HSC Chemistry cheat sheet

Electron configuration fills in order 1s 2s 2p 3s 3p 4s 3d 4p; Cr and Cu are exceptions, adopting [Ar]3d5 4s1 and [Ar]3d10 4s1 for extra stability.

Core charge = nuclear charge minus inner-shell electrons; it increases left to right across a period, explaining smaller atomic radius and higher ionisation energy.

Ionisation energy increases across a period and decreases down a group; successive ionisation energies jump sharply at the shell from which the electron is removed.

First ionisation energy drops at group 13 (s to p block) and group 16 (extra electron repulsion in a half-filled p subshell).

Metallic character decreases across a period; oxides shift from basic (Na2O, MgO) through amphoteric (Al2O3) to acidic (SO3, Cl2O7).

Bond polarity follows electronegativity difference: <0.4 nonpolar covalent, 0.4-1.7 polar covalent, >1.7 predominantly ionic (Pauling scale).

VSEPR electron-domain count fixes geometry: 2 linear, 3 trigonal planar, 4 tetrahedral, 5 trigonal bipyramidal, 6 octahedral; lone pairs compress bond angles.

n(NaOH) = c x V and n = m/M underpin titration and gravimetric stoichiometry; the limiting reagent gives the maximum theoretical yield.

Dispersion forces arise from instantaneous dipoles and strengthen with polarisability, which rises with larger electron clouds and greater surface contact.

Dipole-dipole forces act between permanent dipoles; their strength depends on dipole moment magnitude and molecular orientation.

Hydrogen bonds require H bonded to N, O or F and a lone pair on an electronegative atom; typical energy 10-40 kJ/mol, far below covalent bonds.

Boiling point rises with stronger intermolecular forces and larger molar mass; branching lowers boiling point by reducing surface contact.

Like dissolves like: polar solutes dissolve in polar solvents via ion-dipole or hydrogen bonding; nonpolar solutes dissolve in nonpolar solvents.

Solubility of gases in water falls as temperature rises and rises with partial pressure (Henry's law).

Chromatography separates by differential affinity for mobile and stationary phases; Rf = distance travelled by solute / distance travelled by solvent.

Distillation exploits boiling-point differences; fractional distillation is effective when component boiling points differ by roughly 25 degrees Celsius or more.

Enthalpy change: delta H = H(products) - H(reactants); exothermic reactions release energy (delta H negative) and endothermic reactions absorb it (delta H positive).

Hess's law: the enthalpy change of a reaction is independent of pathway, so delta H can be summed over any sequence of steps.

Bond enthalpy route: delta H = sum of bonds broken - sum of bonds formed; a positive result indicates an endothermic reaction.

Collision theory requires sufficient energy (activation energy) and correct orientation; catalysts lower activation energy without changing delta H.

Rate increases with concentration, surface area, temperature and pressure (for gases); temperature raises both collision frequency and the fraction of molecules exceeding activation energy.

Equilibrium is dynamic; Le Chatelier's principle predicts the shift when concentration, pressure or temperature changes, but a catalyst does not shift position.

Equilibrium constant K depends only on temperature; for aA + bB <=> cC + dD, K = [C]^c[D]^d / ([A]^a[B]^b).

Gibbs free energy delta G = delta H - T delta S; a reaction is spontaneous when delta G < 0, favouring high temperature when delta S > 0.

Percentage yield = (actual yield / theoretical yield) x 100; atom economy = (molar mass of desired product / total molar mass of reactants) x 100.

A synthesis pathway is planned backwards from the target molecule, selecting reagents that give high yield, few steps and good selectivity.

Functional group interconversion examples: alcohol to aldehyde with acidified dichromate under distillation; alcohol to carboxylic acid with excess oxidant under reflux.

Reflux heats a reaction mixture with a condenser to prevent loss of volatile reagents; distillation removes a product as it forms to drive equilibrium forward.

Purification sequence: filtration, solvent extraction, drying with anhydrous salt, then recrystallisation or distillation; melting point confirms purity.

Green chemistry principles include atom economy, less hazardous reagents, renewable feedstocks, energy efficiency and waste minimisation.

Safety design: choose the least hazardous reagent and solvent, use a fume cupboard for volatile or toxic substances, and add concentrated acid to water slowly.

Analytical confirmation uses infrared spectroscopy (functional groups), mass spectrometry (molar mass and fragments) and NMR (hydrogen environments).