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Solved 2025: FYUG Even Semester Exam, 2025

CHEMISTRY (4th Semester) Course No.: CHMDSM-252 Time: 3 Hours | Full Marks: 70 | Pass Marks: 28

UNIT-I

1. (a) What is inorganic benzene? Write its structure. 2

Borazine (B3N3H6) is known as inorganic benzene because its structure is isoelectronic and isostructural with benzene

. It consists of a hexagonal ring of alternating Boron and Nitrogen atoms with one Hydrogen atom attached to each.

1. (b) Discuss briefly the catenation property of carbon. 2

Catenation is the ability of an element to form covalent bonds with other atoms of the same element, resulting in long chains or rings

. Carbon exhibits the highest catenation property due to its small size and high C-C bond energy, allowing for the vast diversity of organic compounds.

1. (c) Write two structural differences between diamond and graphite. 2

Feature Diamond Graphite Hybridization sp³ sp² Structure 3D tetrahedral network 2D hexagonal layered sheets

2. (a) What happens when... 10

(i) Diborane is hydrolyzed: It reacts with water to form Boric acid and Hydrogen gas

.
B2H6 + 6H2O → 2H3BO3 + 6H2

(ii) Diborane is treated with NH3:

  • At low temperatures: Forms an unsymmetrical adduct [H2B(NH3)2]+ [BH4]-.
  • On heating (1:2 ratio): Forms Borazine (B3N3H6).

    • (iii) Calcium carbide is treated with water: It produces Ethyne (Acetylene) and Calcium hydroxide

    .
    CaC2 + 2H2O → C2H2 + Ca(OH)2

    (iv) Excess CO2 is passed through Ca(OH)2: The initial milkiness (CaCO3) disappears due to the formation of soluble Calcium bicarbonate

    .
    CaCO3 + H2O + CO2 → Ca(HCO3)2

    (v) BF3 is treated with NaBH4 in diglyme: It produces Diborane gas

    .
    3NaBH4 + 4BF3 → 2B2H6 + 3NaBF4

    2. (b) (ii) Discuss the bonding in B2H6. 3

    Diborane (B2H6) features unique 3-center-2-electron (3c-2e) bonds, often called "banana bonds"

    .
  • There are four terminal B-H bonds which are normal 2-center-2-electron bonds.
  • The two bridging Hydrogen atoms are bonded to both Boron atoms, where two electrons are shared over three centers (B-H-B).

    • UNIT-II

    3. (a) What are open system and closed system? 2

  • Open System: A system that can exchange both matter and energy with its surroundings.
  • Closed System: A system that can exchange energy (heat or work) but not matter with its surroundings.

    • 3. (c) State the first law of thermodynamics. Write its mathematical form. 2

    The First Law of Thermodynamics states that energy can neither be created nor destroyed, only transformed from one form to another

    .
    ΔU = q + w

    Where ΔU is the change in internal energy, q is heat added to the system, and w is work done on the system

    .

    4. (b) (i) Derive Kirchhoff's equation. 6

    Kirchhoff's equation describes how the enthalpy of a reaction changes with temperature

    .

    For a reaction: ΔH = H(products) - H(reactants)

    .

    Differentiating with respect to Temperature (T) at constant Pressure (P):

    d(ΔH)/dT = dH(prod)/dT - dH(react)/dT

    .

    Since (dH/dT)p = Cp (heat capacity at constant pressure):

    d(ΔH)/dT = ΔCp

    Integrating from T1 to T2:

    ΔH2 - ΔH1 = ΔCp(T2 - T1)

    UNIT-III

    5. (a) Distinguish between ideal and non-ideal solutions. 2

    Ideal Solution Non-Ideal Solution Obeys Raoult's law at all concentrations. Does not obey Raoult's law. ΔH_mixing = 0. ΔH_mixing ≠ 0. ΔV_mixing = 0. ΔV_mixing ≠ 0.

    5. (c) Define Gibbs' phase rule. Explain terms. 2

    F = C - P + 2
  • F (Degrees of Freedom): Number of independent variables (P, T, concentration) that must be specified to define the system.
  • C (Components): Minimum number of chemically independent constituents.
  • P (Phases): Homogeneous, physically distinct part of the system.

    • 6. (b) (i) Draw the phase diagram of water system. 6

    The water system is a one-component system with three possible phases: ice, water, and vapor

    .
  • Triple Point: Point where all three phases coexist in equilibrium (F=0).
  • Curves: Represent two phases in equilibrium (F=1) e.g., Vaporization curve.
  • Areas: Represent a single phase (F=2) e.g., Liquid region.

    • UNIT-IV

    7. (b) Propene treated with HBr (Peroxide effect). 2

    • Absence of Peroxide (Markownikoff's Rule): H attaches to the carbon with more hydrogens. Product: 2-Bromopropane.
    • Presence of Peroxide (Anti-Markownikoff's Rule): Br attaches to the carbon with more hydrogens. Product: 1-Bromopropane.
    CH3-CH=CH2 + HBr → CH3-CHBr-CH3 (No Peroxide)
    CH3-CH=CH2 + HBr (Peroxide) → CH3-CH2-CH2Br

    8. (a) (ii) Friedel-Crafts Alkylation and Acylation. 6

    Alkylation: Introduction of an alkyl group into the benzene ring using an alkyl halide and AlCl3

    .
    C6H6 + CH3Cl (AlCl3) → C6H5-CH3 + HCl

    Acylation: Introduction of an acyl group (RCO-) using an acid chloride and AlCl3

    .
    C6H6 + CH3COCl (AlCl3) → C6H5-COCH3 + HCl

    UNIT-V

    9. (a) Why does CH3X favour SN2 reaction? 2

    Methyl halides (CH3X) favour SN2 because they have the least steric hindrance

    . The nucleophile can easily attack the central carbon atom from the backside. SN1 is not favored because the methyl carbocation is highly unstable.

    10. (a) (i) Discuss SN1 mechanism. 6

    SN1 (Unimolecular Nucleophilic Substitution) is a two-step process

    :
  • Formation of Carbocation: The leaving group departs (slowest step).
  • Nucleophilic Attack: The nucleophile attacks the flat carbocation from either side.

    • Racemic Mixture: Since the carbocation intermediate is planar, the nucleophile can attack from the front or back with equal probability, leading to an equal mixture of two enantiomers (racemization)

    .

    Exam Focus Enhancements

    Exam Tips

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  • Always write balanced chemical equations for "What happens when" questions to secure full marks[span_49](end_span).
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  • For mechanisms, clearly show electron movement using curved arrows[span_51](end_span).
  • In Phase Rule questions, define all variables (F, C, P) before solving.

    • Common Mistakes

  • Confusing the peroxide effect (Anti-Markownikoff) with normal addition in propene.
  • Forgetting to include the "2" in the phase rule formula (F = C - P + 2).
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  • In SN1/SN2 questions, failing to mention steric hindrance or carbocation stability[span_56](end_span).

    • Important Formulas List

  • First Law: ΔU = q + w
  • Kirchhoff's: ΔH2 - ΔH1 = ΔCp(T2 - T1)
  • Phase Rule: F = C - P + 2
  • Raoult's Law: P_total = P°A*XA + P°B*XB

    • Knowlet - Your Academic Companion

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