Introduction

The dissolution of ionic compounds in water is a fundamental process in chemistry that involves several key steps and energy changes. This article explores the mechanisms behind dissolution, from breaking crystal lattices to forming hydrated ions.

Lattice Energy: The Starting Point

Ionic compounds exist in crystalline lattices held together by strong electrostatic forces. Breaking these bonds requires energy equal to the lattice energy - the energy released when the ionic compound forms from its constituent ions. For example:

$Na^+(aq) + Cl^-(aq) \rightleftharpoons NaCl(s) \quad \Delta H = -787\: kJ\:mol^{-1}$

Two main factors influence lattice energy:

The Three-Step Dissolution Process

1. Separation of Water Molecules

The first step involves breaking the hydrogen bonds between water molecules. This endothermic process requires energy to overcome intermolecular forces.

2. Dissociation of Ionic Compounds

The ionic compound's crystal lattice breaks apart, separating the ions. This endothermic step requires sufficient energy to overcome the lattice energy.

3. Hydration of Ions

Water molecules surround the separated ions, forming hydration shells. This exothermic process releases energy through ion-dipole interactions.

Energy Changes During Dissolution

Process	Enthalpy Change (ΔH)	Entropy Change (ΔS)
Water molecule separation	Positive (endothermic)	Positive
Ion dissociation	Positive (endothermic)	Positive
Ion hydration	Negative (exothermic)	Negative

Factors Affecting Solubility

Three main factors determine an ionic compound's solubility:

Overall Energy Changes

The total enthalpy change depends on the balance between: