Chemistry 2022 Paper I 50 marks Explain

Q3

(a) Identify the least stable ion of the following ions and justify your answer. OCN⁻ ONC⁻ SCN⁻ (10 marks) (b) A 74·6 g ice cube floats in the sea. The temperature and pressure of the system and surroundings are 0°C and 1 atm. Calculate ΔS_syst, ΔS_surr and ΔS_univ for the melting of ice cube. What can you conclude about the nature of process from the value of ΔS_univ ? (The molar heat of fusion of water is 6·01 kJ/mol) (10 marks) (c) The graph above shows the distribution of molecular speeds for Argon and Helium at the same temperature. (i) Which curve, 1 or 2 better represents the behavior of Argon ? (ii) Which curve represents the gas that effuses more slowly ? (iii) Which curve more closely represents the behavior of fluorine gas ? Explain. (10 marks) (d) Construct a phase-diagram for a one component system (water) and explain all the three curves. Also describe the significance of critical pressure, critical temperature and triple point. (10 marks) (e) Provide briefly a qualitative account of different forces which influence the speed of an ion in solution of strong electrolyte moving under an externally applied electric field. (10 marks)

हिंदी में प्रश्न पढ़ें

(a) निम्नलिखित आयनों में से कम-से-कम (अल्पतम) स्थायी आयन को पहचानें और अपने उत्तर को उचित सिद्ध करें । OCN⁻ ONC⁻ SCN⁻ (10 अंक) (b) 74·6 g का बर्फ का एक क्यूब समुद्र में तैर रहा है । तंत्र (system) और परिवेश (surroundings) का तापमान और दबाब 0°C और 1 atm है । बर्फ के क्यूब के पिघलने पर ΔS_syst, ΔS_surr और ΔS_univ का परिकलन कीजिए । ΔS_univ के मूल्य से आप इस प्रक्रम के स्वभाव के बारे में क्या निष्कर्ष निकालते हैं ? (जल की ग्रामाणुक संगलन ऊष्मा का मूल्य 6·01 kJ/mol है ।) (10 अंक) (c) नीचे दिए गए ग्राफ में आर्गन और हीलियम का समान तापमान पर आण्विक चाल का वितरण दिया गया है : (i) कौन-सी वक्र रेखा, 1 या 2 आर्गन के बेहतर आचरण का निरूपण करता है ? (ii) कौन-सी वक्र रेखा, गैस के धीरे निस्सरण का निरूपण करता है ? (iii) कौन-सी वक्र रेखा, फ्लुओरिन गैस के आचरण को ज्यादा निकट से निरूपण करता है ? व्याख्या कीजिए । (10 अंक) (d) एक घटक तंत्र (जल) के प्रावस्था आरेख का निर्माण कीजिए और तीनों वक्र रेखाओं की व्याख्या कीजिए । क्रांतिक दाब, क्रांतिक ताप और त्रिक बिंदु की सार्थकता/महत्व की भी व्याख्या कीजिए । (10 अंक) (e) बाहत: अनुप्रयुक्त विद्युत क्षेत्र के अधीन गति कर रहे प्रबल वैद्युत अपघट्य के विलयन में आयन की चाल को प्रभावित करने वाले अलग-अलग बलों का गुणात्मक स्पष्टीकरण करते हुए संक्षिप्त विवरण दें । (10 अंक)

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Directive word: Explain

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How this answer will be evaluated

Approach

Explain the stability trends, entropy calculations, kinetic theory applications, phase diagram construction, and ionic mobility factors across all five parts. Allocate approximately 15-18 minutes each for parts (a), (b), and (d) due to their conceptual depth and diagrammatic requirements; 10-12 minutes each for parts (c) and (e). Begin with a brief introduction acknowledging the interconnected themes of thermodynamics and bonding, then address each part sequentially with clear sub-headings, and conclude with a synthesis of how molecular-level understanding enables prediction of macroscopic behavior.

