Cambridge IAL Chemistry key terms

The enthalpy change when one mole of an ionic solid forms from its gaseous ions under standard conditions. It is always exothermic for formation, and its magnitude indicates ionic bond strength.

The enthalpy change when one mole of gaseous atoms forms from the element in its standard state. For diatomic elements it equals half the bond dissociation enthalpy.

An energy cycle applying Hess's law to link lattice energy with atomisation, ionisation, electron affinity and formation enthalpies. It lets you calculate a value that cannot be measured directly.

The enthalpy change when one mole of gaseous atoms each gains one electron to form singly charged negative ions. First electron affinity is usually exothermic; the second is endothermic.

The enthalpy change when one mole of gaseous ions dissolves in water to form aqueous ions. It is exothermic and becomes more negative as ionic charge density increases.

The enthalpy change when one mole of an ionic solid dissolves in enough water to give a solution so dilute that further dilution causes no heat change. It equals hydration plus the reverse of lattice energy.

A measure of the number of ways energy can be distributed among particles in a system, often described as disorder or dispersal of energy. Units are joules per kelvin per mole.

A thermodynamic quantity defined as enthalpy minus the product of temperature and entropy change. A negative value indicates a reaction is thermodynamically feasible under those conditions.

Whether a reaction has a negative Gibbs free energy change, meaning it can occur spontaneously. Feasibility says nothing about rate, so a feasible reaction may be immeasurably slow.

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

The slowest step in a multi-step reaction mechanism, which controls the overall rate. Its reactants determine the experimentally observed rate equation.

A substance that increases reaction rate by providing an alternative pathway of lower activation energy, without being used up overall. It is not included in the overall equation.

A state in a closed system where forward and reverse reactions occur at equal rates, so concentrations of reactants and products remain constant even though both reactions continue.

The value obtained by multiplying product concentrations (each raised to its stoichiometric power) and dividing by the equivalent for reactants, at equilibrium. It depends only on temperature.

When a system at equilibrium is disturbed by a change in conditions, the position of equilibrium shifts to oppose that change. Used to predict effects of temperature, pressure and concentration.

A species that donates a proton to another species. After donation it becomes its conjugate base, which can accept a proton back.

Two species differing by one proton, such as an acid and the base formed when it loses that proton. Every Brønsted-Lowry reaction involves two such pairs.

The negative logarithm to base ten of the hydrogen ion concentration in mol dm-3. It measures acidity on a scale where values below 7 at 25 degrees Celsius are acidic.

A solution that resists pH change when small amounts of acid or alkali are added. It contains a weak acid with its conjugate base, or a weak base with its conjugate acid.

The potential of a half-cell measured under standard conditions, relative to a standard hydrogen electrode assigned zero volts. It is measured with the half-cell as the right-hand electrode.

A device producing electrical energy from a redox reaction, made of two half-cells joined by a salt bridge and a wire. Electrons flow from the more negative electrode to the more positive one.

A redox reaction in which the same element is simultaneously oxidised and reduced. For example, copper(I) ions form copper metal and copper(II) ions in aqueous solution.

An ion or molecule that donates one or more lone pairs of electrons to a central metal ion, forming coordinate bonds. Examples include water, ammonia, chloride and cyanide.

A charged species consisting of a central metal ion surrounded by ligands bonded through coordinate bonds. Its shape and coordination number depend on the ligands present.

A reaction in which one ligand in a complex ion is replaced by another ligand. The coordination number may change if the new ligand is larger or smaller.

The repeating pattern of physical and chemical properties across periods of the periodic table. It arises from the repeating pattern of outer electron configurations.

The energy needed to remove one electron from each atom in one mole of gaseous atoms, forming singly charged positive ions. It shows trends across periods and down groups.

The ability of an atom to attract the pair of electrons in a covalent bond. It increases across a period and decreases down a group in the periodic table.

Attractive forces between separate molecules, including hydrogen bonding, permanent dipole-dipole attraction and instantaneous dipole-induced dipole forces. They are much weaker than covalent bonds.

A form of isomerism in which molecules have the same molecular formula but different structural formulae. Subtypes include chain, position and functional group isomerism.

Isomerism in which molecules have the same structural formula but different spatial arrangements of atoms. It includes geometric (E-Z) isomerism and optical isomerism.

The property of a molecule that is not superimposable on its mirror image, usually because a carbon atom is bonded to four different groups. Such molecules rotate plane-polarised light.

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.

An atom or group of atoms responsible for the characteristic chemical reactions of a molecule. Examples include hydroxyl, carbonyl, carboxyl and amine groups.

A reaction in which a nucleophile attacks an electron-deficient carbon atom and replaces a leaving group. Haloalkanes react this way with hydroxide or cyanide ions.

A species with a lone pair of electrons that attacks an electron-deficient carbon atom. Examples include hydroxide ions, ammonia and cyanide ions.

An electron-deficient species that accepts a pair of electrons from another species. Examples include positive ions and polar molecules such as bromine in electrophilic addition.

A reaction in which a small molecule such as water or hydrogen halide is removed from an organic compound, producing a double bond. Haloalkanes form alkenes this way with hot ethanolic hydroxide.

A procedure in which a solution of known concentration is added from a burette to a measured volume of another solution until the reaction is just complete, often shown by an indicator.

A solution whose concentration is known accurately, prepared by dissolving a weighed mass of solute in a volumetric flask and making up to a fixed volume.

The range within which a measured value is likely to lie, often expressed as plus or minus half the smallest scale division for a single reading or apparatus tolerance.

The digits in a number that carry meaningful information about its precision. A final answer should be quoted to the same number of significant figures as the least precise measurement used.

The uncertainty of a measurement expressed as a percentage of the measured value. It is used to compare the precision of measurements of different sizes.