(a) Indicating 'Fluorescence' and 'Phosphorescence' processes in Jablonski diagram, distinguish between both processes. (10 marks)

(b) (i) How does water condense onto glass? (5 marks)
(ii) How long will it take for about one-half of a monolayer to

Question

(a) Indicating 'Fluorescence' and 'Phosphorescence' processes in Jablonski diagram, distinguish between both processes. (10 marks)

(b) (i) How does water condense onto glass? (5 marks)
(ii) How long will it take for about one-half of a monolayer to form on the surface if the surface is exposed to a pressure of 2·0 × 10⁻¹¹ torr? Assume that a monolayer formation needs an exposure of 1 Langmuir. Also give reason for using such low pressure in cleaning the surface. (5 marks)

(c) Write the IUPAC nomenclature of the following complexes: (10 marks)
(i) Na₂[ZnCl₄]
(ii) [PtCl₆]²⁻
(iii) [Pt(py)₄] [PtCl₄]
(iv) [(H₃N)₄Co Co(NH₃)₄]⁴⁺
        H₂N
           \
            O
           /
        H
(v) Ni(CO)₄

(d) What is Zeise's salt? Outline its synthesis method. Describe its structure. (10 marks)

(e) (i) Explain lanthanide contraction. (5 marks)
(ii) Hf is placed below Zr in the periodic table. Justify the statement. (5 marks)

UPSC Answer Check · Accepted Answer

The directive 'distinguish' in part (a) demands clear differentiation between fluorescence and phosphorescence using the Jablonski diagram as the primary tool. Allocate approximately 20% time to (a) focusing on the diagram and tabular distinction, 20% to (b) covering condensation mechanism and Langmuir calculation, 20% to (c) for systematic IUPAC naming of all five complexes, 20% to (d) for Zeise's salt synthesis and structure, and 20% to (e) for lanthanide contraction explanation and Hf-Zr justification. Structure each part with definition → mechanism/process → specific details → concluding significance.
- Part (a): Jablonski diagram showing S₀, S₁, T₁ states with radiative transitions; distinction table covering spin-allowed vs spin-forbidden, lifetime (10⁻⁸-10⁻⁴ s vs 10⁻⁴-10² s), and temperature dependence
- Part (b)(i): Water condensation via hydrogen bonding, surface silanol groups, and thin film formation; (b)(ii) Calculation: 1 Langmuir = 10⁻⁶ torr·s, so time = 0.5 × 10⁻⁶ / (2×10⁻¹¹) = 2.5×10⁴ s ≈ 7 hours; ultra-high vacuum prevents contamination
- Part (c): Correct IUPAC names—sodium tetrachloridozincate(II), hexachloridoplatinate(IV), tetrapyridineplatinum(II) tetrachloridoplatinate(II), μ-amido-μ-hydroxido-bis[tetraamminecobalt(III)] or μ-amido-μ-hydroxido-bis[tetraamminecobalt(III)] with bridging ligands, tetracarbonylnickel(0)
- Part (d): Zeise's salt K[PtCl₃(C₂H₄)]·H₂O; synthesis from K₂[PtCl₄] + C₂H₄; structure showing η²-ethylene coordination, Dewar-Chatt-Duncanson model, square planar Pt(II) with back-bonding
- Part (e)(i): Lanthanide contraction—poor shielding by 4f electrons causing 15% atomic radius decrease from La to Lu; (e)(ii) Hf-Zr similarity due to lanthanide contraction offsetting expected size increase, nearly identical radii (Hf 159 pm, Zr 160 pm) and properties

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	25%	12.5	Precise definitions across all parts: singlet/triplet states in (a), physisorption/chemisorption in (b), oxidation states and coordination numbers in (c), η²-coordination in (d), and effective nuclear charge explanation in (e); no conceptual errors	Generally correct definitions with minor inaccuracies; e.g., confuses fluorescence/phosphorescence lifetime order, or states lanthanide contraction without explaining 4f shielding mechanism	Fundamental misconceptions; e.g., claims both processes involve same spin states, or states Hf is larger than Zr contradicting periodic trends
Mechanism / equation	20%	10	Clear mechanistic explanations: ISC and phosphorescence pathway in (a), hydrogen bonding mechanism in (b)(i), stepwise synthesis of Zeise's salt in (d), and nuclear charge accumulation across lanthanide series in (e)	Partial mechanisms described; mentions intersystem crossing without explaining spin-orbit coupling, or gives synthesis steps without conditions	Missing mechanisms entirely; no explanation of how ethylene coordinates to Pt or why lanthanide contraction occurs
Numerical accuracy	15%	7.5	Correct calculation in (b)(ii): recognizes 0.5 monolayer needs 0.5 Langmuir, converts units properly (1 L = 10⁻⁶ torr·s), obtains t = 2.5×10⁴ s or ~7 hours with proper reasoning for UHV use	Correct formula but arithmetic error; or correct final answer with wrong units or incomplete reasoning for low pressure necessity	Incorrect formula application; uses 1 Langmuir directly without 0.5 factor, or order-of-magnitude error in time calculation
Diagram / structure	25%	12.5	Accurate Jablonski diagram in (a) with labeled S₀, S₁, S₂, T₁ and vertical arrows for absorption/fluorescence/phosphorescence; clear structure of Zeise's salt anion showing Pt-C₂H₄ interaction with Dewar-Chatt-Duncanson bonding description in (d)	Diagram present but missing labels or incorrect transition arrows; structure drawn but missing back-bonding explanation or incorrect geometry	No diagram in (a) despite explicit requirement; or diagram shows forbidden transitions incorrectly; no attempt at Zeise's salt structure
Application context	15%	7.5	Connects concepts to practical relevance: fluorescence/phosphorescence in OLEDs and bioimaging; UHV in surface science/semiconductor fabrication; lanthanide contraction in Zr-Hf separation difficulty and TiO₂ photocatalysis applications	Mentions one or two applications superficially; e.g., states phosphorescence is used in glow-in-the-dark materials without elaboration	No application context provided; treats all parts as purely theoretical without real-world significance

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Directive word: Distinguish

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