HSC Chemistry key terms

The attraction holding atoms together within a single molecule or formula unit, such as a covalent bond, ionic bond or metallic bond. These forces are much stronger than the attractions between separate molecules.

The attraction or repulsion between separate molecules or particles, including dispersion forces, dipole-dipole attraction and hydrogen bonding. These forces determine melting point, boiling point, viscosity and solubility.

A bond formed when two atoms share one or more pairs of electrons. The shared pair is attracted to both nuclei, holding the atoms together in molecules or network structures.

The electrostatic attraction between positively and negatively charged ions arranged in a three-dimensional lattice. It forms when electrons are transferred from a metal to a non-metal.

The attraction between a lattice of positively charged metal ions and a sea of delocalised valence electrons. This bonding explains electrical conductivity, malleability and high melting points of metals.

An uneven sharing of bonding electrons caused by a difference in electronegativity between the bonded atoms. It creates partial positive and partial negative charges along the bond.

A measure of how strongly an atom attracts the shared electrons in a chemical bond. Values increase across a period and decrease down a group in the periodic table.

A quantity describing the separation of charge in a polar bond or molecule, calculated from charge size and separation distance. It is a vector, so molecular shape determines the net value.

A strong intermolecular attraction between a hydrogen atom bonded to nitrogen, oxygen or fluorine and a lone pair on a nearby electronegative atom. It raises boiling points and shapes biomolecules.

A weak, temporary attraction arising from instantaneous fluctuations in electron distribution that create short-lived dipoles. It acts between all molecules and strengthens as electron number increases.

The energy released when one mole of an ionic solid forms from its gaseous ions, or absorbed when the solid is broken apart. Larger charges and smaller ions give more negative values.

The heat change when one mole of a solute dissolves in a large excess of solvent. It is the sum of breaking the lattice and forming solute-solvent attractions.

The mass of one mole of a substance, expressed in grams per mole. It is calculated by summing the relative atomic masses of all atoms in the formula.

The amount of substance containing 6.022 × 10²³ representative particles, such as atoms, molecules or ions. It links mass, particle number and gas volume in calculations.

The quantitative relationship between amounts of reactants and products in a balanced chemical equation. It allows prediction of masses, volumes and concentrations involved in a reaction.

The reactant that is completely consumed first in a reaction, thereby determining the maximum amount of product that can form. Other reactants are said to be in excess.

The actual mass of product obtained expressed as a percentage of the theoretical mass predicted by stoichiometry. Values below 100 percent reflect losses, side reactions or incomplete conversion.

The volume occupied by one mole of a gas under specified conditions, approximately 24.8 litres per mole at 25 °C and one atmosphere. It allows conversion between gas volume and amount.

The relationship pV = nRT linking pressure, volume, amount and absolute temperature of a gas through the gas constant. Real gases approximate it at low pressure and high temperature.

When a system at equilibrium is disturbed by a change in concentration, pressure or temperature, it shifts in the direction that partly opposes the disturbance until equilibrium is restored.

A state in a closed system where forward and reverse reaction rates are equal, so concentrations remain constant even though both reactions continue at the molecular level.

The ratio of product concentrations to reactant concentrations, each raised to its stoichiometric coefficient, at equilibrium for a given temperature. It indicates how far a reaction proceeds.

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

A substance that increases reaction rate by providing an alternative pathway with lower activation energy, without being consumed overall. It does not change the equilibrium position or enthalpy change.

The model stating that particles must collide with sufficient energy and correct orientation for a reaction to occur. It explains how concentration, temperature and surface area affect rate.

The change in concentration of a reactant or product per unit time. It is influenced by concentration, temperature, surface area, pressure and the presence of a catalyst.

The high-energy, unstable arrangement of atoms at the peak of the energy profile, where old bonds are breaking and new bonds are forming. It cannot be isolated.

The heat energy exchanged with the surroundings at constant pressure during a reaction, given the symbol ΔH. Negative values indicate exothermic reactions and positive values endothermic ones.

The total enthalpy change of a reaction is independent of the route taken, provided initial and final states are the same. It allows calculation of enthalpy changes from known values.

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

A measure of the dispersal of energy and matter in a system, related to the number of accessible arrangements. It increases for solids melting, liquids vaporising and gases expanding.

A thermodynamic quantity combining enthalpy and entropy, ΔG = ΔH − TΔS. A negative value indicates a spontaneous process at constant temperature and pressure.

An acid that ionises almost completely in water, producing a high concentration of hydronium ions. Hydrochloric and nitric acids are common examples with very small pH values.

An acid that only partially ionises in water, establishing an equilibrium between intact molecules and ions. Its conjugate base is relatively strong.

Two species differing by a single proton, such as ammonia and the ammonium ion. The stronger the acid, the weaker its conjugate base.

A logarithmic scale of hydrogen ion activity, calculated as the negative logarithm of the hydronium ion concentration. Values below 7 are acidic and above 7 basic at 25 °C.

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

A process in which a species loses electrons, causing its oxidation number to increase. Oxidation of one species is always accompanied by reduction of another in a redox reaction.

A process in which a species gains electrons, causing its oxidation number to decrease. The oxidising agent accepts these electrons and is itself reduced.

A bookkeeping value assigned to an atom assuming bonds are purely ionic, used to track electron transfer. Rules assign zero to elements and sums consistent with overall charge.

A reaction involving simultaneous oxidation and reduction, in which electrons are transferred from a reducing agent to an oxidising agent. Half-equations can describe each process separately.

A specific group of atoms within an organic molecule that gives it characteristic chemical properties and reactivity, such as hydroxyl, carboxyl, amino or ester groups.

A family of organic compounds with the same functional group and general formula, differing by successive CH₂ units and showing gradually changing physical properties.

Compounds with the same molecular formula but different arrangements of atoms, giving different physical or chemical properties. Types include chain, position and functional group isomers.

A large molecule formed by joining many small monomer units through covalent bonds. Addition polymers form from alkenes, while condensation polymers release a small molecule such as water.

A small molecule containing a reactive double bond or two functional groups that can link repeatedly to form a polymer chain during addition or condensation polymerisation.