NZ Scholarship Chemistry key terms

The amount of substance containing 6.022 × 10²³ specified entities (atoms, molecules, ions). It links particle counts to measurable masses and volumes, and is the basis for stoichiometric calculation.

The number of constituent particles in one mole, 6.022 × 10²³ mol⁻¹. It converts between counted particles and amount of substance, and its modern definition fixes the mole in the SI system.

The mass of one mole of a substance in grams per mole, numerically equal to its relative molecular mass. It converts between mass and amount of substance in calculations.

The simplest whole-number ratio of atoms of each element in a compound. It is found from percentage composition or combustion data, and may differ from the molecular formula.

The actual number of atoms of each element in one molecule of a compound. It is a whole-number multiple of the empirical formula, found by comparing empirical formula mass with molar mass.

The quantitative relationship between amounts of reactants and products as given by a balanced equation. It allows prediction of masses, volumes, or concentrations consumed or formed in a reaction.

The reactant that is completely consumed first, determining the maximum amount of product formed. Others are in excess; identifying it is essential in stoichiometric calculations.

The actual mass of product obtained expressed as a percentage of the theoretical maximum from stoichiometry. Losses arise from incomplete reactions, side reactions, and handling losses.

The amount of solute per unit volume of solution, commonly in mol L⁻¹. It links measurable volumes to amounts of substance, enabling titration and equilibrium calculations.

A solution whose concentration is accurately known, prepared by weighing a pure solid or diluting a stock solution. It is required for reliable volumetric analysis.

A volumetric procedure in which a solution of known concentration is added from a burette until the reaction with an analyte is just complete. The volume used gives the analyte amount.

The point in a titration where the amounts of reactants added are exactly in the stoichiometric ratio. It is identified experimentally by an indicator colour change or an instrument such as a pH meter.

The observable change, such as an indicator colour change, taken to signal completion of a titration. It should closely match the equivalence point for accurate results.

A weak acid or base whose colour differs between protonated and deprotonated forms. It is chosen so its colour-change range brackets the pH at the equivalence point of a titration.

The heat transferred at constant pressure during a reaction, symbol ΔH. Negative values indicate exothermic reactions releasing energy; positive values indicate endothermic reactions absorbing energy.

The total enthalpy change of a reaction is independent of the route taken. It allows calculation of unknown enthalpy changes by combining known steps, including formation and combustion data.

The enthalpy change when one mole of a compound forms from its elements in their standard states under standard conditions. Values are tabulated and used with Hess's law to find reaction enthalpies.

The average energy required to break one mole of a particular bond in the gaseous state. Summing bonds broken minus bonds formed estimates the enthalpy change of a gas-phase reaction.

The minimum energy colliding particles must possess for a reaction to occur. It corresponds to the energy barrier between reactants and the transition state on a reaction pathway diagram.

The highest-energy arrangement along a reaction pathway, existing momentarily as bonds break and form. Its energy relative to reactants relates to activation energy and reaction rate.

A substance that increases reaction rate by providing an alternative pathway of lower activation energy, without being consumed. It is regenerated and does not change the position of equilibrium.

The change in concentration of a reactant or product per unit time. It depends on concentration, temperature, surface area, and catalysts, and is described by a rate law.

An equation relating reaction rate to reactant concentrations raised to experimentally determined powers. The powers give the order with respect to each reactant and are not read from the equation.

The power to which a reactant concentration is raised in the experimental rate law. It may be zero, first, second, or fractional, and determines how rate responds to concentration changes.

The time taken for the concentration of a reactant to fall to half its initial value. For first-order reactions it is constant and independent of starting concentration, giving half-life = ln2/k.

The slowest step in a multi-step reaction mechanism, which controls the overall rate. The experimental rate law reflects the species involved up to and including this step.

A state in a reversible reaction where forward and reverse rates are equal, so concentrations remain constant while both processes continue. It requires a closed system.

The ratio of product concentrations to reactant concentrations, each raised to its stoichiometric coefficient, at equilibrium. Its value is temperature-dependent and indicates the extent of reaction.

The same ratio as the equilibrium constant but calculated with concentrations at any moment. Comparing it with the equilibrium constant predicts the direction in which the system will shift.

When a system at equilibrium is disturbed by a change in concentration, pressure, or temperature, it shifts to oppose the change. It predicts the direction of shift but not the new constant.

A species that donates a proton to another species. Every donation produces a conjugate base, so acids and bases are defined in pairs within a proton-transfer reaction.

Two species differing by one proton, such as NH₄⁺ and NH₃. Stronger acids have weaker conjugate bases, which explains the direction of proton-transfer equilibria.

The negative logarithm (base 10) of the hydrogen ion concentration in mol L⁻¹. It quantifies acidity, with values below 7 acidic, 7 neutral, and above 7 basic at 25 °C.

The equilibrium constant for loss of a proton from a weak acid, symbol Ka. A larger Ka means a stronger acid; pKa equals −log Ka and is useful for buffer and titration calculations.

A solution of a weak acid and its conjugate base that resists pH change on adding small amounts of acid or base. Its pH is set by the ratio of the two components.

Loss of electrons by a species, accompanied by an increase in oxidation number. Oxidation and reduction always occur together in a redox reaction.

Gain of electrons by a species, accompanied by a decrease in oxidation number. The oxidising agent is itself reduced while causing another species to be oxidised.

A bookkeeping value showing the charge an atom would carry if bonding electrons were fully assigned to the more electronegative atom. It tracks electron transfer in redox reactions.

The potential of a half-cell measured against the standard hydrogen electrode under standard conditions. Values predict whether a redox reaction is spontaneous when two half-cells are combined.

A device converting chemical energy into electrical energy (galvanic) or the reverse (electrolytic). It comprises two half-cells whose electrode potentials determine the cell potential.

An atom or group of atoms within a molecule that gives characteristic chemical reactions and properties, such as hydroxyl, carbonyl, carboxyl, or amine groups. It guides reaction prediction and naming.

The existence of compounds with the same molecular formula but different arrangements of atoms, giving different properties. Types include structural, geometric (cis–trans), and optical isomerism.

The property of a molecule that is not superimposable on its mirror image, usually due to a carbon bearing four different groups. Enantiomers differ in their interaction with plane-polarised light.

A reaction in which two species combine to form one product, with no atom lost. It is characteristic of unsaturated compounds such as alkenes, where the double bond becomes a single bond.

A reaction in which one atom or group in a molecule is replaced by another. It is typical of alkanes and haloalkanes, and may proceed by radical or nucleophilic mechanisms.

The study of how matter interacts with electromagnetic radiation. Techniques such as infrared, nuclear magnetic resonance, and mass spectrometry reveal functional groups, bonding frameworks, and molecular mass.

A technique identifying functional groups from absorption of infrared radiation at frequencies matching specific bond vibrations. Absorptions are reported as wavenumbers and compared with reference data.

A technique in which nuclei such as ¹H absorb radiofrequency radiation in a magnetic field. Chemical shift and splitting patterns reveal the environments and connectivity of atoms in a molecule.

A technique ionising molecules and separating fragments by mass-to-charge ratio. The molecular ion gives molecular mass, and fragmentation patterns help deduce structure.