UPSC Combined Geo-Scientist
Chemist
- 3 descriptive papers
- 200 marks each
- 3 h per paper
Mains Paper II covers physical chemistry.
Photochemistry: every question, 2016 to 2025
40 parts worth 237 marks have been set on this unit in UPSC Geo-Scientist Chemistry Mains Paper II from 2016 to 2025. Each one opens on its own page with the question, the answer in brief and the key points.
Franck-Condon principle and vibrational structure of electronic spectra. Bond dissociation and principle of determination of dissociation energy. Decay of excited states by radiative and non-radiative paths. Fluorescence and phosphorescence, Jablonski diagram. Laws of photochemistry: Grotthus-Draper law, Stark-Einstein law of photochemical equivalence; quantum yield and its measurement for a photochemical process, actinometry. Photostationary state. Photosensitized reactions. Kinetics of HI decomposition, H₂-Br₂ reaction, dimerisation of anthracene.
Notes: Photochemistry: quantum yield, Jablonski diagram, H₂–halogen reactions (full notes) · short notes
Notes: Vibrational and electronic spectra: anharmonicity and Franck–Condon (full notes) · short notes
2025
- Q1(o) Measuring light intensity with a chemical actinometer5 marks Answered
- Q1(p) Why watch hands glow in the dark5 marks Answered
- Q10 Photochemical H₂–Br₂ reaction: rate ∝ √Iabs10 marks Answered
2024
- Q1(o) Fluorescence rate constant of tryptophan5 marks Answered
- Q1(p) Why 511 nm light dissociates Br₂ but not H₂5 marks Answered
- Q10(a) The Franck–Condon principle5 marks Question only
- Q10(b) Dissociation energy of Br₂ from the convergence limit5 marks Question only
- Q14(d) Quantum efficiency in the uranyl oxalate actinometer5 marks Answered
- Q15(b) Photolysis of HI: steady-state rate law and quantum yield10 marks Answered
2023
- Q1(o) Stark–Einstein law and the quantum yield5 marks Answered
- Q1(p) Decay routes of a photoexcited molecule5 marks Answered
- Q9(d) Resonance fluorescence of atoms2 marks Question only
- Q9(e) Phosphorescence delayed, fluorescence prompt2 marks Question only
- Q10(a) Photosensitisation and how it differs from catalysis5 marks Answered
- Q10(b) Quantum yield of the photodecomposition of HI5 marks Answered
2022
- Q1(o) Why radiative transitions are drawn vertical5 marks Answered
- Q1(p) Energy of one einstein at 600 nm and 300 nm5 marks Answered
- Q9(a) Fluorescence quantum yield by the relative method5 marks Question only
- Q9(b) Factors that control the fluorescence quantum yield5 marks Question only
- Q15(a) Rate of formation of CO from the quantum yield10 marks Answered
2021
- Q1(o) Photosensitised reactions5 marks Answered
- Q1(p) Energy of one mole of 2500 Å photons5 marks Answered
- Q9 Franck–Condon principle: which v′ an electronic transition reaches10 marks Question only
- Q13(b) Photochemical decomposition of hydrogen iodide10 marks Question only
2020
- Q1(n) Wavelength and the einstein5 marks Answered
- Q1(o) Uranyl-sensitised decomposition5 marks Answered
- Q1(p) HI photolysis yield5 marks Answered
- Q10(a) Fluorescence and phosphorescence are photophysical5 marks Question only
- Q10(b) Glowing of fireflies: chemical and physical steps5 marks Question only
- Q15(c) Photodimerisation of anthracene: [A₂] independent of [A]10 marks Question only
2019
- Q1(m) Energy of one einstein5 marks Answered
- Q9(a) Chain-reaction quantum yield5 marks Answered
- Q11(b) Decay of an excited molecule5 marks Answered
2018
- Q1(m) Anthracene photodimerization5 marks Answered
- Q11(c) Bond dissociation energy from band convergence10 marks Question only
2017
- Q6(b) Photosensitized reactions: photosynthesis8 marks Question only
- Q12(b) HI photolysis5 marks Answered
- Q13(b) Fluorescence and phosphorescence lifetimes5 marks Answered
2016
- Q1(m) Photochemical quantum yield5 marks Answered
- Q7 Jablonski diagram from photophysical data10 marks Question only
UPSC Combined Geo-Scientist (Main) Chemistry Paper II: Model Answers 2021 to 2025
- All fifteen questions of each paper, with the question printed above its answer
- Numericals worked from formula to result with units; graphs with labelled axes
- Topic-wise weightage and a topic-wise index