FYUG Even Semester Exam, 2025
CHEMISTRY: Fundamentals of Chemistry
Course No.: CHMDSM-151 | Semester: 2nd | Full Marks: 70 | Pass Marks: 28 | Time: 3 Hours
Instructions: The figures in the margin indicate full marks for the questions.
UNIT-I
1. (a) Mention two important postulates of Bohr's theory. 2
- Electrons revolve around the nucleus in specific circular paths called orbits or stationary states with fixed energy.
- An electron can move from a lower energy level to a higher energy level by absorbing a definite amount of energy, and vice versa by emitting energy.
1. (b) What is meant by dual nature of particles in motion? 2
According to de Broglie, every moving particle (like an electron) behaves both as a particle and as a wave.
This is known as the wave-particle duality, expressed by the equation: wavelength = h / mv.
1. (c) Draw the radial probability distribution curves for 1s and 2s electrons. 2
For 1s, the curve shows a single peak.
For 2s, there is one node and two peaks, with the larger peak further from the nucleus.
2. (a) (i) State and explain Heisenberg uncertainty principle. Mention its significance. 3
It is impossible to determine simultaneously and precisely both the position and the momentum (or velocity) of a microscopic particle like an electron.
Mathematical expression: (Δx)(Δp) ≥ h / 4π. Its significance lies in the fact that it rules out the existence of definite paths or trajectories for electrons and other similar particles.
2. (a) (ii) Numerical: de Broglie wavelength of an electron moving with speed of light. 2
Given: h = 6.62 x 10^-34 J.s, mass (m) = 9.1 x 10^-31 kg, velocity (v) = 3 x 10^8 m/s
Formula: wavelength = h / mv
Calculation: wavelength = (6.62 x 10^-34) / (9.1 x 10^-31 x 3 x 10^8)
Final Answer: 2.42 x 10^-12 meters.
2. (a) (iii) Time-independent Schrödinger wave equation. 3
The equation is: (δ²ψ/δx²) + (δ²ψ/δy²) + (δ²ψ/δz²) + (8π²m/h²)(E - V)ψ = 0
- ψ (psi): Wave function
- m: Mass of the particle
- E: Total energy
- V: Potential energy
- h: Planck's constant
2. (b) (i) State and explain Aufbau principle with example. 3
In the ground state of an atom, orbitals are filled in order of their increasing energies.
Orbitals with lower (n + l) value are filled first.
Example: 4s (4+0=4) is filled before 3d (3+2=5) because 4 < 5.
UNIT-II
3. (a) Draw the structures of BF3 and NH3. 2
BF3: Trigonal planar geometry with sp2 hybridization.
NH3: Pyramidal geometry due to one lone pair and sp3 hybridization.
3. (c) Draw resonance structures of isoelectronic NO2- and O3. 2
Both species have bent structures with delocalized pi electrons.
4. (a) (i) Discuss geometry of XeF4 and ClF3 using VSEPR theory. 4
- XeF4: Xenon has 8 valence electrons. 4 bond pairs + 2 lone pairs = 6 electron pairs (Octahedral arrangement). Geometry is Square Planar.
- ClF3: Chlorine has 7 valence electrons. 3 bond pairs + 2 lone pairs = 5 electron pairs (Trigonal bipyramidal arrangement). Geometry is T-shaped.
4. (a) (ii) Draw the MO diagram of N2 molecule. 4
Nitrogen (Z=7) has 14 electrons. Configuration: σ1s² σ*1s² σ2s² σ*2s² (π2px² = π2py²) σ2pz².
Magnetic Property: Since all electrons are paired, N2 is diamagnetic.
UNIT-III
5. (a) Two postulates of Kinetic Theory of Gases. 2
- Gases consist of large numbers of tiny particles (atoms or molecules) which are in constant random motion.
- The actual volume of the gas molecules is negligible compared to the total volume occupied by the gas.
5. (b) Calculate most probable velocity of oxygen at 27°C. 2
Formula: v = sqrt(2RT / M)
T = 27 + 273 = 300 K; R = 8.314 J/mol.K; M = 32 x 10^-3 kg/mol
v = sqrt(2 * 8.314 * 300 / 0.032) = 394.8 m/s.
6. (a) (iii) Difference between Real Gas and Ideal Gas. 2
| Ideal Gas |
Real Gas |
| Obeys gas laws at all T and P. |
Obeys gas laws only at low P and high T. |
| No intermolecular forces. |
Intermolecular forces are present. |
UNIT-IV
7. (a) What is viscosity? Variation with temperature. 2
Viscosity is the internal resistance to flow in a liquid.
As temperature increases, kinetic energy increases, weakening intermolecular forces, so viscosity decreases.
8. (a) (iii) Explain Schottky and Frenkel defects. 3
- Schottky Defect: Equal number of cations and anions are missing from their lattice sites.
Decreases density.
- Frenkel Defect: An ion (usually cation) leaves its site and occupies an interstitial position.
Density remains constant.
UNIT-V
9. (a) Conditions for Homolytic and Heterolytic cleavage. 2
- Homolytic: Occurs in non-polar bonds, usually in the presence of UV light or high temperature.
Forms free radicals.
- Heterolytic: Occurs in polar bonds or in the presence of polar solvents.
Forms carbocations and carbanions.
10. (a) (i) What is Hyperconjugation? Stability of carbocations. 3
Hyperconjugation is the delocalization of sigma electrons of a C-H bond into an adjacent empty p-orbital or pi-system.
More the number of alpha-hydrogens, more the hyperconjugative structures, and higher the stability.
Order of stability: (CH3)3C+ (Tertiary) > (CH3)2CH+ (Secondary) > CH3CH2+ (Primary) > CH3+ (Methyl).