Concept Explanation: Matching Transformations with Properties

This question involves matching chemical transformations (Column I) with their correct descriptions (Column II). We need to analyze each transformation for type of change, enthalpy change (ΔH), and entropy change (ΔS).

Step-by-Step Matching:

Step 1: Analyze Transformation (A) CO₂(s) → CO₂(g)

This is sublimation (solid to gas). It is a phase transition (p). Energy is absorbed (endothermic), so ΔH > 0 (r). Disorder increases (gas > solid), so ΔS > 0 (s).

Matches: (p), (r), (s)

Step 2: Analyze Transformation (B) CaCO₃(s) → CaO(s) + CO₂(g)

This is decomposition. It is not a phase transition or allotropic change. It is endothermic (requires heat), so ΔH > 0 (r). A solid produces a solid and a gas, so disorder increases, ΔS > 0 (s).

Matches: (r), (s)

Step 3: Analyze Transformation (C) 2H → H₂(g)

This represents formation of H₂ molecule from atoms. Bond formation releases energy, so exothermic: ΔH < 0 (not positive). Two free atoms form one molecule, freedom decreases, so ΔS < 0 (t).

Matches: (t)

Step 4: Analyze Transformation (D) P_{(white, solid)} → P_{(red, solid)}

This is a change between allotropes of phosphorus (different solid forms). It is an allotropic change (q). White phosphorus is less stable than red; conversion is exothermic (ΔH < 0, not positive). White P has higher disorder than red P, so ΔS < 0 (t).

Matches: (q), (t)

Final Answer:

(A) → (p), (r), (s); (B) → (r), (s); (C) → (t); (D) → (q), (t)

Related Topics:

• Phase Transitions: Changes between solid, liquid, gas states (e.g., melting, vaporization, sublimation).
• Allotropy: Existence of an element in multiple physical forms (e.g., carbon: diamond, graphite; phosphorus: white, red).
• Enthalpy Change (ΔH): Heat change at constant pressure; ΔH > 0 for endothermic, ΔH < 0 for exothermic processes.
• Entropy Change (ΔS): Measure of disorder; ΔS > 0 for increase in randomness, ΔS < 0 for decrease.

Key Formulae and Theory:

Gibbs Free Energy: $\Delta G=\Delta H-T\Delta S$

Spontaneity depends on ΔG < 0. For phase changes like sublimation (A), ΔG = 0 at equilibrium.

Entropy Sign: ΔS > 0 for processes increasing molecular freedom (e.g., solid→gas, dissolution, number of gas molecules increase).

Enthalpy Sign: ΔH > 0 for bond breaking (endothermic), ΔH < 0 for bond formation (exothermic).