Q3(c). Photostationary dimer concentration
A molecule A in solution absorbs UV light at intensity Iabs and follows: (i) A + hν → A*; (ii) A* + A → A₂ (rate constant k₂); (iii) A* → A + hν′ (fluorescence, k₃); (iv) A₂ → 2A (thermal decomposition, k₄). Derive the concentration of A₂ at equilibrium when the concentration of A is large.
Model answer
The free solution for 2024 answers questions 1, 2, 4, 5 and 8, the ones picked in its time plan. Question 3 is answered in full in the IFoS Chemistry Paper I, 2021 to 2025 book, with every part of every paper.
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.
UPSC Indian Forest Service (Main) Chemistry Paper I: Model Answers 2021 to 2025
- All eight questions of each paper, with the question printed above its answer
- Physical chemistry numericals worked with units; inorganic structures drawn
- Topic-wise weightage and a topic-wise index
Coversphotostationary statephotodimerisationsteady-state approximationfluorescencephotochemical kinetics
All parts of Question 3
- 3(a) Thermodynamic transition-state theory15 marks
- 3(b) Heat capacity of an Einstein crystal10 marks
- 3(c) Photostationary dimer concentration10 marks
- 3(d) Cubic diffraction selection rules5 marks