Lattice Enthalpy

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Lattice enthalpy is the enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions.

It is an exothermic change. Lattice enthalpies have a negative sign because energy is given out when ionic bonds are being formed from gaseous ions.

Lattice enthalpy indiactes the strength of an ionic lattice and is a measure of the ionic bond strength. A large negative value for lattice enthalpy shows that there are strong electrostatic forces of attraction between the oppositely charged ions in the lattice.

A covalent substance does not have a lattice enthalpy because there are no ions in its structure.

Lattice enthalpies cannot be measured directly because it is impossible to form one mole of an ionic lattice from gaseous ions.

The standard enthalpy change of formation

One mole of a compound is formed from its constituent elements in their standard states.

This is usually an exothermic process for an ionic compound.

The standard enthalpy change of atomisation

One mole of gaseous atoms is formed from its element in its standard state.

It is always an endothermic process because bonds have to be broken.

First ionisation energy

One mole of gaseous 1+ ions is formed from gaseous atoms.

It is an endothermic process because the electron being lost has to overcome the attraction from the nucleus in order to leave the atom.

Second ionisation energy

One mole of gaseous 2+ ions are formed from one mole of gaseous 1+ ions.

This is an endothermic process as the electron being lost has to overcome the attraction from the nucleus.

First electron affinity

One mole of gaseous 1- ions is formed from gaseous atoms.

This is an exothermic process because the electron is attracted into the outer shell of an atom by the nucleus.

Second electron affinity

One mole of gaseous 2- ions is formed from gaseous 1- ions.

This is an endothermic process because the electron is repelled by the 1- ion. This repulsion has to be overcome.

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