Chemistry 2022 Paper I 50 marks Explain

Q8

(a) Which one is more stable between the two isomers? Explain. [(H₃N)₅ Cr – CN – Cr(CN)₅] [(H₃N)₅ Cr – NC – Cr(CN)₅] 10 marks (b) Draw the active site structure of Hemocyanin (Hc) in deoxyhemocyanin and oxyhemocyanin forms, and write the colour of Hemocyanin in these two forms. Write the functions of Hemocyanin. 10 marks (c) Define stationary and non-stationary (branching) chain reaction. Non-stationary chain reactions always lead to explosion under certain conditions. Give a detailed account of these conditions. 10 marks (d) What is meant by steady-state approximation? How this approximation helps in deriving the kinetics of following photochemical reaction? H₂(g) + Cl₂(g) → 2HCl(g) The quantum yield of this reaction is extremely large. Justify or criticize this statement. 10 marks (e) Derive Langmuir adsorption isotherm. How does Langmuir adsorption isotherm help in elucidation of kinetics of a gaseous reaction on solid surface? 10 marks

हिंदी में प्रश्न पढ़ें

(a) दो समावयवों में से कौन-सा ज्यादा स्थायी है? व्याख्या कीजिए। [(H₃N)₅ Cr – CN – Cr(CN)₅] [(H₃N)₅ Cr – NC – Cr(CN)₅] 10 (b) डीऑक्सीहेमोसायनिन और ऑक्सीहेमोसायनिन रूपों में हेमोसायनिन के सक्रिय स्थल संरचना को खींचे और दोनों रूपों में हेमोसायनिन के रंग को लिखे। हेमोसायनिन के प्रकार्य को लिखे। 10 (c) स्थिर और गैर-स्थिर (शाखन) श्रृंखला अभिक्रियाओं की परिभाषा दें। गैर-स्थिर श्रृंखला अभिक्रिया कुछ शर्तों के अधीन हमेशा विस्फोट की ओर अग्रसर होता है। इन शर्तों का विस्तार पूर्वक स्पष्टीकरण दें। 10 (d) स्थिर-अवस्था सन्निकटन से क्या अभिप्राय है? निम्नलिखित प्रकाशरासायनिक अभिक्रिया की बलगतिकी को व्युत्पन्न करने में यह सन्निकटन कैसे सहायक है? H₂(g) + Cl₂(g) → 2HCl(g) इस अभिक्रिया की कांतम लंबी अत्यधिक बड़ी है। इस कथन को उचित सिद्ध करें या आलोचित (आलोचना) करें। 10 (e) लैंगम्युर अधिशोषण समतापी वक्र को व्युत्पन्न करें। लैंगम्युर अधिशोषण समतापी वक्र, गैसीय अभिक्रिया की बलगतिकी का ठोस पृष्ठीय/धरातल पर स्पष्टीकरण करने में कैसे सहायक है? 10

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Directive word: Explain

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How this answer will be evaluated

Approach

Begin with a brief introduction linking bioinorganic and surface chemistry themes. For part (a), explain the ambidentate nature of cyanide and apply HSAB principle to justify Cr-CN-Cr linkage stability. For (b), draw accurate active site structures showing μ-η²:η² peroxo dicopper(II) in oxyhemocyanin and colorless deoxy form with Cu(I), noting color changes. For (c), define both chain types with examples like H₂-O₂ explosion, explaining branching ratio and critical explosion conditions. For (d), state steady-state approximation, derive H₂+Cl₂ kinetics showing chain propagation, and justify high quantum yield via chain mechanism. For (e), derive Langmuir isotherm from rate considerations, then apply to unimolecular/bimolecular surface reactions. Allocate approximately 2-2.5 minutes per mark, with proportional time for each 10-mark sub-part.

