Chemical Principles & Laboratory Techniques

Chemistry bridges atomic structure to macroscopic behavior. Mastery requires understanding bonding, rxn mechanisms, thermo/kinetics, and safe lab practice.

## Atomic Structure & Periodicity

Electron configuration follows Aufbau principle: fill lowest energy orbitals first (1s→2s→2p→3s→3p→4s→3d→4p...). Hund's rule: maximize spin in degenerate orbitals before pairing. Pauli exclusion: max 2e⁻ per orbital w/ opposite spin. Exceptions: Cr is [Ar]3d⁵4s¹ (half-filled stability), Cu is [Ar]3d¹⁰4s¹ (filled stability).

Periodic trends: atomic radius decreases L→R (increasing Zeff), increases top→bottom (new shells). IE increases L→R, decreases top→bottom. EA generally increases L→R (halogens highest). EN follows same trend as IE — F is most electronegative (3.98 Pauling). Metallic character increases bottom-left.

## Chemical Bonding

Ionic: ΔEN > 1.7 typically. Lattice energy (Born-Haber cycle) determines stability. Higher charges and smaller ions → stronger lattice (MgO >> NaCl). Crystal structures: NaCl (6:6 coord), CsCl (8:8), ZnS (4:4).

Covalent: sharing of e⁻ pairs. Lewis structures: count VE, arrange atoms (least EN = central), distribute e⁻ to satisfy octets. Formal charge = VE - lone pairs - ½(bonding e⁻). Minimize FC, prefer negative FC on more EN atom. Resonance structures when multiple valid arrangements exist (NO₃⁻, benzene, ozone).

VSEPR: electron geometry determined by steric number (SN = bonding + lone pairs around central atom). SN2=linear, SN3=trigonal planar, SN4=tetrahedral, SN5=trigonal bipyramidal, SN6=octahedral. Lone pairs compress bond angles (H₂O: SN4 tetrahedral geometry, bent molecular shape, 104.5° not 109.5°).

MO theory: atomic orbitals combine to form bonding (σ, π) and antibonding (σ*, π*) MOs. Bond order = (bonding e⁻ - antibonding e⁻)/2. O₂ has 2 unpaired e⁻ in π* → paramagnetic (Lewis structure incorrectly predicts diamagnetic). Heteronuclear: MOs shifted toward more EN atom.

## Chemical Equilibrium

Law of mass action: for aA + bB ⇌ cC + dD, K = [C]ᶜ[D]ᵈ/([A]ᵃ[B]ᵇ). K >> 1 favors products, K << 1 favors reactants. Kp = Kc(RT)^Δn for gas-phase rxns. Solids and pure liquids excluded from K expression.

Le Chatelier's principle: system at EQ responds to stress by shifting to counteract it. Add reactant → shift right. Increase T: exothermic shifts left, endothermic shifts right. Pressure increase (or volume decrease) shifts toward fewer moles of gas. Catalyst does NOT shift EQ — reaches EQ faster.

Acid-base EQ: pH = -log[H⁺], pOH = -log[OH⁻], pH + pOH = 14 (at 25°C). Strong acids (HCl, HNO₃, H₂SO₄) fully dissociate. Weak acids: Ka = [H⁺][A⁻]/[HA]. Henderson-Hasselbalch: pH = pKa + log([A⁻]/[HA]). Buffer capacity max when pH = pKa ± 1. Titration: equivalence point when moles acid = moles base. Strong acid/strong base → pH 7 at EP. Weak acid/strong base → pH > 7 at EP (conjugate base hydrolysis).

Solubility: Ksp = product of ion concentrations raised to stoichiometric powers. Precipitation occurs when Q > Ksp. Common ion effect decreases solubility. Complex ion formation increases solubility (AgCl dissolves in excess NH₃ via [Ag(NH₃)₂]⁺).

