UPSC Combined Geo-Scientist, Mains · 2025 · Mains Paper II
Q1(n). Rotational activity of CO and CO₂
Question
Which one of the following molecules is rotational spectrum active and why? CO₂ and CO
Answer in brief
CO is rotationally active because it has a permanent dipole moment. CO₂ (μ = 0) is inactive.
Key points
- Gross selection rule: the molecule must have a permanent dipole moment.
- CO (heteronuclear), μ = 0.11 D. Its rotation gives an oscillating dipole that exchanges energy with the microwave field.
- CO₂: linear and symmetric (D∞h). Bond dipoles cancel, μ = 0 in every orientation ⇒ no interaction.
- CO lines: ν̃ = 2B(J + 1) with B = 1.93 cm⁻¹. CO₂ is seen only in the rotational Raman spectrum.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
Coversmicrowave spectroscopyrotational spectrumpermanent dipole momentgross selection ruleCO and CO2
All parts of Question 1
- 1(a) Why Z > 1 for H₂ and He at all pressures5 marks
- 1(b) Corresponding state of H₂S for N₂ at 25 °C and 1 atm5 marks
- 1(c) Band structure of n-type and p-type semiconductors5 marks
- 1(d) Symmetry elements of orthorhombic Mg₂SiO₄5 marks
- 1(e) Fall of G with temperature at constant pressure5 marks
- 1(f) Z > 1 and f > P for the gas P(Vm − b) = RT5 marks
- 1(g) Overall equation and the role of each species5 marks
- 1(h) log k against pH for specific H⁺ catalysis5 marks
- 1(i) KCl salt bridge for a silver–lead cell5 marks
- 1(j) Equilibrium constant from standard potentials5 marks
- 1(k) Conditions for an acceptable wave function5 marks
- 1(l) No solution with E = 0 for the particle in a box5 marks
- 1(m) Discrete levels and the resonance condition5 marks
- 1(n) Rotational activity of CO and CO₂5 marks
- 1(o) Measuring light intensity with a chemical actinometer5 marks
- 1(p) Why watch hands glow in the dark5 marks