthanide separation relevance to Indian Rare Earths Limited (IREL) processing of monazite sands from Kerala and Odisha; industrial significance of valency change method in Indian nuclear fuel reprocessingMentions generic industrial uses of silicones without Indian context; describes lanthanide separation without reference to Indian mineral resources; adequate but not specific application examplesNo mention of practical applications; completely misses industrial relevance; fails to connect coordination chemistry to catalysis or materials science applications relevant to India

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