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UPSC Chemistry — Previous Year Questions

Every question from Chemistry papers in UPSC Civil Services Mains (2021–2026), organised by year. 96 questions in total, each with directive-word analysis, model answer points, and a full evaluation rubric.

96Questions
6Years covered

2026 Mains

16 questions · 800 marks

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Paper I 8 Qs · 400 marksPaper II 8 Qs · 400 marks
All 16 questions from 2026

Paper I

  1. Q1 (a) One real gas equation is van der Waals equation (P + (n² a)/(V²))(V - nb) = nRT Which parameter in the above… 50 marks
  2. Q2 (a) The Maxwell distribution of molecular speed of a gaseous system is dN_c = 4πN ( M / (2πRT) )³/2 c² e^-Mc²/2RT dc… 50 marks
  3. Q3 (a) (i) Derive the expressions of chemical potential of a component in a mixture system in terms of extensive… 50 marks
  4. Q4 (a) Draw and discuss the polarographic wave obtained in the case of a dropping mercury electrode in polarography. (10… 50 marks
  5. Q5 (a) What is the de Broglie wavelength for a baseball (140 g) moving at 40 m s⁻¹? Comment on your finding. (5 marks) (b)… 50 marks
  6. Q6 (a) (i) Predict the Lewis structures and geometry of the following as per VSEPR theory : (1) SO₂, (2) I₃⁻, (3) SF₄, (4)… 50 marks
  7. Q7 (a) (i) Show that ψ(x) = e^(αx) is an eigenfunction for the operator  = d^n/dx^n. What is the eigenvalue? (5 marks)… 50 marks
  8. Q8 (a) Differentiate between T and R conformation of hemoglobin. (10 marks) (b) (i) Describe the structure and bonding in… 50 marks

Paper II

  1. Q1 (a) (i) Draw the structures of three possible geometrical isomers of [10] annulene and explain the aromaticity. (5… 50 marks
  2. Q2 In the following reaction, intermediate [A] is formed : CHCl₃ + ᵗBuOK ——> [A] (a) Write the structures of the major… 50 marks
  3. Q3 (a) (i) Write the structure of the major products of the following reaction and write the mechanism of their formation… 50 marks
  4. Q4 (a) (i) Write the structure of the stereochemical product formed in the reaction of (2E, 4E)-hexa-2,4-diene under… 50 marks
  5. Q5 (a) Proteins are dynamic molecules rather than static structures. Explain this phenomenon using hemoglobin as an… 50 marks
  6. Q6 (a) Write the structures of the products formed in the following reactions : 2×5=10 (i) (Cyclohex-2-en-1-one)… 50 marks
  7. Q7 (a) (i) What are the phenomena that result in the emission wavelength being longer than the excitation wavelength in… 50 marks
  8. Q8 (a) (i) Calculate the rotational constant of a diatomic molecule if the moment of inertia is 13·97 × 10⁻⁴⁷ kg m²… 50 marks

2025 Mains

16 questions · 800 marks

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Paper I 8 Qs · 400 marksPaper II 8 Qs · 400 marks
All 16 questions from 2025

Paper I

  1. Q1 (a) Calculate the ratio of probability of finding the 1s electron of hydrogen atom at r = a₀ and at r = 10a₀, where 'r'… 50 marks
  2. Q2 (a) Find the probability of existence of a particle in a one-dimensional box of length 'a' in the region 0 ≤ x ≤ a/4… 50 marks
  3. Q3 (a) Calculate the number of collisions that oxygen makes per second on 1·00 cm² of the surface of the vessel containing… 50 marks
  4. Q4 (a) Draw the molecular orbital (MO) diagram of NO molecule. The experimental bond dissociation energy of NO is 626 kJ… 50 marks
  5. Q5 (a) The gas-phase reaction between methane (CH₄) and diatomic sulphur (S₂) is given by the following reaction: CH₄ (g)… 50 marks
  6. Q6 (a) The following molecule shows the rigid or fluxional behaviour at higher temperature or in the presence of a base… 50 marks
  7. Q7 (a) Distinguish the "T" (tense) and 'R' (relax) conformations of Hemoglobin on reversible binding of oxygen (O₂). 15… 50 marks
  8. Q8 (a) Draw the structures of S₂N₂, S₄N₂, S₁₁N₂, S₅N₆ and I₂Cl₆. 10 marks (b) What are silicones ? Mention some of their… 50 marks