Key points expected

  • Part (a): Cyanide is ambidentate; Cr(III) is borderline acid preferring C-end (soft) over N-end (hard); first isomer [(H₃N)₅Cr-CN-Cr(CN)₅] is more stable due to better HSAB match and reduced steric repulsion
  • Part (b): Deoxyhemocyanin has two Cu(I) centers (colorless, 3-coordinate); oxyhemocyanin has μ-η²:η² peroxo dicopper(II) (blue, 4-5 coordinate); functions include oxygen transport in arthropods and molluscs, not hemoglobin
  • Part (c): Stationary chain: chain carriers remain constant (rate of initiation = termination); non-stationary/branching chain: carriers multiply (α > 1, branching factor); explosion when chain branching exceeds termination, depends on pressure limits (lower and upper explosion limits)
  • Part (d): Steady-state approximation: d[intermediate]/dt = 0; for H₂+Cl₂, derive rate = k[H₂][Cl₂]¹/² using this approximation; quantum yield ~10⁶ justified by long chain length, not criticized as this is characteristic of chain photoreactions
  • Part (e): Langmuir isotherm derivation from adsorption equilibrium: θ = KP/(1+KP) or equivalent; application to surface kinetics: unimolecular (rate ∝ θ) and bimolecular (rate ∝ θ_Aθ_B or θ_Aθ_vacant) reactions, leading to Langmuir-Hinshelwood and Eley-Rideal mechanisms

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5Demonstrates precise understanding of HSAB principle for (a), copper oxidation states and coordination geometry for (b), branching factor and explosion limits for (c), steady-state mathematical validity for (d), and monolayer assumption for (e); no conceptual errors across any sub-partShows basic understanding of most concepts but misapplies HSAB vaguely, confuses Cu oxidation states in hemocyanin, or presents incomplete definitions of chain reactions; minor errors in steady-state applicationFundamental misconceptions such as calling cyanide unidentate, assigning Fe instead of Cu in hemocyanin, or confusing stationary with non-stationary chain reactions; fails to grasp Langmuir assumptions
Mechanism / equation25%12.5Presents complete stepwise mechanisms: initiation, propagation, termination for H₂+Cl₂ with proper radical intermediates; derives Langmuir isotherm from kinetic rate laws; clearly distinguishes chain branching steps; all equations balanced with proper stoichiometryWrites most key equations but misses termination steps or branching reactions; derivation of Langmuir isotherm present but skips key assumptions; mechanisms present but incompleteMissing essential equations; no derivation attempted; confused mechanisms with incorrect intermediates; fails to show chain nature of photochemical reaction
Numerical accuracy10%5Correctly cites typical quantum yield values (~10⁶) for H₂+Cl₂; provides accurate explosion limit pressures or branching ratios where relevant; mathematical derivation of isotherm algebraically correctMentions quantum yield is large without specific value; order-of-magnitude estimates acceptable; minor algebraic slips in derivation that don't affect final resultIncorrect numerical values; serious algebraic errors in derivation; no quantitative treatment where expected; confuses orders of magnitude
Diagram / structure25%12.5Clear, labeled diagrams: both isomers showing Cr-CN vs Cr-NC connectivity; accurate μ-η²:η² peroxo dicopper core with Cu-Cu distance ~3.6 Å and O-O bond; proper Langmuir isotherm plot showing saturation; neat structural representations with correct geometryDiagrams present but lacking labels or detail; hemocyanin structures show correct connectivity but poor geometry; isotherm sketch shows trend but not labeled axes; acceptable but not preciseMissing essential diagrams; incorrect connectivity in isomers; wrong metal center or ligand arrangement in hemocyanin; no graphical representation of isotherm; messy or misleading structures
Application context15%7.5Connects to real-world significance: industrial HCl synthesis via Deacon process, respiratory proteins in Indian biodiversity (horseshoe crabs, scorpions), explosion safety in chemical industry, heterogeneous catalysis in Indian petrochemical sector; demonstrates interdisciplinary awarenessBrief mention of applications without elaboration; standard examples without Indian context; functional descriptions without connecting to broader significanceNo application context provided; fails to mention biological role of hemocyanin or industrial importance of surface catalysis; purely theoretical treatment

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