(a) Determine the electronic and molecular geometry of [BrF₅] and [ICl₂]⁻ interhalogen compounds. 10

(b) Answer the following questions based on the phase diagram given below :
(i) How many components does this phase diagram represent?
(ii) Identify

Question

(a) Determine the electronic and molecular geometry of [BrF₅] and [ICl₂]⁻ interhalogen compounds. 10

(b) Answer the following questions based on the phase diagram given below :
(i) How many components does this phase diagram represent?
(ii) Identify the points A to D with corresponding degrees of freedom.
(iii) Explain the changes expected in the paths E → B → H and E → F → G → H. 20

(c) Consider the reaction H₂(g) + ½O₂(g) → H₂O(l), which occurs in a H₂–O₂ fuel cell.
(i) Identify the elements that undergo oxidation and reduction.
(ii) Calculate the standard reaction Gibbs free energy (ΔᵣG°) at 25 °C.
(iii) Write down the two reduction half-reactions for the cell.
(iv) Calculate the E_cell.
Given that
Δ_fH°(H₂O, l) = –285.83 kJ mol⁻¹
S°_m(H₂O, l) = 69.91 J K⁻¹ mol⁻¹
S°_m(H₂, g) = 130.68 J K⁻¹ mol⁻¹
S°_m(O₂, g) = 205.14 J K⁻¹ mol⁻¹ 20

UPSC Answer Check · Accepted Answer

Calculate and derive the required parameters across all three parts, allocating approximately 20% time to part (a) for VSEPR analysis, 40% to part (b) for phase diagram interpretation with clear identification of invariant points and path descriptions, and 40% to part (c) for thermodynamic calculations and electrochemical equations. Structure the answer with clear headings for each sub-part, showing step-by-step working for all numerical derivations, and conclude with brief contextual significance where applicable.
- For (a): Apply VSEPR theory to determine BrF₅ has square pyramidal molecular geometry (AX₅E) with octahedral electronic geometry, and [ICl₂]⁻ has linear molecular geometry (AX₂E₃) with trigonal bipyramidal electronic geometry; include lone pair counts and hybridization (sp³d² and sp³d respectively)
- For (b)(i): Identify the phase diagram represents a one-component system (typically water or similar single component)
- For (b)(ii): Correctly identify points A-D as triple point, critical point, normal boiling point, normal freezing point (or equivalent) with corresponding degrees of freedom F = 0, 0, 1, 1 using Gibbs phase rule
- For (b)(iii): Explain E→B→H as heating at constant pressure through liquid-vapor equilibrium to superheated vapor, and E→F→G→H as compression at constant temperature showing phase transitions from gas to liquid to solid
- For (c)(i): Identify H₂ undergoes oxidation (0 to +1) and O₂ undergoes reduction (0 to -2)
- For (c)(ii)-(iv): Calculate ΔᵣS° = -163.17 J K⁻¹ mol⁻¹, ΔᵣG° = -237.13 kJ mol⁻¹, write correct half-reactions (cathode: O₂ + 4H⁺ + 4e⁻ → 2H₂O; anode: 2H₂ → 4H⁺ + 4e⁻), and E°cell = 1.23 V using ΔG° = -nFE°

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	20%	10	Demonstrates flawless application of VSEPR theory for both interhalogen compounds with correct electron pair geometry, molecular geometry, and lone pair positioning; correctly applies Gibbs phase rule (F = C - P + 2) for all points in part (b); accurately identifies oxidation states and spontaneity conditions in fuel cell thermodynamics	Shows basic understanding of VSEPR with minor errors in geometry assignment; applies phase rule with occasional miscalculation of degrees of freedom; identifies oxidation/reduction correctly but may confuse sign conventions in thermodynamics	Confuses electronic and molecular geometries; fails to apply phase rule correctly; makes fundamental errors in identifying oxidation/reduction or misunderstands spontaneity criteria
Mechanism / equation	20%	10	Writes balanced half-reactions with correct stoichiometry, phase labels, and electron transfer; clearly shows the relationship between ΔG°, ΔH°, and ΔS° through proper equations; explicitly states Gibbs phase rule and its application to each invariant point	Writes correct half-reactions but may miss phase labels or balancing; shows thermodynamic equations with minor errors in unit handling; states phase rule but applies inconsistently	Unbalanced or incorrect half-reactions; omits key thermodynamic relationships; fails to state or apply governing equations
Numerical accuracy	25%	12.5	Computes ΔᵣS° = -163.17 J K⁻¹ mol⁻¹, ΔᵣG° = -237.13 kJ mol⁻¹ (or -237.15 kJ mol⁻¹), and E°cell = 1.23 V with correct significant figures, unit conversions, and algebraic manipulation; all intermediate steps shown with proper sign conventions	Correct methodology but arithmetic errors leading to slightly incorrect final values; unit conversion errors (kJ vs J) or sign errors in ΔG° calculation; correct final E°cell but incorrect intermediate values	Major calculation errors, incorrect formula application (e.g., using ΔH° directly for ΔG°), or missing numerical answers; fails to convert units appropriately
Diagram / structure	20%	10	Draws clear 3D representations of BrF₅ (square pyramidal with axial lone pair) and [ICl₂]⁻ (linear with three equatorial lone pairs) showing bond angles and lone pair positions; accurately sketches/references phase diagram with all labeled points (A-D) and paths clearly traced; includes labeled axes and phase regions	Basic 2D representations of molecular geometries without clear 3D perspective; phase diagram description without clear visualization or missing some labeled points; paths described but not clearly illustrated	Absent or incorrect structural drawings; no phase diagram reference or completely misidentified points; fails to visualize molecular geometries or phase transitions
Application context	15%	7.5	Connects interhalogen compounds to their use as fluorinating agents in organic synthesis and rocket propellants; relates phase diagram to industrial processes like freeze-drying or metallurgical phase transformations; discusses H₂-O₂ fuel cell significance for clean energy, ISRO's use in space missions, and comparison with alkaline fuel cells in Indian context	Brief mention of practical applications without elaboration; generic statements about fuel cells for energy; limited connection to real-world examples	No application context provided; purely theoretical treatment without relevance to industrial, environmental, or technological significance

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