Paper II

  1. Q1 (a) (i) Tropolone is aromatic, but fulvene is non-aromatic. Why? (ii) Explain with example pseudo-aromaticity. (b)… 50 marks
  2. Q2 (a) (i) In the presence of sodium ethoxide, the following transformation occurs. Explain : (ii) Propose a suitable… 50 marks
  3. Q3 (a) (i) Write the structure of the product formed in the following reactions: (1) (2) (10 marks) (ii) Describe the… 50 marks
  4. Q4 (a) (i) After polymerization of p-hydroxybenzoic acid, IR analysis shows 0·2% unreacted —COOH. Calculate the molecular… 50 marks
  5. Q5 (a) Write the structure of the product(s) and the intermediate formed in the following reaction: Cl₂CH—COCl 1) Et₃N → ?… 50 marks
  6. Q6 (a) (i) Elucidate the structure of the product and the intermediate (if any) in the following reactions: (A)… 50 marks
  7. Q7 (a) (i) Acetone shows a weak absorption at 280 nm and a strong absorption at 190 nm in the UV spectrum. Account for the… 50 marks
  8. Q8 (a) (i) Estimate the expected splitting (coupling constant J in Hz) for the lettered protons in the ¹H NMR spectrum of… 50 marks

2024 Mains

16 questions · 800 marks

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Paper I 8 Qs · 400 marksPaper II 8 Qs · 400 marks
All 16 questions from 2024

Paper I

  1. Q1 (a) (i) Determine the distance from the nucleus at which the electron is most expected in the hydrogen atom in its… 50 marks
  2. Q2 (a) State Heisenberg's uncertainty principle. Show that for a particle in a one-dimensional box having length from 0 to… 50 marks
  3. Q3 (a) (i) Calculate the coefficient of viscosity of air at temperatures : (I) 298 K and (II) 0 K. Assume that the… 50 marks
  4. Q4 (a) A new drug has been synthesized and its phase diagram is explored. It is found that near its triple point, vapour… 50 marks
  5. Q5 (a) Indicating 'Fluorescence' and 'Phosphorescence' processes in Jablonski diagram, distinguish between both processes… 50 marks
  6. Q6 (a) (i) Calculate the number of moles of HCl (g) produced by the absorption of one Joule of radiant energy of… 50 marks
  7. Q7 (a) Show the splitting of 'd' orbitals in square planar field according to Crystal Field Theory (CFT). Comment on the… 50 marks
  8. Q8 (a) 'All the F atoms appear indistinguishable in the ¹⁹F NMR spectrum of sp³d hybridized PF₅ molecule.' Explain how… 50 marks

Paper II

  1. Q1 (a) (i) Classify the following as aromatic, nonaromatic or antiaromatic : (1) Azulene (2) Pyridine (3) Sydnone (4)… 50 marks
  2. Q2 (a) (i) Consider the following reactions : PhCH₂CH₂Br + EtO⁻ → PhCH=CH₂ (Rate = k_H) and PhCD₂CH₂Br + EtO⁻ → PhCD=CH₂… 50 marks
  3. Q3 (a) (i) Predict the reaction mechanism and the major product formed when methyl cyanide is heated with dilute… 50 marks
  4. Q4 (a) (i) Consider the following reaction : furan + (CH₃CO)₂O →(BF₃, CH₃COOH) X. Name the product X and outline the… 50 marks
  5. Q5 (a) Write the structures of the bases present in DNA and RNA. Compare the stability of DNA and RNA. (10 marks) (b)… 50 marks
  6. Q6 (a) (i) Explain the various steps involved in benzoyl peroxide-initiated polymerization of ethylene to give… 50 marks
  7. Q7 (a) Calculate the value of λ_max in the following compounds using Woodward-Fieser rules : three tricyclic… 50 marks
  8. Q8 (a) Ethanolic solution of compound I on irradiation leads to the formation of compounds II, III and IV [structures in… 50 marks

2023 Mains

16 questions · 800 marks

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Paper I 8 Qs · 400 marksPaper II 8 Qs · 400 marks
All 16 questions from 2023

