Chemistry 2024 Paper I 50 marks Explain

Q8

(a) 'All the F atoms appear indistinguishable in the ¹⁹F NMR spectrum of sp³d hybridized PF₅ molecule.' Explain how. (10 marks) (b) (i) Draw all the possible structural dispositions of ClF₃ molecule. Establish logically which will be the most favoured disposition. Comment on the shape of ClF₃ molecule. (10 marks) (ii) Draw the structure of B₂H₆. Explain the bonding in B₂H₆ on the basis of hybridization approach. (10 marks) (c) Briefly explain the principles involved in the following methods of separation of the lanthanides : (20 marks) (i) Repeated fractional crystallisation (ii) Solvent extraction (iii) Fractional precipitation (iv) Change of oxidation state

हिंदी में प्रश्न पढ़ें

(a) 'sp³d संकरित PF₅ अणु के ¹⁹F NMR स्पेक्ट्रम में सभी F परमाणु अविभेद्य दिखाई पड़ते हैं ।' व्याख्या कीजिए कि ऐसा क्यों है । (10 अंक) (b) (i) ClF₃ अणु की सभी संभव संरचनात्मक स्थितियों को बनाइए । तर्कसंगत प्रमाणित कीजिए कि इनमें से कौन-सी सबसे अनुकूल/स्वीकारात्मक स्थिति होगी । ClF₃ अणु के आकार पर टिप्पणी कीजिए । (10 अंक) (ii) B₂H₆ की संरचना को बनाइए । संकरण दृष्टिकोण के आधार पर B₂H₆ में आबंधन की व्याख्या कीजिए । (10 अंक) (c) निम्नलिखित दी गई लैन्थेनाइडों के पृथक्कन की विधियों में लगने वाले नियमों की संक्षिप्त व्याख्या कीजिए : (20 अंक) (i) बारंबार प्रभाजी क्रिस्टलन (ii) विलायक निष्कर्षण (iii) प्रभाजी अवक्षेपण (iv) ऑक्सीकरण अवस्था का बदलना

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Directive word: Explain

This question asks you to explain. The directive word signals the depth of analysis expected, the structure of your answer, and the weight of evidence you must bring.

See our UPSC directive words guide for a full breakdown of how to respond to each command word.

How this answer will be evaluated

Approach

The directive 'explain' demands clear reasoning with cause-effect relationships across all sub-parts. Allocate approximately 20% time to part (a) on PF₅ fluxional behavior, 40% to part (b) covering ClF₃ VSEPR dispositions and B₂H₆ 3c-2e bonding, and 40% to part (c) on four lanthanide separation methods. Structure with brief introductions for each part, detailed explanatory body with diagrams, and concluding remarks on significance.

Key points expected

  • Part (a): Berry pseudorotation mechanism in PF₅ showing rapid interconversion of axial and equatorial fluorines making all ¹⁹F NMR equivalent; mention of low temperature coalescence
  • Part (b)(i): Three possible T-shaped dispositions of ClF₃ (lone pairs in equatorial positions); VSEPR-based logical elimination to find most stable arrangement with minimal lone pair-bond pair repulsion
  • Part (b)(ii): Diborane structure with two bridging hydrogens; sp³ hybridization of boron and formation of 3-center-2-electron B-H-B bonds distinct from conventional 2c-2e bonds
  • Part (c)(i): Fractional crystallization based on slight solubility differences of double salts (e.g., magnesium ammonium nitrates of Ce group vs Y group lanthanides)
  • Part (c)(ii): Solvent extraction using TBP (tributyl phosphate) or D2EHPA in Indian rare earth plants at Udyogamandal/Aluva; distribution coefficient differences based on ionic radii
  • Part (c)(iii): Fractional precipitation using oxalates/hydroxides with controlled pH; basicity differences across lanthanide series
  • Part (c)(iv): Oxidation state change exploiting Ce⁴⁺/Ce³⁺ and Eu²⁺/Eu³⁺ stability for selective separation; mention of Indian monazite sand processing

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5Demonstrates accurate understanding of fluxional molecules, VSEPR theory with lone pair repulsion hierarchy, 3c-2e bonding uniqueness, and lanthanide contraction effects on separation; correctly identifies why T-shape is favored for ClF₃ and why Ce/Eu oxidation state changes are exploitableShows basic familiarity with concepts but confuses axial/equatorial positions, misapplies VSEPR to ClF₃ disposition ranking, or presents generic lanthanide separation without specific chemical principlesFundamental errors such as claiming PF₅ has static structure, drawing incorrect Lewis structures, stating B₂H₆ has conventional B-B bond, or confusing fractional crystallization with precipitation
Mechanism / equation20%10Clearly articulates Berry pseudorotation mechanism with 120° and 90° rotation steps; explains 3c-2e bond formation through orbital overlap diagrams; writes distribution ratio equations for solvent extraction and redox equations for Ce⁴⁺/Eu²⁺ separationMentions mechanisms superficially without stepwise clarity; presents equations without proper conditions or states; describes 3c-2e bonding vaguely without orbital involvementOmits mechanisms entirely or presents chemically impossible steps; fails to provide any equations for redox or extraction processes; confuses mechanisms between different methods
Numerical accuracy10%5Provides correct bond angles (90°, 120°, 180° in ClF₃), accurate coordination numbers, and precise oxidation states; cites realistic separation factors or distribution coefficients where relevant; correct electron counts in molecular orbital descriptionsApproximate angles or missing precise values; minor errors in oxidation state assignments; vague references to 'small differences' without quantitative senseIncorrect bond angles, wrong oxidation states (e.g., B in B₂H₆), or invented numerical values; serious errors in electron counting for hybridization schemes
Diagram / structure25%12.5Clear trigonal bipyramidal PF₅ with labeled axial/equatorial positions and fluxional arrows; all three ClF₃ dispositions drawn with lone pairs explicitly shown; accurate B₂H₆ with dashed lines for bridging hydrogens and distinct 2c-2e vs 3c-2e bonds; flow diagram for lanthanide separation methodsDiagrams present but missing key labels or unclear distinction between bond types; only one or two ClF₃ dispositions shown; generic block diagrams for separation without chemical detailAbsent or seriously flawed diagrams; incorrect molecular geometries (e.g., planar PF₅, linear ClF₃); no attempt to illustrate 3c-2e bonding; diagrams contradict text description
Application context20%10References Indian rare earth industry (IREL, Kerala monazite sands), mentions specific extractants (TBP, D2EHPA) used in Indian plants, connects lanthanide separation to strategic materials for electronics/magnets; explains why fluxional behavior matters for spectroscopic timescalesGeneric mention of industrial importance without Indian context; superficial connection between theory and application; no mention of why certain separation methods are preferred for specific lanthanidesNo application context provided; fails to explain practical significance of any concept; irrelevant or fabricated industrial references

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