GCE A/L Chemistry key terms

The energy needed to remove one mole of electrons from one mole of gaseous atoms of an element, leaving gaseous 1+ ions. It measures how strongly the nucleus holds the outermost electron.

Half the distance between the nuclei of two identical atoms joined by a single bond, or half the distance between neighbouring atoms in a solid lattice. It decreases across a period and increases down a group.

A measure of the power of an atom in a molecule to attract the shared pair of electrons in a covalent bond towards itself. Values rise across a period and fall down a group.

The regular repeating pattern in physical and chemical properties of elements when they are arranged in order of increasing proton number. It arises because outer electron configurations repeat at intervals.

A covalent bond in which the bonding electrons are shared unequally because the two atoms differ in electronegativity, giving the bond a permanent dipole with partial positive and partial negative ends.

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, ammonia and hydrogen fluoride.

Weak, short-range attractions between molecules caused by temporary or permanent dipoles, including instantaneous dipole–induced dipole forces. They strengthen as the number of electrons and the contact area increase.

A pair of electrons in the outer shell of an atom that is not shared in a covalent bond. Lone pairs repel bonding pairs more strongly, reducing bond angles and allowing the atom to act as a ligand or base.

A covalent bond in which both shared electrons come from the same atom, one donating a lone pair into an empty orbital of another atom or ion. The resulting bond is identical to an ordinary covalent bond.

The number of particles in one mole of a substance, approximately 6.02 × 10²³ per mole. It links the mass of a sample, counted in moles, to the actual number of atoms, ions or molecules present.

The amount of substance that contains as many elementary entities as there are atoms in exactly 12 g of carbon-12. It is the chemist's counting unit linking mass, gas volume and concentration.

The volume occupied by one mole of any gas under stated conditions, about 24 dm³ at room temperature and pressure. It allows volumes of gases to be converted directly into moles.

The relationship pV = nRT, linking pressure, volume, amount in moles, the gas constant and absolute temperature. Real gases approach it at low pressure and high temperature.

The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions. Values are used with Hess's law to find enthalpy changes that cannot be measured directly.

The total enthalpy change for a reaction is the same whatever route is taken, provided the initial and final conditions are identical. It follows from the fact that enthalpy is a state function.

The enthalpy change when one mole of a substance is completely burned in excess oxygen under standard conditions, with all reactants and products in their standard states. It is always exothermic for combustible substances.

The average energy required to break one mole of a particular covalent bond in the gaseous state, with fragments also gaseous. Comparing bonds broken with bonds formed allows estimation of reaction enthalpy.

A state in a reversible reaction where the forward and reverse rates are equal, so concentrations of reactants and products remain constant although both reactions continue. It requires 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 how yields respond to altered conditions.

The ratio of the product of product concentrations to the product of reactant concentrations, each raised to its stoichiometric coefficient, measured at equilibrium. Its value depends only on temperature for a given reaction.

A solution that resists changes in pH when small amounts of acid or base are added or on dilution. It usually contains a weak acid with its conjugate base, or a weak base with its conjugate acid.

The negative logarithm to base ten of the hydrogen ion concentration in mol dm⁻³. It gives a convenient scale from about 0 to 14 for expressing acidity and alkalinity in aqueous solution.

The equilibrium constant for the partial ionisation of a weak acid in water, written Ka. A larger value indicates a stronger acid; its negative logarithm, pKa, is often used instead.

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

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

A substance that increases the rate of a reaction by providing an alternative pathway of lower activation energy, without being consumed in the overall reaction. It does not alter the position of equilibrium.

The potential of a half-cell measured against the standard hydrogen electrode under standard conditions, with all species at unit activity. More positive values indicate greater tendency to be reduced.

The loss of electrons by a species, accompanied by an increase in oxidation number. It occurs at the anode in any electrochemical cell and is always matched by reduction elsewhere.

A reaction in which one species is oxidised and another is reduced, with electrons transferred from the reducing agent to the oxidising agent. Oxidation numbers change during the process.

A device in which a redox reaction either generates electrical energy (galvanic cell) or is driven by an external supply (electrolytic cell). It consists of two electrodes in contact with electrolytes.

An atom or group of atoms within a molecule that gives the compound its characteristic chemical reactions and physical properties, such as the hydroxyl, carbonyl, carboxyl or amino group.

A family of organic compounds with the same functional group and general formula, in which each member differs from the next by a CH₂ unit. Members show gradually changing physical properties and similar chemistry.

The existence of compounds with the same molecular formula but different arrangements of atoms, giving different structural formulae. Types include chain, position and functional group isomerism.

A reaction in which a nucleophile attacks an electron-deficient carbon atom and replaces a leaving group. Haloalkanes reacting with hydroxide ions to form alcohols is a typical example.

A reaction in which an electrophile attacks a region of high electron density, such as a carbon–carbon double bond, and two species add across it. Alkenes reacting with hydrogen halides are typical examples.

An ion or molecule that donates one or more lone pairs to a central metal ion to form a coordinate bond in a complex. Common examples are water, ammonia, chloride and cyanide.

A quantitative procedure in which a solution of known concentration is added from a burette to a measured volume of analyte until the equivalence point is reached, allowing the analyte concentration to be calculated.

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