Paper I

  1. Q1 (a) (i) Find the wavelength of the emitted light when 1.0×10⁻²⁷ g particle in a one-dimensional box of length 3 Å goes… 50 marks
  2. Q2 (a) Using the Gibbs equation for a closed system in the absence of non-expansion work at constant composition, answer… 50 marks
  3. Q3 (a) Determine the electronic and molecular geometry of [BrF₅] and [ICl₂]⁻ interhalogen compounds. 10 (b) Answer the… 50 marks
  4. Q4 (a) The volume of a certain gas is found to be 5.0×10⁻⁴ m³ mol⁻¹ at 273 K and 3.0×10⁶ Pa. This gas obeys the van der… 50 marks
  5. Q5 (a) The decomposition of AB₂ to AB and B is a first-order reaction with k = 2·8 × 10⁻⁷ s⁻¹ at T = 1000 K. The atomic… 50 marks
  6. Q6 (a) Complete the following reactions and draw the structure of the major product(s) formed: (i) 2XeF₆ + SiO₂ → ____ (50… 50 marks
  7. Q7 (a) Explain the common structural features of two major iron-containing proteins—haemoglobin and cytochrome c present… 50 marks
  8. Q8 (a) Sketch the synthesis of [Fe(η⁵-C₅H₅)(η⁵-C₅H₄COCH₃)] and [Fe(η⁵-C₅H₅)(η⁵-C₅H₄COOH)] complexes starting from… 50 marks

Paper II

  1. Q1 1.(a)(i) pKₐ value of cyclopentadiene is almost similar to water. Explain. 5 marks 1.(a)(ii) Rate of hydrogen exchange… 50 marks
  2. Q2 2.(a)(i) Based on Hückel rule, predict the above compounds as aromatic, antiaromatic, and nonaromatic. [(A)… 50 marks
  3. Q3 3.(a) Would you expect the above conversion to require heat or light? Explain using molecular orbital diagram… 50 marks
  4. Q4 4.(a)(i) Complete the above reaction and write the steps involved in the reaction. [norbornene + D–Br/H2O → (A) + (B)]… 50 marks
  5. Q5 (a) How many signals would you expect in the ¹H NMR spectrum of above compounds ? Mark these protons. 10 marks (b)… 50 marks
  6. Q6 (a) (i) The frequencies of vibration of the following molecules in their v = 0 states are HCl : 2885 cm⁻¹; D₂ : 2990… 50 marks
  7. Q7 (a) (i) Draw the structure of 2'-deoxycytidine-3'-monophosphate. (ii) Why nucleotides and nucleic acids are acidic ?… 50 marks
  8. Q8 (a)(i) For the following compound : (i) Identify the site of initial ionization under EI conditions. (ii) Draw the… 50 marks

2022 Mains

16 questions · 800 marks

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Paper I 8 Qs · 400 marksPaper II 8 Qs · 400 marks
All 16 questions from 2022

Paper I

  1. Q1 1.(a) Compare average and most probable values of position of an electron in the ground state of hydrogen atom. Explain… 50 marks
  2. Q2 2.(a) An automobile tyre contains air at 320×10³ Pa at 20°C. The stem valve is removed and the air is allowed to expand… 50 marks
  3. Q3 (a) Identify the least stable ion of the following ions and justify your answer. OCN⁻ ONC⁻ SCN⁻ (10 marks) (b) A 74·6 g… 50 marks
  4. Q4 (a) By using the following given data, find that under what conditions is H₂ in the state corresponding to N₂ at 126 K… 50 marks
  5. Q5 (a) The rate Law for the reaction N₂O₂(g) → 2NO(g) is of first order in the concentration of N₂O₂. Derive an expression… 50 marks
  6. Q6 (a) (i) Calculate the number of metal-metal bond in Cp₂Fe₂(CO)₄. (ii) Determine the structural type of the metal atom… 50 marks
  7. Q7 (a) Cite one example of an optically active tetracoordinated complex compound where the metal ion and donor atoms lie… 50 marks
  8. Q8 (a) Which one is more stable between the two isomers? Explain. [(H₃N)₅ Cr – CN – Cr(CN)₅] [(H₃N)₅ Cr – NC – Cr(CN)₅] 10… 50 marks

