Chemistry: Physical, Inorganic and Organic Reasoning
Chemistry in JEE Advanced is divided into physical, inorganic, and organic chemistry, but questions often cross boundaries: an equilibrium problem may require thermodynamics, and an organic reaction may hinge on acid–base strength. The exam tests quantitative reasoning, structure–reactivity relationships, and mechanism-level explanations rather than isolated facts.
Core rules and concepts
- Thermodynamics: ΔG = ΔH − TΔS. A reaction is spontaneous when ΔG < 0; at equilibrium ΔG = 0 and Q = K.
- Equilibrium: K_p = K_c(RT)^Δn_gas. For weak acids, K_a ≈ Cα² only when α ≪ 1.
- Electrochemistry: E_cell = E°_cell − (0.0591/n) log Q at 298 K. A positive E_cell means a spontaneous cell reaction.
- Inorganic: periodic trends follow effective nuclear charge and shielding; coordination compounds follow crystal-field splitting and the spectrochemical series.
- Organic: identify the nucleophile and electrophile, then track electron flow; stereochemistry follows from the geometry of the transition state.
Worked example
For a reaction with ΔH = +50.0 kJ mol⁻¹ and ΔS = +100 J K⁻¹ mol⁻¹, find the temperature at which the system is at equilibrium under standard conditions.
Use ΔG = ΔH − TΔS. At equilibrium, ΔG = 0, so T = ΔH / ΔS = 50000 / 100 = 500 K. Below 500 K, TΔS < 50.0 kJ mol⁻¹, so ΔG > 0 and the forward reaction is non-spontaneous. Above 500 K, ΔG < 0. The units must be matched before dividing.
Traps that cost marks
- Using ΔG° to predict spontaneity under non-standard conditions.
- Forgetting that common-ion addition suppresses solubility.
- Assigning aromaticity without checking planarity, cyclic conjugation, and Hückel’s 4n + 2 count.
- Inverting nucleophile/electrophile labels in carbonyl addition.
- Ignoring n in the Nernst equation.