WBCHSE HS Chemistry key terms

Rule stating that electrons occupy the lowest-energy orbitals available before filling higher-energy ones, giving the order 1s, 2s, 2p, 3s and so on, and thereby predicting ground-state configurations of atoms and ions.

Rule that degenerate orbitals of a subshell are singly occupied with parallel spins before any pairing occurs, because this arrangement minimises electron-electron repulsion and gives the most stable configuration.

Statement that no two electrons in an atom can share all four quantum numbers, so an orbital holds at most two electrons and they must have opposite spins.

Mixing of atomic orbitals of similar energy on the same atom to give new equivalent orbitals that explain observed molecular geometry, such as sp3 tetrahedral, sp2 trigonal planar and sp linear arrangements.

Model predicting molecular shape by arranging electron pairs around a central atom as far apart as possible, then distinguishing bonding pairs from lone pairs, which repel more strongly and compress bond angles.

Number of electron pairs shared between two bonded atoms; in molecular orbital theory it equals half the difference between bonding and antibonding electrons, and higher values mean shorter, stronger bonds.

Product of the magnitude of a charge separation and the distance between the charges, measured in debye; it indicates molecular polarity and is zero for symmetric molecules whose bond dipoles cancel.

Attractive force between a hydrogen atom covalently bound to a highly electronegative N, O or F atom and a lone pair on a neighbouring such atom, explaining the high boiling point of water and alcohols.

Energy conservation statement for a system: the change in internal energy equals heat supplied to the system plus work done on it, conventionally written ΔU = q + w with sign conventions fixed.

Heat exchanged at constant pressure, defined as H = U + pV; the change in enthalpy equals the heat absorbed or released in most laboratory reactions, making it the usual measure of reaction heat.

Because enthalpy is a state function, the enthalpy change of a reaction is the same whether it occurs in one step or several, so unknown reaction enthalpies can be found from known formation or combustion data.

Measure of the dispersal of energy in a system, related to the number of accessible microstates; it increases for spontaneous processes in an isolated system, as when a solid melts or a gas expands.

Combined thermodynamic function G = H − TS; a process at constant temperature and pressure is spontaneous when ΔG is negative, which requires a favourable balance of enthalpy and entropy terms.

When a system at equilibrium is disturbed by a change in concentration, pressure or temperature, the position of equilibrium shifts in the direction that opposes the change, as far as the system can.

Ratio of product concentrations to reactant concentrations, each raised to its stoichiometric coefficient, at equilibrium for a given temperature; a large value means products are favoured.

Solution that resists change in pH on adding small amounts of acid or base, usually made from a weak acid and its conjugate base, or a weak base and its conjugate acid.

Relationship giving the pH of a buffer as pKa plus the logarithm of the ratio of conjugate base concentration to weak acid concentration; it also predicts the pH of salt solutions of weak acids.

Equilibrium constant for the dissolution of a sparingly soluble salt, equal to the product of the concentrations of its ions raised to their stoichiometric powers in a saturated solution at a stated temperature.

Suppression of ionisation of a weak electrolyte when a strong electrolyte supplying one of the same ions is added, because the concentration quotient now exceeds the equilibrium constant and equilibrium shifts back.

Formal charge assigned to an atom in a compound by treating bonds as fully ionic, with rules such as oxygen −2 and hydrogen +1; changes in it identify oxidation and reduction in a reaction.

Procedure that splits a redox reaction into oxidation and reduction half-reactions, balances atoms and charges by adding water and electrons, then combines them so electrons cancel.

Equation expressing reaction rate as a constant times reactant concentrations raised to experimentally determined powers; the sum of these powers gives the order of the reaction.

Time required for the concentration of a reactant to fall to half its initial value; for a first-order reaction it is independent of starting concentration and equals 0.693 divided by the rate constant.

Minimum extra energy reactant molecules must possess for collisions to produce products; it equals the height of the energy barrier between reactants and the transition state, and catalysts lower it.

Two kinds of adsorption: physisorption involves weak van der Waals forces, is reversible and has low enthalpy; chemisorption involves chemical bond formation, is often irreversible and much stronger.

Properties of dilute solutions depending on the number of solute particles rather than their nature: relative lowering of vapour pressure, elevation of boiling point, depression of freezing point and osmotic pressure.

Ratio of the observed colligative effect to the value expected for a non-dissociating solute; it exceeds one for electrolytes that dissociate and falls below one for solutes that associate.

Excess pressure that must be applied to a solution to stop solvent passing through a semipermeable membrane into it; for dilute solutions it equals CRT, where C is molar concentration.

Potential of a half-cell measured against the standard hydrogen electrode at 298 K with unit activities; more positive values indicate greater tendency to be reduced and act as oxidising agents.

Relation giving electrode potential as standard potential minus RT/nF times the natural logarithm of the reaction quotient, showing how concentration and temperature shift cell voltage from standard conditions.

List of elements or ions arranged by standard reduction potential; it predicts whether a displacement reaction occurs, which electrode is the anode, and the feasibility of a cell reaction.

Splitting of the d orbitals of a metal ion into different energy levels by the electrostatic field of surrounding ligands; the size of the gap decides colour and magnetic behaviour of complexes.

Ordering of ligands by their ability to split d-orbital energies, from weak-field iodide to strong-field cyanide; strong-field ligands favour low-spin configurations in octahedral complexes.

Number of donor atoms directly bonded to the central metal ion in a complex; common values are six for octahedral and four for tetrahedral or square planar geometries.

Ion or molecule that donates one or more electron pairs to a central metal ion, forming a coordinate bond; examples include ammonia, water, chloride and ethylenediamine.

Two substitution pathways: SN1 proceeds through a carbocation intermediate in two steps with racemisation, while SN2 is a single-step backside attack with inversion and depends strongly on steric hindrance.

When hydrogen halide adds to an unsymmetrical alkene, the hydrogen attaches to the carbon bearing more hydrogens, giving the more stable carbocation intermediate and hence the major product.

Reaction of a carboxylic acid with an alcohol in the presence of an acid catalyst to form an ester and water; it is reversible and slow, with concentrated sulphuric acid absorbing water.

Test using benzenesulphonyl chloride to distinguish primary, secondary and tertiary amines: primary gives a soluble sulphonamide with alkali, secondary an insoluble one, tertiary no reaction.

Ion carrying both positive and negative charges on different sites within the same molecule, as in amino acids at their isoelectric point, where the net charge is zero.

Amide linkage formed between the carboxyl group of one amino acid and the amino group of another with loss of water; repeated linkages build the polypeptide chain of proteins.

Two routes to polymers: addition joins unsaturated monomers without by-products, while condensation joins monomers with loss of small molecules such as water, as in nylon and polyester.

Process of heating natural rubber with sulphur to form cross-links between polymer chains, increasing strength, elasticity and resistance to temperature change.

Polymer that microorganisms can break down into harmless products, such as polyhydroxybutyrate or polylactic acid, offering an alternative to persistent plastics derived from petroleum.

Unusual properties of water, including maximum density at 4 °C, high boiling point and high specific heat, explained by extensive hydrogen bonding among molecules.

Scattering of light by colloidal particles, making a beam visible as a bright path; it distinguishes a colloid from a true solution, whose particles are too small to scatter light.

Model in which atomic orbitals combine to give bonding and antibonding molecular orbitals spread over the whole molecule; it explains paramagnetism of oxygen and bond order in diatomic species.