Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 12.16 Enthalpy of solution and hydration-Study Notes - New Syllabus

12.16 Enthalpy Change of Solution and Hydration

These enthalpy changes describe the energy changes that occur when ionic compounds dissolve in water. They are essential for understanding solubility and are closely related to lattice energy and ion–water interactions.

(i) Enthalpy Change of Solution (\( \mathrm{\Delta_{sol}H} \))

The enthalpy change of solution is the enthalpy change when one mole of an ionic solid dissolves in water to form an infinitely dilute solution under standard conditions.

Key Features

• Produces aqueous ions.
• Involves both breaking the ionic lattice and hydrating the ions.
• Can be endothermic or exothermic.
• Depends on the balance between lattice energy and hydration enthalpy.

Example

\( \mathrm{NaCl(s) \rightarrow Na^+(aq) + Cl^-(aq)} \)

(ii) Enthalpy Change of Hydration (\( \mathrm{\Delta_{hyd}H} \))

The enthalpy change of hydration of an ion is the enthalpy change when one mole of gaseous ions becomes surrounded by water molecules to form aqueous ions under standard conditions.

Key Features

• Always involves gaseous ions → aqueous ions.
• Always exothermic (\( \mathrm{\Delta H < 0} \)).
• Energy is released due to ion–dipole attractions between ions and water molecules.
• Stronger for ions with higher charge and smaller size.

Examples

\( \mathrm{Na^+(g) \rightarrow Na^+(aq)} \)

\( \mathrm{Cl^-(g) \rightarrow Cl^-(aq)} \)

Relationship Between the Two

\( \mathrm{\Delta_{sol}H = \Delta_{latt}H (breaking) + \sum \Delta_{hyd}H} \)

• Lattice energy (breaking lattice) is endothermic.
• Hydration enthalpies are exothermic.
• The overall enthalpy change depends on which effect is greater.

Key Features

• \( \mathrm{\Delta_{sol}H} \) describes the overall dissolving process.
• \( \mathrm{\Delta_{hyd}H} \) describes interaction between ions and water.
• Hydration is always exothermic.
• Solution enthalpy can be positive or negative.