Enthalpy of Solution & Hydration

Enthalpy of Solution

The enthalpy of solution is the energy change when 1 mole of an ionic solid dissolves in water to form an aqueous solution of its ions.

It can be calculated using a Hess cycle, because it’s difficult to measure directly.

What steps are involved?

To dissolve an ionic solid like , we imagine it happening in two stages:

1. Break up the solid lattice into separate gaseous ions

→ This is the lattice enthalpy ().

→ Always endothermic ( is positive).

2. Hydrate the gaseous ions by surrounding them with water molecules

→ This is enthalpy of hydration ().

→ Always exothermic ( is negative).

→ This is where the water molecules and ions interact through ion/dipole attractions. Note the on the oxygen oriented towards the cation and the on the anion oriented towards the anion.

Where:

is positive (energy in to break the lattice)

is the sum of negative hydration values

What does the sign of tell you?

• If is negative, the solution process is exothermic → often dissolves easily.

• If is positive, dissolving requires energy → may not dissolve well unless entropy favours it.