Paper II

  1. Q1 (a) Ionization of 3,4-dichloro-1,2,3,4-tetramethylcyclobutene in SbF₅-SO₂ at –75 °C produces a reaction intermediate of… 50 marks
  2. Q2 (a) Predict the products X and Y in the following reactions: (5×3=15 marks) (i) 1-allyl-2,2-dimethylcyclopropan-1-ol… 50 marks
  3. Q3 (a) The compound A, acetal of acetaldehyde, on reaction with B, under given reaction conditions, yields C and the final… 50 marks
  4. Q4 (a) (i) Complete the following reaction by showing stepwise reaction mechanism for the formation of products … 50 marks
  5. Q5 (a) Write the structures of nucleosides and nucleotides, and discuss the primary structures of DNA and RNA. (10 marks)… 50 marks
  6. Q6 (a) Predict the products in the following reactions : (i) PhCO–CH₂CH₂CH₂CH₃ (valerophenone) + CH₃CO–CH₂CH₂–CD₂–CH₃… 50 marks
  7. Q7 (a) Discuss the different types of secondary structures of proteins and compare these structures with tertiary… 50 marks
  8. Q8 (a) (i) Among the following compounds I and II, which has more carbonyl stretching frequency in IR spectra? Explain: I… 50 marks

2021 Mains

16 questions · 800 marks

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Paper I 8 Qs · 400 marksPaper II 8 Qs · 400 marks
All 16 questions from 2021

Paper I

  1. Q1 (a) Calculate ⟨x⟩, the expectation value of position of a particle, in the ground state of one-dimensional box having… 50 marks
  2. Q2 (a) Hydrogen atoms are observed to have radiative transitions from n = 101 to n = 100 to occur. (i) What are the… 50 marks
  3. Q3 (a) A drop of water, 0·4 cm in radius, is split up into 125 tiny drops. Find the increase in surface energy. [γwater… 50 marks
  4. Q4 (a) Calculate the e.m.f. of the following electrochemical cell at 25 °C : Pt/H₂₍₁ ₐₜₘ₎|H⁺₍C=0.01 M₎‖Cu²⁺₍C=0.1 M₎|Cu(s)… 50 marks
  5. Q5 (a) Derive an equation for rate constant of a zero-order reaction. Show that half-life period of the reaction is… 50 marks
  6. Q6 (a) How does the bonding in cyclic phosphazene differ from that of benzene and borazine? (10 marks) (b) What are the… 50 marks
  7. Q7 (a) Draw the structures of various iron-sulphur proteins and their corresponding redox states. (10 marks) (b) How would… 50 marks
  8. Q8 (a) Justify that the interhalogen compound BrF₃ acts as an aprotic solvent and undergoes acid-base and neutralization… 50 marks

Paper II

  1. Q1 (a) In the reaction R—COO⁻ + Br—CN → R—CN + Br⁻ + CO₂↑ What is the origin of —CN group in the product ? Explain by… 50 marks
  2. Q2 (a) Involving the stereochemical concept, compare the elimination behaviour of compounds A and B in the presence of… 50 marks
  3. Q3 (a) Complete the following reactions with mechanisms: (i) Conc. H₂SO₄ (10 marks) (ii) EtO⁻ (10 marks) (b) (i) Write the… 50 marks
  4. Q4 (a) (i) Heating of 3-deuterioindene causes scrambling of the deuterium. Explain with mechanism. (5 marks) (ii) Write… 50 marks
  5. Q5 (a) ¹H NMR spectrum me 2,4-pentadione paanch signal darsata hai. Is prashn ka lekh do v anumanit rasayanik vishapan… 50 marks
  6. Q6 (a)(i) The molecule obtained on treatment of acetone with dilute sodium hydroxide exhibits the following spectral data… 50 marks
  7. Q7 7.(a) Complete the above reaction sequence by writing the structures of A, B and C. I. C₆H₅—CH = CH₂ n-BuLi A… 50 marks
  8. Q8 8.(a) (i) Write the products X/Y in the above chemical transformations. I. MCPBA → X II. OsO₄ → X NaIO₄ → Y III. OH |… 50 marks

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Answer skeleton

  1. Concept
  2. Structure or mechanism
  3. Reasoning
  4. Result

What earns the marks

  • Draw structures and arrow-pushing mechanisms
  • Give correct IUPAC names and stereochemistry
  • Justify with electronic and steric reasoning
  • Numerical parts: show the full working

What earns the extra mark

  • Named reaction
  • Reagent and conditions

Traps that cost marks

  • Product without mechanism
  • Wrong nomenclature
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