UPSC Civil Services (Mains)
Chemistry optional
- 2 papers
- 250 marks each
- 3 h per paper
- 5 of 8 questions, Q1 and Q5 compulsory
Paper I covers physical and inorganic chemistry.
All topics
Paper I, physical and inorganic chemistry
Photochemistry: every question, 2021 to 2025
11 parts worth 120 marks have been set on this unit in UPSC CSE Chemistry Paper I from 2021 to 2025. Each one opens on its own page with the question, the answer in brief and the key points.
Syllabus
Absorption of light; decay of excited state by different routes; photochemical reactions between hydrogen and halogens and their quantum yields.
Notes: Photochemistry: quantum yield, Jablonski diagram, H₂–halogen reactions (full notes) · short notes
2025
- Q5(b) Light absorbed by a dye solution5 marks Answered
- Q6(b) Photochemical H₂ + Br₂: rate law and quantum yield20 marks Answered
2024
- Q5(a) Fluorescence and phosphorescence on a Jablonski diagram10 marks Answered
- Q6(a)(i) Moles of HCl formed per joule absorbed at 480 nm10 marks Answered
- Q6(a)(ii) Molar extinction coefficient from 50 % transmission5 marks Answered
2023
- Q5(b) Radiative and non-radiative processes; Jablonski diagram10 marks Answered
- Q8(c) Light absorbed when the path length is halved10 marks Answered
2022
- Q5(b) Jablonski diagram (A to E); the route to photochemistry10 marks Answered
- Q8(d) Steady-state approximation; photochemical H₂ + Cl₂; quantum yield10 marks Answered
2021
- Q5(b) Lambert–Beer law: statement and derivation10 marks Answered
- Q7(c) Quantum yield for the photodecomposition of oxalic acid20 marks Answered
Printed copy and Kindle
UPSC Civil Services (Main) Chemistry Paper I: Model Answers 2021 to 2025
- All eight questions of each paper, with the question printed above its answer
- Question choice and time plan for each paper
- Topic-wise weightage and a topic-wise index