3.6 Enthalpy changes for solids and solutions

Lattice enthalpy, ΔH latt

The energy released when one mole of ionic compound is formed from its constituent ions in the gaseous state under standard conditions

Standard enthalpy change of formation

Enthalpy change that occurs when a mole of a compound is formed from its constituent elements in their standard states

Standard enthalpy change of atomisation, ΔHө

at

The enthalpy change that occurs when one mole of gaseous atoms is formed from the elements in their standard state

1st ionisation energy

The energy required to remove one mole of electrons from one mole of atoms in the gaseous state.

1st electron affinity, ΔHө EA1

The enthalpy change that occurs when one mole of gaseous 1- ions are formed from gaseous atoms.

Standard enthalpy change of solution, ΔHөs

The enthalpy change when one mole of a compound dissolves in water under standard conditions.

When an ionic compound dissolves the following happens:

The ionic lattice breaks down into gaseous ions

These gaseous ions are hydrated

Hydration

The formation of solvation shells around the gaseous ions, with the δ+ hydrogen side of water surrounding the cation and the δ- oxygen side of water surrounding the anion

AN EXOTHERMIC PROCESS

Standard enthalpy of hydration, ΔHө hyd

The enthalpy change that occurs when one mole of aqueous ions are formed from their gaseous ions under standard conditions

How does ion size affect lattice enthalpy

Lattice enthalpy becomes less exothermic as the size of the negative ion increases. This is because the ions will be further apart and as such the electrostatic attraction has to act over a greater distance thus reducing its effect.

Smaller ions will have a greater charge density so the force of attraction between the ions will be greater.

How does ion charge affect lattice enthalpy

As the ionic charge increases the lattice enthalpy increases.

As there will be a greater charge density on each positive ion the ions will get closer so the force of attraction will be greater between the ions, making more exothermic

How does ion size affect enthalpy of hydration

As the ion size increases ΔH hyd becomes less exothermic ) as the charge density will decrease making it less attractive to the water molecules and less likely to form a solvation shell

How does ion charge affect enthalpy of hydration

As the ion charge increases ΔH hyd becomes more exothermic

As the ions have a greater charge density, they become more attractive to the water molecules and can get closer to the water molecules forming a tighter solvation shell.