## Thermochemistry & Kinetics

Enthalpy: ΔHrxn = ΣΔHf(products) - ΣΔHf(reactants). Hess's Law: ΔH is state function, path-independent. Bond energies: ΔHrxn ≈ Σ(bonds broken) - Σ(bonds formed). Exothermic: ΔH < 0 (bonds formed > bonds broken).

Gibbs free energy: ΔG = ΔH - TΔS. Spontaneous when ΔG < 0. Four cases: ΔH<0/ΔS>0 always spontaneous, ΔH>0/ΔS<0 never spontaneous, mixed cases depend on T. ΔG° = -RTlnK relates thermodynamics to EQ. ΔG = ΔG° + RTlnQ; at EQ, Q=K, ΔG=0.

Kinetics: rate = k[A]ᵐ[B]ⁿ. Order determined experimentally (method of initial rates). Zero order: [A] = [A]₀ - kt, t½ = [A]₀/2k. First order: ln[A] = ln[A]₀ - kt, t½ = 0.693/k (independent of conc). Second order: 1/[A] = 1/[A]₀ + kt, t½ = 1/(k[A]₀).

Arrhenius equation: k = Ae^(-Ea/RT). Plot ln(k) vs 1/T → slope = -Ea/R. Higher T → faster rxn (more molecules exceed Ea). Catalyst lowers Ea by providing alternative pathway. Two-point form: ln(k₂/k₁) = (Ea/R)(1/T₁ - 1/T₂).

Rxn mechanisms: elementary steps combine to give overall rxn. Rate-determining step (RDS) = slowest step, controls overall rate. Intermediates appear in mechanism but not overall rxn. Steady-state approximation: d[intermediate]/dt ≈ 0 for short-lived intermediates.

## Organic Chemistry Essentials

Functional groups determine reactivity: -OH (alcohol), -COOH (carboxylic acid), -NH₂ (amine), C=O (carbonyl), -X (halide). SN1 vs SN2: SN2 favors primary substrates, strong nucleophile, aprotic solvent (DMF, DMSO, acetone). SN1 favors tertiary, weak nucleophile, protic solvent (water, MeOH). E1/E2 compete w/ substitution.

Stereochemistry: chirality requires 4 different groups on sp3 carbon. R/S assignment via CIP priority rules. SN2 gives inversion (Walden). SN1 gives racemization (planar carbocation intermediate). E/Z for alkene geometric isomers based on CIP priority.

## Laboratory Techniques

Titration: standardize buret, rinse w/ titrant. Read meniscus bottom. Record to 0.01 mL. Calculate molarity from stoichiometry. Indicators: phenolphthalein (pH 8.2-10), methyl orange (pH 3.1-4.4). For weak acid/strong base, use phenolphthalein (EP > 7).

Spectroscopy for identification: IR — broad O-H ~3300 cm⁻¹, sharp C=O ~1700 cm⁻¹, N-H ~3400 cm⁻¹ (two peaks for primary amine). ¹H NMR: chemical shift (δ) indicates environment (TMS reference = 0), splitting pattern reveals neighboring H's (n+1 rule), integration gives H ratio. MS: molecular ion peak gives MW, fragmentation pattern aids structural ID.

Safety: always wear PPE (goggles, gloves, lab coat). Know SDS for all chemicals. Acids to water (never reverse — exothermic mixing). Fume hood for volatile/toxic substances. Know locations: eyewash, safety shower, fire extinguisher, spill kit. Never pipette by mouth. Dispose waste in proper containers — never down the drain without verification.

Gravimetric analysis: precipitate analyte, filter (vacuum filtration faster), wash precipitate, dry to constant mass, calculate % composition. Sources of error: coprecipitation, incomplete precipitation, loss during transfer. Use 10% excess precipitating agent.

Chromatography: TLC for quick analysis — Rf = distance(compound)/distance(solvent front). Column chromatography for preparative separation. HPLC for quantitative analysis — retention time identifies compound, peak area gives quantity. GC for volatile compounds.