Why is enthalpy change of hydration exothermic

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.

The approximate hydration energies of some typical ions are listed here. From the above table, an estimate can be made for the hydration energy of sodium chloride.

The hydration energy of an ionic compound consists of two inseparable parts. The first part is the energy released when the solvent forms a coordination compound with the ions. The processes related to these energies are shown below:. The second step is to disperse the ions or hydrated ions into the solvent medium, which has a dielectric constant different from vacuum.

That bears little relationship to the value calculated here! I have no idea what the source of this discrepancy is. One or more of the figures I am using is obviously inaccurate. Trying to find reliable values for energy terms like lattice enthalpies or hydration enthalpies has been a total nightmare throughout the whole of this energetics section. Virtually every textbook I have available and I have quite a few!

Virtually every textbook that you can access via Google Books has different values. Virtually every website that you look at seems to have its own combination of values which may or may not agree with any of the books.

Despite the fact that I am now totally fed up with this whole topic, it shouldn't affect you as a student or a teacher working towards an exam the equivalent of UK A level.

You have to work with whatever values your examiners give you. What is important is that you understand what you are doing. Enthalpies of the solution can be either negative or positive.

In other words, we can say that some ionic substances dissolve endothermically for example, NaCl ; others dissolve exothermically for example NaOH. An infinitely dilute solution can be defined as one where there is a sufficiently large excess of water that adding any more does not cause any further heat to be absorbed or evolved.

The change is slightly endothermic. So it can be said that the temperature of the solution will be slightly lower than that of the original water.

Hydration enthalpy: Hydration enthalpy can be defined as the measure of the energy released when attractions are set up between positive or negative ions and water molecules. The size of the hydration enthalpy is usually governed by the amount of attraction between the ions and the water molecules. The attractions are generally stronger for the smaller ions. For example, the hydration enthalpies fall as and when we go down a group in the Periodic Table.

Solubility depends on dissolution of the solute into the solvent and, like all chemical reactions, is governed by the laws of thermodynamics. In order for any chemical reaction to proceed, it must be thermodynamically favorable. Many factors influence how thermodynamically favorable a given reaction is, including the heat of hydration, or hydration energy released when water solvates, or surrounds, an ion, and the amount of energy required to overcome the attractive forces between solute molecules, known as lattice energy.

Since the coulombic forces that bind ions and highly polar molecules into solids are quite strong, we might expect these solids to be insoluble in most solvents. The attractive interactions between ionic molecules are called the lattice energy, and they must be overcome for a solution to form. Ionic solids are insoluble in the majority of non-aqueous solvents, but they tend to have high solubility specifically in water. The key factor that determines solubility is the interaction of the ions with the solvent.

The electrically-charged ions undergo ion-dipole interactions with water to overcome strong coulombic attraction, and this produces an aqueous solution. The water molecule is polar; it has a partial positive charge on the hydrogens while oxygen bears a partial negative charge.