Indian Forest Service (Mains)
Chemistry optional
- 2 papers
- 200 marks each
- 3 h per paper
Paper I covers physical and inorganic chemistry.
Chemical bonding: every question, 2021 to 2025
14 parts worth 135 marks have been set on this unit in UPSC IFoS 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, factors affecting stability 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 homonuclear molecules: H₂⁺, H₂ to Ne₂, NO, CO, HF, CN, CN⁻, BeH₂ and CO₂. 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: Periodic properties: Slater's rules, radii, ionisation and electron-gain enthalpy, electronegativity (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(a) Equivalent bonds in nitrate8 marks Answered
- Q2(b)(ii) Dissociation energies of dioxygen species10 marks Answered
- Q2(c)(i) Ordering molecular dipoles5 marks Answered
- Q2(c)(ii) Bonding and magnetism of B₂5 marks Answered
2024
- Q5(a) Solubility of alkali-metal halides8 marks Answered
- Q7(a) Why CO and N₂ differ as ligands15 marks Question only
2023
- Q1(b) Formation enthalpy of lithium fluoride8 marks Answered
- Q2(b) NO and the nitrosonium ion15 marks Answered
- Q2(c) Bond orders of oxygen and carbon monoxide10 marks Answered
2022
- Q1(b) Dipole moments of ammonia and nitrogen trifluoride8 marks Answered
- Q2(b) Second ionisation energy of sodium15 marks Answered
- Q3(a) Molecular orbitals of linear BeH₂10 marks Question only
2021
- Q1(c) C–F and C–I potential-energy curves8 marks Answered
- Q2(b) Dissociation of an ionic KF molecule10 marks Answered
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