Thermodynamics definitions

Lattice enthalpy and Born–Haber cycles

Lattice enthalpy of formation

The enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions under standard conditions.

Lattice enthalpy of dissociation

The enthalpy change when one mole of a solid ionic compound is separated into its gaseous ions under standard conditions.

Enthalpy of atomisation

The enthalpy change when one mole of gaseous atoms is formed from the element in its standard state.

First ionisation energy

The enthalpy change when one mole of gaseous atoms each lose one electron to form one mole of gaseous 1+ ions.

Second ionisation energy

The enthalpy change when one mole of gaseous 1+ ions each lose one electron to form one mole of gaseous 2+ ions.

First electron affinity

The enthalpy change when one mole of gaseous atoms each gain one electron to form one mole of gaseous 1− ions.

Second electron affinity

The enthalpy change when one mole of gaseous 1− ions each gain one electron to form one mole of gaseous 2− ions.

Enthalpy of formation

The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions.

Perfect ionic model

A model of an ionic lattice in which the ions are perfect spheres with evenly spread charge and the only interactions are electrostatic attractions and repulsions.

Polarisation

The distortion of the electron cloud of an anion by a neighbouring cation, giving the bond some covalent character.

Polarising power

The ability of a cation to distort the electron cloud of an anion; it is greater for a smaller cation with a larger charge, a higher charge density.

Polarisability

How easily the electron cloud of an anion is distorted; it is greater for a larger anion with a larger charge.

Enthalpies of solution and hydration

Enthalpy of solution

The enthalpy change when one mole of an ionic solid dissolves in enough water that the dissolved ions are well separated and do not interact with one another.

Enthalpy of hydration

The enthalpy change when one mole of gaseous ions dissolves in water to form one mole of aqueous ions.

Ion–dipole attraction

The electrostatic attraction between an ion and the oppositely charged end of a polar molecule such as water.

Charge density

The charge on an ion relative to its size; a small ion with a large charge has a high charge density.

Solvation

The surrounding of a dissolved ion or molecule by solvent molecules attracted to it; hydration is solvation by water.

Hess's law

The enthalpy change of a reaction is independent of the route taken, provided the initial and final conditions are the same.

Hydrated ion

An ion in aqueous solution surrounded by water molecules held to it by ion–dipole attractions.

Entropy and Gibbs free-energy change

Entropy

A measure of the disorder of a system: the number of ways the particles and their energy can be arranged.

Standard entropy

The entropy of one mole of a substance in its standard state at 100 kPa and a stated temperature, usually 298 K.

Entropy change of a reaction

The sum of the standard entropies of the products minus the sum of the standard entropies of the reactants, each multiplied by its number of moles in the equation.

Gibbs free-energy change

The quantity ΔG = ΔH − TΔS, which combines the enthalpy change, the entropy change and the temperature; a reaction is feasible when it is zero or negative.

Feasible

Able to take place without a continuous input of energy from outside, which is so when ΔG is zero or negative.

Kinetically stable

Describes a feasible reaction mixture that does not react at a measurable rate because its activation energy is high.

Thermodynamically stable

Describes a substance or mixture for which the possible reaction has a positive ΔG, so the reaction is not feasible under the conditions stated.

Entropy change of the surroundings

The entropy change of everything outside the reacting system, equal to −ΔH/T; it is positive for an exothermic reaction.

Total entropy change

The sum of the entropy changes of the system and of the surroundings; a change takes place naturally when it is positive.