MATSEC IM Chemistry key terms

The amount of substance containing 6.02 × 10²³ specified particles (atoms, molecules or ions). It links mass to particle number and is the basis of stoichiometric calculation.

The number of particles in one mole, approximately 6.02 × 10²³ per mole. It converts between particle counts and amounts in moles.

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

The weighted mean mass of the naturally occurring atoms of an element compared with one-twelfth the mass of a carbon-12 atom. It is dimensionless and reflects isotope abundance.

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

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 reactant that is completely used up first and therefore determines the maximum amount of product formed. Other reactants are in excess.

The actual mass of product obtained expressed as a percentage of the theoretical maximum mass predicted by stoichiometry. It is always less than or equal to 100%.

Atoms of the same element with the same number of protons but different numbers of neutrons, hence different mass numbers. They have identical chemical properties but may differ in physical behaviour.

The total number of protons and neutrons in the nucleus of an atom. It is written as a superscript before the element symbol, e.g. ²³Na.

The number of protons in the nucleus of an atom, which defines the element and its position in the periodic table. In a neutral atom it equals the number of electrons.

The electrostatic attraction between oppositely charged ions formed by transfer of electrons from a metal to a non-metal. The resulting lattice is held together by these attractions.

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.

A measure of the tendency of an atom in a molecule to attract the shared pair of electrons towards itself. Differences in electronegativity determine bond polarity.

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. It explains the high boiling points of water and alcohols.

Attractive forces between separate molecules, such as London dispersion forces, permanent dipole–dipole attractions and hydrogen bonds. They are weaker than covalent bonds and govern physical properties.

The enthalpy change when one mole of an ionic compound is formed from its gaseous ions, or the energy needed to separate one mole of the solid into gaseous ions. It indicates ionic bond strength.

The loss of electrons by a species, accompanied by an increase in oxidation number. In terms of oxygen it is gain of oxygen; in terms of hydrogen it is loss of hydrogen.

The gain of electrons by a species, accompanied by a decrease in oxidation number. In terms of oxygen it is loss of oxygen; in terms of hydrogen it is gain of hydrogen.

A reaction in which oxidation and reduction occur simultaneously, with electrons transferred from a reducing agent to an oxidising agent. Oxidation numbers of some elements change.

The charge an atom would carry if its bonds were treated as fully ionic, calculated by assigning shared electrons to the more electronegative atom. It tracks electron transfer in redox reactions.

The voltage of a half-cell measured against a standard hydrogen electrode at 298 K, 1 atm and 1 mol dm⁻³ ion concentration. It is used to predict cell voltages and reaction feasibility.

A device converting chemical energy into electrical energy (galvanic) or the reverse (electrolytic), consisting of two electrodes in contact with electrolytes. Electrons flow through an external circuit.

The decomposition of a compound by passing a direct electric current through it, either molten or in solution. Oxidation occurs at the anode and reduction at the cathode.

The heat energy transferred at constant pressure during a reaction, given the symbol ΔH. It is negative for exothermic and positive for endothermic reactions.

A reaction that releases heat to the surroundings, so the temperature rises and ΔH is negative. The products have less enthalpy than the reactants.

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

A substance that increases the rate of a reaction by providing an alternative pathway with lower activation energy, while remaining chemically unchanged at the end. It does not alter the enthalpy change.

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

The state in a reversible reaction where the forward and reverse rates are equal, so concentrations remain constant although both reactions continue. It is reached only in a closed system.

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

A family of organic compounds with the same functional group, similar chemical properties and a general formula, in which successive members differ by CH₂. Physical properties vary gradually along the series.

An atom or group of atoms in an organic molecule that determines its characteristic chemical reactions, such as –OH in alcohols or –COOH in carboxylic acids.

The existence of two or more compounds with the same molecular formula but different structural or spatial arrangements of atoms, giving different properties. Includes structural and stereoisomerism.

A reaction in which two molecules combine to form a single product, typical of alkenes where the double bond opens. No atom is lost from either reactant.

A reaction in which an atom or group in a molecule is replaced by another atom or group, characteristic of alkanes and halogenoalkanes. A small molecule is often eliminated.

The joining of many small monomer molecules into a large molecule, either by addition across double bonds or by condensation with loss of a small molecule such as water.

An acid that dissociates almost completely in aqueous solution, releasing hydrogen ions, e.g. hydrochloric acid. It gives a low pH and high electrical conductivity.

An acid that only partially dissociates in aqueous solution, establishing an equilibrium between undissociated molecules and ions. It has a higher pH than a strong acid of equal concentration.

A logarithmic scale from 0 to 14 measuring hydrogen ion concentration, defined as –log₁₀[H⁺]. Values below 7 are acidic and above 7 are alkaline at 25 °C.

A solution that resists changes in pH when small amounts of acid or alkali are added, usually containing a weak acid and its conjugate base. It works by removing added H⁺ or OH⁻ ions.

A procedure in which a solution of known concentration is added gradually from a burette to a measured volume of another solution until the reaction is complete, indicated by a colour change.