HKDSE Chemistry key terms

The amount of substance containing 6.02 × 10^23 specified particles (Avogadro's constant). It links mass, number of particles and gas volume in quantitative calculations.

The number of particles in one mole, approximately 6.02 × 10^23 per mole. It converts between counted particles and amount of substance.

The mass of one mole of a substance in grams per mole, numerically equal to its relative molecular mass. Used to convert mass to amount of substance.

The volume occupied by one mole of gas, about 24 dm³ at room temperature and pressure. Used to relate gas volume to amount of substance.

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

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

The weighted average mass of an atom of an element compared with one-twelfth the mass of a carbon-12 atom. It accounts for isotopic abundance.

Atoms of the same element with the same number of protons but different numbers of neutrons, giving different mass numbers but identical chemical properties.

The reactant that is completely used up first in a reaction, determining the maximum amount of product formed. Identified by comparing mole ratios.

The actual mass of product obtained expressed as a percentage of the theoretical maximum mass calculated from the balanced equation. Indicates reaction efficiency.

The strong electrostatic attraction between oppositely charged ions formed when electrons are transferred from a metal to a non-metal. Gives high melting points.

A bond formed by the sharing of one or more pairs of electrons between atoms. It may be single, double or triple, and may be polar or non-polar.

The tendency of an atom to attract the shared electrons in a covalent bond. Differences between atoms determine bond polarity and intermolecular forces.

A strong intermolecular attraction between a hydrogen atom bonded to N, O or F and a lone pair on a nearby N, O or F atom. Raises boiling points.

Weak intermolecular attractions arising from temporary or permanent dipoles between molecules. Their strength increases with molecular size and surface area.

A three-dimensional network of atoms joined by covalent bonds, as in diamond and silicon dioxide. Such substances have very high melting points and are insoluble.

Different structural forms of the same element in the same physical state, such as diamond and graphite for carbon. They differ in bonding and properties.

A state in a reversible reaction where the forward and reverse rates are equal, so concentrations remain constant although both reactions continue.

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

The ratio of product concentrations to reactant concentrations, each raised to its stoichiometric power, at equilibrium. It varies with temperature only.

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

The minimum energy that colliding particles must possess for a reaction to occur. It is lowered by a catalyst.

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

An acid that ionises almost completely in water, such as hydrochloric acid. It gives a low pH and high concentration of hydrogen ions.

An acid that ionises only partially in water, establishing an equilibrium, such as ethanoic acid. It gives a higher pH than a strong acid of equal concentration.

A measure of hydrogen ion concentration, defined as the negative logarithm to base ten of that concentration. Values below 7 indicate acidity.

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 reaction between an acid and a base to form a salt and water. The hydrogen ions and hydroxide ions combine to form water.

A procedure in which a solution of known concentration is added from a burette to a measured volume of another solution until the end-point is reached.

A reaction involving both oxidation and reduction, identified by changes in oxidation numbers or by electron transfer between species.

The charge an atom would carry if electrons in bonds were assigned to the more electronegative atom. Used to identify oxidation and reduction.

A species that accepts electrons and is itself reduced, causing another species to be oxidised. Examples include potassium permanganate and chlorine.

The decomposition of an electrolyte by passing a direct electric current through it, with oxidation at the anode and reduction at the cathode.

A conductor through which current enters or leaves an electrolyte. The anode is where oxidation occurs and the cathode is where reduction occurs.

A family of organic compounds with the same functional group and general formula, showing gradual changes in physical properties.

An atom or group of atoms that determines the characteristic chemical reactions of a molecule, such as hydroxyl, carboxyl or carbonyl groups.

The existence of compounds with the same molecular formula but different arrangements of atoms, giving different structures and properties.

Isomerism in which atoms are joined in the same order but arranged differently in space, including cis-trans and optical isomerism.

A reaction in which unsaturated monomers join together without losing any atoms, forming a long-chain polymer with the same empirical formula as the monomer.

A reaction in which monomers with two functional groups join with the loss of a small molecule such as water, forming a polymer.

A small molecule that can join with many identical or similar molecules to form a polymer through polymerisation.