Chemistry 2021 Paper I 50 marks Explain

Q8

(a) Justify that the interhalogen compound BrF₃ acts as an aprotic solvent and undergoes acid-base and neutralization reactions by giving examples. (10 marks) (b) The observed magnetic moments of lanthanide ions in general differ from observed magnetic moments of first row transition metal ions. Explain by giving reason(s). Identify the lanthanide ions having magnetic moments corresponding to spin-only value, and those which are diamagnetic. (20 marks) (c) In the coordination compound [Ru(PPh₃)₂Cl(NO)₂]⁺, one NO ligand bonds linearly while the other is bent. Explain the different modes of bonding of NO ligands in this molecule and expected M—N bond orders. (20 marks)

हिंदी में प्रश्न पढ़ें

(a) उदाहरण देते हुए उचित सिद्ध कीजिए कि अंतरहैलोजन यौगिक BrF₃, ऐप्रोटिक विलायक के रूप में क्रिया करता है और अम्ल-क्षारक तथा निष्प्रभावन/उदासीनिकरण अभिक्रियाओं को सहता है/से गुजरता है। (10 अंक) (b) लैन्थेनाइड आयनों के प्रेक्षित चुंबकीय आघूर्ण सामान्य तौर पर प्रथम पंक्ति संक्रमण धातु आयनों के प्रेक्षित चुंबकीय आघूर्ण से अलग हैं। कारण सहित इसकी व्याख्या कीजिए। ऐसे लैन्थेनाइड आयनों को पहचानिए, जिनके चुंबकीय आघूर्ण केवल प्रचक्रण के मान के अनुरूप हैं और जो प्रतिचुंबकीय हैं। (20 अंक) (c) उपसहसंयोजन यौगिक [Ru(PPh₃)₂Cl(NO)₂]⁺ में एक NO लिगैंड (संलगी) बंध रैखिकतः है जबकि दूसरा बंकित है। इस अणु में NO लिगैंड की अलग-अलग आबंधक विधा और अनुमानित M—N आबंध क्रमों की व्याख्या कीजिए। (20 अंक)

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Directive word: Explain

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How this answer will be evaluated

Approach

Begin with a brief introduction acknowledging the diverse nature of the three sub-parts covering interhalogen chemistry, lanthanide magnetism, and coordination chemistry. Allocate approximately 20% effort to part (a) on BrF₃ solvent chemistry, 40% to part (b) on lanthanide magnetic moments including calculations, and 40% to part (c) on NO bonding modes with structures. For each part, define key terms first, then provide explanations with equations or diagrams, and conclude with specific examples. Use chemical equations for acid-base reactions in (a), show μeff calculations for (b), and draw structures for (c).

Key points expected

  • Part (a): BrF₃ undergoes autoionization as 2BrF₃ ⇌ BrF₂⁺ + BrF₄⁻, establishing it as an aprotic ionizing solvent; identification of BrF₂⁺ as acid and BrF₄⁻ as base
  • Part (a): Specific acid-base reaction examples such as SbF₅ + BrF₃ → BrF₂⁺ + SbF₆⁻ (acid) and KF + BrF₃ → K⁺ + BrF₄⁻ (base), plus neutralization: BrF₂⁺ + BrF₄⁻ → 2BrF₃
  • Part (b): Explanation that lanthanide magnetic moments differ due to strong spin-orbit coupling (J states) versus quenched orbital contribution in first-row TM; μeff = gJ√[J(J+1)] for lanthanides vs spin-only μso = √[n(n+2)] BM for first-row TM
  • Part (b): Lanthanide ions with spin-only values: La³⁺ (4f⁰), Gd³⁺ (4f⁷, L=0, J=S=7/2), Lu³⁺ (4f¹⁴); diamagnetic ions: La³⁺ and Lu³⁺ (both μ = 0 BM)
  • Part (c): Linear NO bonding as NO⁺ (nitrosyl, 2-electron donor, triple bond character, M-N≡O ~180°) with M-N bond order ~2-3; bent NO as NO⁻ (nitrosyl anion, 1-electron donor, M-N=O ~120°) with M-N bond order ~1-2
  • Part (c): Application of Enemark-Feltham notation to [Ru(PPh₃)₂Cl(NO)₂]⁺: {Ru(NO⁺)(NO⁻)}⁺ or {Ru(NO)₂}⁷ configuration; electron counting shows one NO⁺ (linear) and one NO⁻ (bent) to satisfy 18-electron rule with Ru(II) or Ru(III) oxidation state analysis

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5Precisely defines autoionization of BrF₃, distinguishes aprotic vs protic solvents correctly; accurately explains spin-orbit coupling dominance in lanthanides versus crystal field quenching in first-row TM; correctly identifies NO⁺/NO⁻ formulations and their geometric consequences with proper oxidation state assignment for RuBasic understanding of solvent properties shown but confuses aprotic with protic characteristics; mentions spin-orbit coupling but doesn't contrast with first-row TM mechanism clearly; identifies linear/bent NO but with incorrect electron counting or oxidation stateFundamental errors such as calling BrF₃ protic, ignoring spin-orbit coupling entirely, or misassigning NO as neutral NO radical in both positions without distinction
Mechanism / equation20%10Writes balanced autoionization equation for BrF₃; provides clear acid-base reaction mechanisms with proper arrow pushing or ionic equations; shows μeff calculation formula with correct gJ factor for lanthanides; explains bent vs linear NO through π-acceptor strength and back-bonding differencesWrites some equations but misses autoionization; shows spin-only formula but not full J-state calculation; describes bonding modes without mechanistic detail on π-backbonding differencesMissing critical equations, unbalanced reactions, incorrect magnetic moment formulas, or no explanation of how NO geometry relates to metal d-orbital interactions
Numerical accuracy15%7.5Calculates or states exact μeff values: La³⁺ (0 BM), Gd³⁺ (~7.94 BM, matches spin-only), Lu³⁺ (0 BM); provides Landé g-factor expression; correctly counts electrons in [Ru(PPh₃)₂Cl(NO)₂]⁺ to verify 18-electron configurationStates approximate μeff values without calculation; mentions Gd³⁺ has spin-only value but doesn't explain why (L=0); attempts electron counting with minor errorsIncorrect μeff values, confuses spin-only with observed moments for wrong ions, or fails to perform any electron counting verification for the coordination compound
Diagram / structure20%10Draws clear T-shaped BrF₃ structure showing lone pairs; illustrates 4f orbital shielding for lanthanides; sketches distinct linear (Ru-N≡O, ~180°) and bent (Ru-N-O, ~120-140°) NO geometries with approximate bond angles labeled; shows orbital overlap diagrams for π-backbondingDescribes structures in words without diagrams, or draws basic shapes without angles; mentions shielding but no orbital diagram; distinguishes linear/bent without structural drawingsNo diagrams despite structural question; incorrect geometries (e.g., tetrahedral for BrF₃); fails to distinguish NO bonding modes visually or descriptively
Application context20%10Cites specific examples: BrF₃ used for fluorination of UO₂ to UF₆ in Indian nuclear fuel cycle; notes relevance of lanthanide magnetism in MRI contrast agents (Gd³⁺) and permanent magnets (Nd₂Fe₁₄B); connects NO linkage isomerism to biological NO signaling and metallodrug designMentions general applications without specific Indian or contemporary examples; notes nuclear industry for BrF₃ but without detail; aware of biomedical relevance but vagueNo real-world applications cited; purely theoretical treatment missing connection to nuclear chemistry, materials science, or medicinal inorganic chemistry

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