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NEET · Cheatsheet

Chemistry Quick Revision

Physical + Inorganic + Organic — complete revision from mole to biomolecules.

Some basic concepts

Moles

n = m/M = N/Nₐ

Molarity

M = mol/L soln

Molality

m = mol/kg solvent

Normality

N = eq/L

Mole fraction

x_A = n_A/(n_A+n_B)

% by mass

(m_solute/m_soln)×100

Ideal gas

PV = nRT

Tip: Molarity depends on T (volume expands); molality doesn't. STP: 1 mol = 22.4 L (old NCERT) or 22.7 L (new).

Atomic structure

Quantum numbers: n (shell), l (0..n−1: s,p,d,f), m_l (−l..+l), m_s (±½).
Aufbau (n+l rule), Hund (max multiplicity), Pauli (no two same 4 QN).
Filling: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d.
Cr = [Ar] 3d⁵ 4s¹, Cu = [Ar] 3d¹⁰ 4s¹ (half/full stability).
Photon E = hν; Rydberg 1/λ = R(1/n₁² − 1/n₂²).
Nodes: radial = n−l−1, angular = l, total = n−1.

Chemical bonding

Octet rule + exceptions (BF₃, PCl₅, SF₆, NO).
Fajans: covalent character ↑ with small cation, large anion, high charge.
Hybridisation: sp (180°), sp² (120°), sp³ (109.5°), sp³d (TBP), sp³d² (oct).
VSEPR: lone pairs repel more (bp–bp < lp–bp < lp–lp).
MO order (till N₂): σ1s σ*1s σ2s σ*2s π2p_x=π2p_y σ2p_z π*...; after O₂ σ2p_z below π.
Bond order = (Nb − Na)/2.
H-bond > dipole–dipole > London.

Thermodynamics

1st law

ΔU = q + w

Enthalpy

H = U + PV

Gibbs

ΔG = ΔH − TΔS

ΔG°

−RT ln K = −nFE°

Cp − Cv

R (ideal gas)

Tip: Spontaneous ↔ ΔG < 0. Endothermic entropy-driven if ΔS > ΔH/T. Bond dissociation is endothermic; formation exothermic.

Equilibrium (chemical)

Kc, Kp: Kp = Kc(RT)^Δn.
Le Chatelier: system shifts to relieve stress.
For reaction with Δn=0 (e.g., H₂+I₂ ⇌ 2HI), pressure has no effect.
Reaction quotient Q vs K: Q<K forward, Q>K backward.
Adding inert gas at constant V: no shift.

Ionic equilibrium & pH

[H⁺][OH⁻] = 10⁻¹⁴

−log[H⁺]

pKa + pKb

Buffer (Henderson)

pH = pKa + log([A⁻]/[HA])

Ksp (AB)

s²

Ksp (A₂B)

4s³

Tip: Common ion effect ↓ solubility. Hydrolysis: weak acid strong base salt → basic.

Redox & electrochemistry

Nernst

E = E° − (0.0591/n) log Q

ΔG°

−nFE°

Faraday

Q = It; w = ZIt

Molar conductivity

Λm = κ·1000/M

Kohlrausch

Λ°m = ν₊ λ°₊ + ν₋ λ°₋

Tip: Higher SRP = better oxidising agent. Salt bridge = KCl in agar-agar.

Chemical kinetics

Rate

−d[R]/dt = k[R]ⁿ

1st order k

(2.303/t) log([A]₀/[A])

Half-life (1st)

0.693/k

Arrhenius

k = A e^(−Ea/RT)

Temp coeff

k(T+10)/k(T) ≈ 2–3

Solid state

7 crystal systems, 14 Bravais lattices.
Packing: SC (52%), BCC (68%), FCC/HCP (74%).
Atoms/unit cell: SC=1, BCC=2, FCC=4.
Voids: FCC has 4 octahedral + 8 tetrahedral per cell.
Defects: Schottky (↓ density), Frenkel (density same), F-centre (colour).
Semiconductors: n-type (extra e⁻), p-type (hole).

Solutions & colligative

Raoult

P = x·P°

ΔTb

Kb · m · i

ΔTf

Kf · m · i

Osmotic P

π = iCRT

van't Hoff i

1 + α(n−1)

Tip: Positive deviation: weaker A–B (ethanol+acetone). Negative: stronger (HCl+H₂O).

Surface chemistry

Adsorption: Physi (van der Waals, low ΔH, reversible), Chemi (covalent, high ΔH).
Freundlich: x/m = k P^(1/n).
Colloids: sol, gel, emulsion, aerosol; Tyndall effect, Brownian motion.
Coagulation power ∝ charge (Hardy–Schulze): Al³⁺ > Ba²⁺ > Na⁺ for −ve sol.
Catalysis: homogeneous vs heterogeneous; enzymes = biocatalysts.

