Chemistry: Physical, Inorganic and Organic
Assam CEE Chemistry covers physical, inorganic and organic chemistry at NCERT Class 11–12 depth. Physical chemistry supplies numericals, inorganic questions test trends and exceptions, and organic questions test IUPAC naming, stereochemistry and reaction mechanisms.
Core rules
- Mole concept: n = m/M = N/N_A. One mole of an ideal gas occupies 22.4 L at 273.15 K and 1 atm; at 25 °C and 1 bar it is about 24.8 L.
- Equilibrium: K_p = K_c(RT)^Δn. pH = −log[H₃O⁺]; a buffer obeys pH = pKa + log([A⁻]/[HA]).
- Thermodynamics: ΔG = ΔH − TΔS; a reaction is spontaneous when ΔG < 0.
- Periodic trends: atomic radius decreases across a period and increases down a group; ionisation enthalpy shows the opposite trend. Electron gain enthalpy of Cl is more negative than that of F because F is compact.
- Organic: choose the longest chain and lowest locants. SN1 is favoured by tertiary carbocations; SN2 by primary substrates and strong nucleophiles. Markovnikov addition governs HX addition to alkenes.
- Inorganic: d-d transitions colour transition-metal ions; d⁰ and d¹⁰ ions are colourless.
Worked example
Find the pH of 0.1 M CH₃COOH with Ka = 1.8 × 10⁻⁵.
For a weak acid, [H⁺] = √(KaC) = √(1.8 × 10⁻⁶) = 1.34 × 10⁻³ M. pH = −log(1.34 × 10⁻³) ≈ 2.87. The approximation is valid because dissociation is under 5%. If acetate ion were added, the buffer equation would replace this square-root approximation.
Traps that cost marks
- Using 22.4 L at 25 °C instead of about 24.8 L.
- Omitting Δn when converting Kp and Kc.
- Confusing empirical and molecular formulae.
- Assigning R/S without ranking substituents by CIP priority.
- Assuming every d-block ion is coloured; Zn²⁺ and Cu⁺ are not.
- Ignoring common-ion suppression of weak-acid dissociation.