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.
Chemical kinetics: every question, 2021 to 2025
13 parts worth 135 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.
Differential and integral rate equations for zeroth, first, second and fractional order reactions; rate equations involving reverse, parallel, consecutive and chain reactions; branching chain and explosions; effect of temperature and pressure on rate constant. Study of fast reactions by stop-flow and relaxation methods. Collisions and transition state theories.
Notes: Theories of reaction rate: Arrhenius, collision and transition-state theory (full notes) · short notes
Notes: Rate laws and order: integrated rate equations, half-life, reversible, parallel and consecutive reactions (full notes) · short notes
2025
- Q5(a) Activation energy from two rate constants5 marks Answered
- Q5(d) Showing that a decomposition is first order5 marks Answered
- Q7(d) Second-order rate law with unequal concentrations; units of k10 marks Answered
2024
- Q1(e) Temperature at which the uncatalysed reaction matches catalase at 20 °C10 marks Answered
- Q4(c)(i) Time of maximum concentration of B in A → B → C10 marks Answered
- Q4(c)(ii) Yield of P₁ from three parallel paths5 marks Answered
2023
- Q5(a) First-order decomposition of AB₂10 marks Answered
- Q8(d) Transition-state theory and the pre-exponential factor20 marks Answered
2022
2021
- Q5(a) Rate constant and half-life of a zero-order reaction10 marks Answered
- Q6(b) Limitations of collision theory; the answer of transition-state theory20 marks Answered
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