Periodic properties

Across period →: radius ↓, IE ↑, EN ↑, metallic ↓.
Down group ↓: radius ↑, IE ↓, EN ↓, metallic ↑.
IE anomalies: N > O (half-filled 2p), Be > B (2s filled).
EA: Cl > F (F small, e⁻e⁻ repulsion).
EN order (Pauling): F > O > N ≈ Cl > Br > I > S > C > H.

s-block (alkali & alkaline earth)

Flame colours: Li (crimson), Na (yellow), K (violet), Ca (brick red), Sr (crimson), Ba (apple green).
Solubility of hydroxides down group ↑; sulphates ↓ (BaSO₄ insoluble).
Li resembles Mg (diagonal); Be resembles Al.
NaOH by Castner–Kellner; Na₂CO₃ by Solvay process.

p-block essentials

Group 13: B (non-metal) → Al..Tl (metals); BF₃ Lewis acid.
Group 14: C (allotropes: diamond, graphite, fullerene), Si, Sn, Pb; +2 stability ↑ down.
Group 15: N₂ triple bond, NH₃ pyramidal; HNO₃ (Ostwald).
Group 16: O₃ bent; H₂SO₄ (Contact); allotropes of S (S₈).
Group 17: EN F>Cl>Br>I; oxidising F₂>Cl₂>Br₂>I₂; interhalogens.
Group 18: noble gases; XeF₂ (linear), XeF₄ (square planar), XeF₆.

d & f-block

Variable OS due to close 3d/4s energies.
Coloured ions from d–d transitions; Zn²⁺, Sc³⁺, Ti⁴⁺ colourless (d⁰/d¹⁰).
Magnetic moment μ = √(n(n+2)) BM (spin only).
Lanthanoid contraction → similar sizes of 4d & 5d; separation difficulty.
K₂Cr₂O₇: orange, oxidising; KMnO₄: purple, Mn goes 7→2 in acidic.

Coordination compounds

Werner: primary (ionizable) & secondary (coordination) valencies.
Ligands: mono/bi/poly-dentate; chelate effect ↑ stability.
IUPAC: ligands alphabetical → metal (OS in Roman).
Isomerism: geometrical (cis/trans), optical, linkage (NO₂/ONO), ionization.
CFT: Δ_o splits eg (higher) & t2g (lower); Δ_t = 4/9 Δ_o.
Strong field (CN⁻, CO): low spin; weak (I⁻, Br⁻, F⁻): high spin.
Spectrochemical: I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < CN⁻ < CO.

General organic principles

Inductive effect (through σ): −NO₂, −CN, −COOH withdraw; alkyl donates.
Resonance/M-effect (through π): −OH, −NH₂ donate; −NO₂, −CHO withdraw.
Hyperconjugation: σC–H → empty p; explains stability 3°>2°>1° carbocation.
Aromaticity: cyclic, planar, conjugated, (4n+2)π (Hückel).
Reactive intermediates: carbocation, carbanion, free radical, carbene, nitrene.

Hydrocarbons + substitution

Alkanes: sp³, free-radical halogenation (Cl>Br selectivity).
Alkenes: Markovnikov (HBr in +ve), anti-Mark (peroxide, only HBr).
Alkynes: acidic terminal H (pKa ~25); Baeyer's test (KMnO₄).
Aromatic EAS: nitration, sulphonation, halogenation, Friedel–Crafts.
SN1: 3°>2°>1°, polar protic, racemisation, carbocation intermediate.
SN2: 1°>2°>3°, polar aprotic, inversion (Walden), single step.
E1 favours 3°, heat; E2 favours strong base + 2°/3°; Saytzeff more substituted alkene.

Oxygen-containing functional groups

Alcohols: Lucas test distinguishes 1°, 2°, 3° (ZnCl₂/HCl).
Phenols: acidic (pKa ~10), Kolbe → salicylic; Reimer–Tiemann → salicylaldehyde.
Aldehydes/ketones: Tollens (Ag mirror), Fehling (Cu₂O red), only aldehydes give.
Aldol (α-H + base) → β-hydroxy carbonyl → α,β-unsat on heat.
Cannizzaro: no α-H aldehyde + conc. NaOH → alcohol + carboxylate.
Carboxylic acids: HVZ (α-halogenation), decarboxylation with soda lime.

Nitrogen-containing compounds

Amine basicity (aq): 2°>1°>3° (alkyl, steric+solvation); aromatic weaker.
Carbylamine test (only 1° amine + CHCl₃/KOH → isocyanide, foul smell).
Hinsberg: 1° soluble in KOH, 2° insoluble solid, 3° no reaction.
Diazotisation: ArNH₂ + HNO₂/HCl (0–5°C) → ArN₂⁺; Sandmeyer for halides.
Coupling with phenol/aniline → azo dyes (colour).

Biomolecules & polymers

Carbohydrates: aldose/ketose; glucose (D, dextrorotatory) → pyranose.
Reducing sugars have free CHO/CO (glucose, fructose, maltose); sucrose non-reducing.
Proteins: peptide bond; α-helix, β-sheet (2°); denaturation loses 2°/3°.
Enzymes are globular proteins; specificity + rate.
DNA: A–T (2H), G–C (3H); RNA: U instead of T; ribose sugar.
Vitamins: A,D,E,K fat-soluble; B,C water-soluble.
Polymers: addition (PE, PVC), condensation (nylon-6,6, terylene, bakelite).