Key points expected

For (a): Identify ONC⁻ as least stable based on formal charges, electronegativity placement, and resonance structures; compare with OCN⁻ (cyanate) and SCN⁻ (thiocyanate) stability
For (b): Calculate moles of ice (4.14 mol), ΔS_syst = +22.0 J/K, ΔS_surr = -22.0 J/K, ΔS_univ = 0 for reversible process at equilibrium; conclude process is reversible/equilibrium
For (c)(i)-(iii): Curve 1 = Ar (heavier, slower), Curve 2 = He; slower effusion = Curve 1; F₂ matches Curve 1 due to comparable molar mass (~38 vs 40 g/mol)
For (d): Sketch water's phase diagram with fusion curve (negative slope), vaporization curve, sublimation curve; mark triple point (0.01°C, 0.006 atm) and critical point (374°C, 218 atm)
For (e): Describe interplay of ionic atmosphere (relaxation effect), electrophoretic effect, solvation/hydration shell, interionic attractions, and viscous drag on ionic mobility

Evaluation rubric

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	25%	12.5	Demonstrates flawless conceptual grasp: correctly identifies ONC⁻ instability via formal charge analysis (N with +2), recognizes reversible melting at 0°C gives ΔS_univ = 0, applies Graham's law reasoning for effusion, understands negative Clapeyron slope for water's fusion curve, and comprehensively explains Debye-Hückel-Onsager factors for ionic mobility	Shows reasonable understanding with minor errors: correctly identifies least stable ion but weak justification, calculates entropy values with sign errors, confuses which curve represents which gas, draws approximate phase diagram with misplaced critical point, lists some mobility factors but misses relaxation/electrophoretic distinction	Fundamental misconceptions: selects wrong ion as least stable, treats melting as spontaneous (ΔS_univ > 0), reverses effusion speed relationship, draws incorrect phase diagram slopes, or describes ionic speed without reference to interionic forces
Mechanism / equation	20%	10	Precisely applies all relevant equations: resonance structures with formal charges, ΔS = q_rev/T, ΔS_univ = ΔS_syst + ΔS_surr, Maxwell-Boltzmann distribution principles, Clapeyron and Clausius-Clapeyron equations for phase boundaries, and Debye-Hückel-Onsager equation for conductivity	Uses most equations correctly with minor errors: correct entropy formulas but inconsistent units (J vs kJ), states Graham's law without derivation, mentions but doesn't apply Clapeyron equation, describes ionic atmosphere qualitatively without Onsager equation	Missing or incorrect equations: no formal charge calculations, wrong entropy formula (uses ΔH/T for surroundings incorrectly), confuses effusion with diffusion rates, no thermodynamic basis for phase diagram curves, no mention of ionic atmosphere effects
Numerical accuracy	15%	7.5	All calculations precise: moles ice = 74.6/18.015 = 4.141 mol, ΔS_syst = 6010×4.141/273 = +91.2 J/K (or per mole basis clearly stated), ΔS_surr = -91.2 J/K, ΔS_univ = 0; proper unit handling throughout; correct molar mass comparisons for gases	Generally correct with minor slips: rounding errors in moles, sign convention confusion, unit mixing (J vs kJ), arithmetic errors in final values but correct methodology	Serious numerical errors: wrong molar mass for water, incorrect temperature in Kelvin, magnitude errors >50%, no calculation shown for entropy values, or completely omitted numerical work where required
Diagram / structure	20%	10	Clear, labeled diagrams: Maxwell-Boltzmann distribution showing two curves with Ar peak lower and right-shifted vs He; water phase diagram with all three curves, triple point, critical point, and negative fusion slope explicitly marked; resonance structures for all three ions with formal charges	Adequate diagrams with minor flaws: roughly correct Maxwell-Boltzmann shapes but unlabeled axes, phase diagram with correct general shape but missing labels or misplaced special points, some resonance structures shown	Poor or missing diagrams: no phase diagram, no distribution curves described, no structural representations of ions, or diagrams that contradict verbal explanation
Application context	20%	10	Connects theory to real-world phenomena: discusses why OCN⁻/SCN⁻ are common ligands in coordination chemistry (Indian context: detection in food adulteration), explains significance of ΔS_univ = 0 for equilibrium in natural systems, relates effusion to uranium enrichment or industrial gas separation, describes critical point applications in supercritical fluid extraction (coffee decaffeination in India), and explains conductivity measurements for water quality monitoring	Some contextual links made: mentions ligand chemistry briefly, notes that ΔS_univ = 0 indicates equilibrium, states Graham's law applications without specifics, describes triple point for temperature standardization, mentions conductivity for electrolyte strength	No application context: purely theoretical treatment, no mention of practical significance for any part, or irrelevant examples that demonstrate misunderstanding

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