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 bonding: every question, 2021 to 2025
14 parts worth 145 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.
Ionic bond, characteristics of ionic compounds, lattice energy, Born-Haber cycle; covalent bond and its general characteristics, polarities of bonds in molecules and their dipole moments; valence bond theory, concept of resonance and resonance energy; molecular orbital theory (LCAO method); bonding in H₂⁺, H₂, He₂⁺ to Ne₂, NO, CO, HF and CN⁻; comparison of valence bond and molecular orbital theories, bond order, bond strength and bond length.
Notes: VSEPR, hybridisation and Bent's rule (full notes) · short notes
Notes: Molecular orbital theory: LCAO, homonuclear and heteronuclear diatomics (O₂, N₂, CO, NO, CN⁻, HF), bond order and magnetism (full notes) · short notes
Notes: Ionic bonding: lattice energy, Born–Haber cycle, Born–Landé and Kapustinskii equations, Fajans’ rules (full notes) · short notes
2025
- Q1(b) Born–Haber cycle for NaCl at 298 K5 marks Answered
- Q1(f) Ascending order of dipole moment: NH₃, NF₃, H₂O5 marks Answered
- Q4(a) MO diagram of NO; NO and NO⁺; NO as a radical20 marks Answered
2024
- Q1(a)(ii) Lattice energies of NaBr and NaI above the calculated values5 marks Answered
- Q2(b)(i) Bond order of O₂, O₂⁻, O₂²⁻ and O₂⁺10 marks Answered
- Q2(b)(ii) Molecular orbital diagram of CO10 marks Answered
2023
- Q1(b) Lewis structures of [Br₃]⁻ and thiourea; the most polar bond10 marks Answered
- Q3(a) Electronic and molecular geometry of BrF₅ and [ICl₂]⁻10 marks Answered
- Q5(c) Geometry and hybridisation of carbon in cytosine and thymine10 marks Answered
2022
- Q1(b) Non-bonding interactions between orbitals of A and B10 marks Answered
- Q3(a) The least stable of OCN⁻, ONC⁻ and SCN⁻10 marks Answered
- Q4(e) Percentage ionic character of the Br–Cl bond10 marks Answered
2021
- Q1(b) Born–Haber cycle: can NaCl₂ form?10 marks Answered
- Q2(b) Shapes of the sp³d, sp³d², dsp² and sd³ hybrids and the d orbitals used20 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