What is Hydration Enthalpy?

The quantity of energy produced when a one mole of ions undergoes hydration is hydration energy or hydration enthalpy. Hydration is a kind of solvation. Hydration energy is a form of dissolution of energy where the solvent is water. One of the most challenging components of structural prediction is determining the number of hydration energies. When a salt is dissolved in water, anions and cations react with the water’s positive and negative dipoles. The hydration energy is the result of a trade-off between these interactions and those within the crystalline solid. Wendell Latimer found that hydration energy is related to the ionic radius of anion and cations.

What is Hydration enthalpy?

Hydration enthalpy is defined as the quantity of power/heat/energy produced when 1 mole of the gaseous ions is mixed with H2O (water) to produce hydrated ions.

Let’s take the example of Lithium. It is an alkali metal with a more negative value of power, E–, which denotes high reduction power.

The reducing power is denoted as the sum of 3 energies: sublimation energy, hydration energy, and ionisation energy. 

They are shown below:

First, for sublimation energy

M(s) → M(g) 

Second, for ionisation energy

M(g) → M+( g) 

Third step, for hydration enthalpy

M+(g) + H2O → M+(aq) 

So, here we can say that, 

EO  =  IE+ ΔHn +ΔHs

Where EO =Reducing power

IE = ionisation energy 

ΔHn = Hydration energy 

ΔHs = Sublimation energy

Lithium has a high ionisation enthalpy due to its small size. However, because of its small size, it is heavily hydrated and has a higher hydration enthalpy. The higher enthalpy of the hydration accounts for the significant energy needed to eliminate electrons (in the second step). Hence, Lithium has a greater tendency than other alkali metals to lose electrons in the solution. As a result, Lithium is the most powerful reducing agent.

So, the high reducing power of Lithium is related to its high hydration energy, and Lithium’s large hydration energy is proportional to its smaller size.

Factors affecting the size of hydration enthalpy

Hydration enthalpy is the energy produced when negative or positive ions and water molecules form an attraction. These attractions may be rigorous coordinate covalent connections between the oxygen atoms, positive ions, and water molecules or merely lose ion-dipole attractions. Ion-dipole attractions form between negative ions and positive hydrogen ions in water molecules.

The degree of attraction between both ions and the water molecules usually depends upon the size of the hydration enthalpy.

The force of attraction is generally high in the case of the smaller ions. The hydration enthalpies, for example, decrease as we progress down the Periodic Table. The Lithium ion has the highest hydration enthalpy in Group 1 of the periodic table, whereas the smaller fluoride ion has the maximum hydration enthalpy in Group 7. As the ion size increases, the hydration enthalpy decreases in both groups.

The charge of the ion is directly proportional to the force of attraction between ions, i.e., attraction became stronger with the increase in the charge. For example, Group 2 ions (such as Mg2+) have substantially greater hydration enthalpies than Group 1 ions (such as Na+).

Application of hydration enthalpy

An example of hydration enthalpy is the reaction of water and cement. Because the process is exothermic, a lot of heat is released. In mass constructions such as dams and other structures, where large amounts of cement are utilised to build enormous concrete blocks, the heat emitted becomes significant.

The heat is produced during the setting process. The exterior edges of the bricks cool quicker than the inner edges, resulting in a temperature gradient in the brick that can lead to cracks and structural failures. To avoid this challenge, low-heat types of cement, cement containing pozzolanic admixtures such as fly ash or slag, and concrete prepared with ice rather than water are favoured for enormous constructions.

Conclusion

In the above topic, we discussed hydration enthalpy. It is one of the most crucial concepts of chemistry that is widely used in chemical equations. Hydration enthalpy is defined as the quantity of energy produced when one mole of gaseous ions is mixed with water to produce hydrated ions. The various factors that determine the hydration enthalpy are enthalpy of hydration, ionic radius, and ionic charging. The chemical reactions that produce heat in the presence of water and cement work on the principle of hydration enthalpy.