MATSEC SEC Chemistry key terms

The amount of substance containing 6.02 × 10^23 particles (Avogadro's number); used as the counting unit that links masses of substances to numbers of atoms, ions or molecules in chemical calculations.

The number of particles in one mole of a substance, equal to 6.02 × 10^23 per mole. It converts between numbers of particles and amounts in moles.

The average mass of the atoms of an element compared with one-twelfth the mass of a carbon-12 atom; a ratio with no units, averaged over the natural isotopes of the element.

The sum of the relative atomic masses of all atoms in one molecule of a molecular substance, compared with one-twelfth the mass of a carbon-12 atom; it has no units.

The mass of one mole of a substance, in grams per mole. Numerically it equals the relative atomic or molecular mass of the substance and is used to convert mass to moles.

The volume occupied by one mole of any gas at a stated temperature and pressure; at standard conditions it is about 24 dm^3 per mole.

The formula showing the simplest whole-number ratio of atoms of each element in a compound, obtained from percentage composition or from experimental masses of elements combined.

The formula showing 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 actual mass of product obtained expressed as a percentage of the maximum mass predicted by the balanced equation; it is always less than or equal to 100 percent.

The reactant that is used up first in a reaction and therefore decides the maximum amount of product that can form, even if other reactants remain in excess.

Atoms of the same element with the same number of protons but different numbers of neutrons, so they have the same atomic number but different mass numbers.

The total number of protons and neutrons in the nucleus of an atom; written as a superscript before the element symbol, as in carbon-12.

The number of protons in the nucleus of an atom, which identifies the element and equals the number of electrons in a neutral atom.

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

A bond formed when two atoms share one or more pairs of electrons, usually between non-metal atoms; the shared pair is attracted to both nuclei.

The attraction between positively charged metal ions in a lattice and the delocalised electrons that move freely among them, explaining conduction and malleability.

An atom or group of atoms that has gained or lost electrons and therefore carries a positive charge (cation) or a negative charge (anion).

A measure of the tendency of an atom in a molecule to attract the shared pair of electrons in a covalent bond towards itself; it increases across a period and decreases down a group.

A molecule with an uneven distribution of charge because its polar bonds are not arranged symmetrically, giving it a permanent dipole, as in water.

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

A weak attraction between separate molecules, such as van der Waals forces or hydrogen bonds, which must be overcome when a substance melts or boils.

A reaction in which a substance loses electrons, gains oxygen or loses hydrogen; oxidation and reduction always occur together as a pair.

A reaction in which a substance gains electrons, loses oxygen or gains hydrogen; the opposite of oxidation and always paired with it.

A reaction in which oxidation and reduction take place together, with electrons transferred from the reducing agent to the oxidising agent.

A substance that accepts electrons and so causes another substance to be oxidised; it is itself reduced during the reaction.

A substance that donates electrons and so causes another substance to be reduced; it is itself oxidised during the reaction.

The breakdown of an ionic compound, molten or in aqueous solution, by passing a direct electric current through it using inert or reactive electrodes.

A substance that conducts electricity when molten or dissolved in water because it contains freely moving ions, and is decomposed by the current.

A reaction that releases heat energy to the surroundings, so the temperature rises and the enthalpy change is negative.

A reaction that absorbs heat energy from the surroundings, so the temperature falls and the enthalpy change is positive.

The minimum energy that colliding particles must have for a reaction to occur; it corresponds to the energy barrier between reactants and the transition state on an energy profile.

A substance that speeds up a reaction by providing an alternative route with lower activation energy, but is not used up and is chemically unchanged at the end.

The change in concentration of a reactant or product per unit time; it can be increased by raising temperature, adding a catalyst or increasing surface area.

The state reached in a reversible reaction in a closed system when the forward and reverse reactions occur at equal rates, so concentrations stay constant.

A reaction in which the products can react to re-form the reactants, so it can proceed in both directions and may reach a state of equilibrium.

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

A substance that donates protons (hydrogen ions) to another substance; in water it produces hydrogen ions, giving a pH below 7.

A substance that accepts protons from an acid; soluble bases are called alkalis and give a pH above 7 in water.

A scale from 0 to 14 measuring the concentration of hydrogen ions in a solution: below 7 is acidic, 7 is neutral and above 7 is alkaline.

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

A quantitative procedure in which a solution of known concentration is added from a burette to a measured volume of another solution until the indicator shows the endpoint, allowing concentration to be calculated.

A weak acid or base that changes colour over a particular pH range and is used to show the endpoint of a titration or to test whether a solution is acidic or alkaline.

A family of organic compounds with the same general formula and similar chemical properties, in which each member differs from the next by a CH2 unit.

An atom or group of atoms that gives a family of organic compounds its characteristic chemical properties, such as the hydroxyl group in alcohols or the carboxyl group in acids.

One of two or more compounds with the same molecular formula but different arrangements of atoms, and therefore different structures and properties.

The joining together of many small monomer molecules to form a large molecule called a polymer, either by addition across double bonds or by condensation with loss of small molecules.