Predicating Solubility 18 September 201818 September 2018 Learning Objectives; I can define the term enthalpy of hydration of an ion and use it and lattice energy to calculate the enthalpy of solution of an ionic compound (4.4n) I can demonstrate an understanding of the factors that affect the values of enthalpy of hydration and the lattice energy of an ionic compound (4.4o) I can use entropy and enthalpy of solution values to predict the solubility of ionic compounds (4.4p)

This is an equation for sodium chloride, an ionic substance, dissolving in water.

The enthalpy change accompanying this process is called the enthalpy change of solution. It is the enthalpy change when one mole of a compound dissolves to form an ‘infinitely dilute’ solution in water.

The dissolution of sodium chloride can be divided into two processes The dissolution of sodium chloride can be divided into two processes. The first is the breaking up or dissociation of the crystal lattice.

The enthalpy change accompanying this process is called the lattice dissociation enthalpy. It is the reverse of the lattice enthalpy and is an endothermic process.

The second process is the hydration of the separated ions The second process is the hydration of the separated ions. We can divide this into the hydration of the cations and the hydration of the anions.

The enthalpy change accompanying each of these is called the enthalpy of hydration.

It is the enthalpy change when one mole of isolated gaseous ions is dissolved in water to form one mole of aqueous ions.

We can use the enthalpy changes involved in dissolving an ionic compound to construct an enthalpy cycle.

This enthalpy change is the enthalpy change of solution.

We can divide this into two stages We can divide this into two stages. The first stage involves dissociating the ionic lattice to form gaseous ions.

This enthalpy change is the lattice dissociation enthalpy, or the reverse of the lattice enthalpy.

In the second stage the gaseous ions are hydrated to form a solution In the second stage the gaseous ions are hydrated to form a solution. The enthalpy changes here are the enthalpies of hydration for the cations and anions.

This is the enthalpy cycle for the dissolution of sodium chloride.

There are two enthalpies of hydration: one for sodium ions and one for chloride ions.

If the enthalpy values for two of the steps in the cycle are known, we can work out the value of the third step.

For example, the enthalpy of solution of sodium chloride is 786 + (–364) + (–406) = +16 kJ mol–1. This is an endothermic process.