VKC ScienceChemistry for written exams Telegram

UPSC Combined Geo-Scientist

Chemist

  • 3 descriptive papers
  • 200 marks each
  • 3 h per paper

Mains Paper II covers physical chemistry.

UPSC Combined Geo-Scientist, Mains · 2025 · Mains Paper II

Q10. Photochemical H₂–Br₂ reaction: rate ∝ √Iabs

Question

For the reaction H₂(g) + Br₂(g) → 2HBr(g) (in light, hν), show that the rate of the reaction varies as the square root of the intensity of the absorbed radiation.

Answer in brief

Steady-state treatment of H• and Br• gives [Br•] = (Iabs/k₅)^½, so the rate d[HBr]/dt is proportional to √Iabs.

Part of VKC Basic

Read the key points and the full model answer with VKC Basic

VKC Basic (₹999 for 6 months) opens every solved answer, the notes, the handbook and the daily answer archive. Every Mains test series includes it.

See what is included Already a member? Sign in

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.

The handwritten model answer pages open with the VKC plan.
Read the whole 2025 solution2025 question paper

Topic notes: photochemistry (quantum yield, Jablonski diagram, H₂–halogen reactions), with every Civil Services, IFoS and Geo-Scientist PYQ →

CoversH2–Br2 photochemical reactionchain reactionsteady-state approximationquantum yieldabsorbed intensity