Hydration

Hydration energy

The solution process involves three steps all including a change in enthalpy.

The first delta H (change in enthalpy) is the process by which water molecules overcome attractive forces in the solute particles to break chemical bonds. This process is endothermic.

The second step delta H 2 is the separation of solvent molecules to accommodate the solute. This step also requires energy and is endothermic (delta H is greater than 0)

The final step is the formation of new attractive interactions between solute and solvent particles and is exothermic (delta H is less than 0).

The sum of delta H 1, 2, and 3 is the overall enthalpy of the solution process and this sum is known as the hydration energy. If the sum of delta H 1 and 2 is greater in magnitude than the delta H 3 (which is a negative number) the overall process will be endothermic. If the sum of delta H 1 and 2 is lesser in magnitude than delta H 3 the overall process will be exothermic.



SourceLink

The formation of a solution involves the interaction of solute with solvent molecules. Many different liquids can be used as solvents for liquid solutions, and water is the most commonly used solvent. When water is used as the solvent, the dissolving process is called hydration.

The interaction between water molecules and sodium ion is illustrated as one of the diagram below.

This is a typical ion-dipole interaction. At the molecular level, the ions interact with water molecules from all directions in a 3-dimensional space. This diagram depicts the concept of interaction only.

The above diagram also display hydrogen-bonding, dipole-dipole, ion-induced dipole, and dipole-induced dipole interactions. In the absence of these interactions, solvation takes place due to dispersion. Definitions of these terms are obvious from the diagrams. The meaning of the words used in the term also hints the nature of the interactions.