IB Chemistry HL key terms

The weighted mean mass of an atom of an element compared with one-twelfth the mass of an atom of carbon-12, taking natural isotopic abundance into account.

An atom of the same element with the same number of protons but a different number of neutrons, so it has a different mass number but identical chemical behaviour.

An analytical technique that ionises a sample, separates the ions by mass-to-charge ratio and detects their relative abundance, giving isotopic and structural information.

The sum of the relative atomic masses of all atoms in one molecule of a covalent substance, used to convert between mass and amount in moles.

The amount of substance containing 6.02 × 10^23 specified particles, linking the measurable mass of a sample to the number of atoms, ions or molecules present.

The mass of one mole of a substance in grams per mole, numerically equal to its relative atomic or molecular mass.

The simplest whole-number ratio of atoms of each element in a compound, obtained from percentage composition or combustion data.

The actual number of atoms of each element in one molecule of a compound, which is a whole-number multiple of the empirical formula.

The reactant that is completely consumed first in a chemical reaction and therefore determines the maximum amount of product that can form.

The maximum mass or amount of product calculated from the stoichiometry of the equation when the limiting reactant is fully converted.

The actual yield of product expressed as a percentage of the theoretical yield, reflecting losses and side reactions.

The arrangement of electrons in atomic orbitals, written using subshell notation or an orbital diagram and governed by the Aufbau principle, Hund's rule and the Pauli exclusion principle.

A region of space around a nucleus where the probability of finding an electron is high; each orbital holds at most two electrons of opposite spin.

The minimum energy needed to remove one mole of electrons from one mole of gaseous atoms of an element, giving evidence for shell structure.

A measure of the tendency of an atom in a bond to attract the shared pair of electrons towards itself, increasing across a period and decreasing down a group.

A covalent bond in which the shared electrons are unevenly distributed because the bonded atoms have different electronegativities, creating partial charges.

An attractive force between a hydrogen atom bonded to N, O or F and a lone pair on a nearby electronegative atom, explaining anomalously high boiling points.

A model predicting molecular shape in which electron domains around a central atom repel each other and arrange themselves as far apart as possible.

The description of a species as a hybrid of two or more valid Lewis structures, giving bond lengths intermediate between single and double bonds.

A d-block element that forms at least one ion with a partially filled d subshell, showing variable oxidation states, coloured complexes and catalytic activity.

A species that donates a lone pair of electrons to a central metal ion to form a coordinate bond in a complex.

The number of coordinate bonds from ligands to the central metal ion in a complex, commonly six for octahedral and four for tetrahedral or square planar shapes.

The heat energy exchanged with the surroundings at constant pressure, given a sign by convention: negative for exothermic and positive for endothermic reactions.

The total enthalpy change of a reaction is independent of the route taken, allowing unknown enthalpy changes to be calculated from known ones.

The energy required to break one mole of a particular bond in gaseous molecules, averaged over similar compounds and used to estimate reaction enthalpies.

The minimum kinetic energy colliding particles must possess for a reaction to occur, lowered by catalysts and shown on an energy profile.

The change in concentration of a reactant or product per unit time, often determined from the gradient of a concentration-time graph.

A substance that increases the rate of a reaction by providing an alternative pathway of lower activation energy without being consumed overall.

A state in a closed system where the forward and reverse reactions proceed at equal rates, so concentrations remain constant although both reactions continue.

When a system at equilibrium is disturbed by a change in concentration, pressure or temperature, the position of equilibrium shifts to oppose the change.

The ratio of product concentrations to reactant concentrations, each raised to its stoichiometric coefficient, at equilibrium at a stated temperature.

A species that donates a proton to another species; its conjugate base is what remains after the proton is lost.

The negative logarithm to base ten of the hydrogen ion concentration in mol dm^-3, giving a scale from acidic through neutral to alkaline values.

A solution that resists changes in pH when small amounts of acid or base are added, usually containing a weak acid and its conjugate base.

The potential of a half-cell measured against a standard hydrogen electrode under standard conditions of 298 K, 1 mol dm^-3 and 100 kPa.

A reaction involving simultaneous oxidation, the loss of electrons, and reduction, the gain of electrons, tracked through changes in oxidation number.

A device converting chemical energy into electrical energy, consisting of two half-cells joined by a wire and a salt bridge, with voltage from the difference in electrode potentials.

The range within which a measured value is likely to lie, expressed as an absolute value or percentage and propagated when values are combined.

Variation in measurements caused by unpredictable fluctuations, reducing precision but not necessarily accuracy, and reduced by repeating readings and averaging.

A consistent offset in measurements caused by faulty equipment or flawed method, affecting accuracy in one direction and not removed by repetition.

The digits in a value that carry meaningful information about its precision, used to report calculated results consistently with the measurements they derive from.

A volumetric procedure in which a solution of known concentration is added from a burette until the equivalence point is reached, allowing an unknown concentration to be calculated.

Titres that agree closely, usually within a small fixed range such as 0.10 cm^3, and are averaged to give the most reliable value.

A graph of instrument response against known concentrations, used to determine the concentration of an unknown sample by interpolation.

Identification of the species present in a sample, for example by flame tests or precipitation reactions with specific reagents.

Determination of the amount or concentration of a species in a sample, for example by titration, gravimetric analysis or spectroscopy.

A separation technique in which components distribute differently between a mobile phase and a stationary phase, identified by comparing Rf values with standards.