Hydration Enthalpy and Solubility

The amount of maximum solute that can diffuse in a solvent at a particular temperature is known as solubility. Hydration enthalpy is defined as the quantity of energy produced when 1 (one) mole of the gaseous ions is mixed with H2O (water) to produce hydrated ions. Hydration energy is a major component in the brief analysis of solvation. When salt is dissolved in H2O (water), the topmost ions break away from its lattice & are covered by the water molecules around them. A salt is water-soluble when the hydration value is equal to or greater than lattice energy. 

Hydration Enthalpy and Solubility

The energy needed to convert the one of an ionic solid into its gaseous state is known as the Lattice energy. It is used to determine the ionic bond in the ionic solutions.  Lattice energies could be used to determine the compound solubilities as we assume that Born-Haber theory for dissolution of the salt in H2O (water), and we can predict it as the combination of the two mechanisms:

• the vaporisation of salt to generate gaseous ions, determined by lattice enthalpy
• the process of hydrating the ions to create a solution

The coulombic force of attraction binds the ions in the solute together; to dissolve that soluble substance (in this case, water), the water molecule must overcome the gravitational forces. Lattice enthalpy is the force necessary to cross lines of attraction. Most ionic compounds may not break in waterless solutions yet have high water solubility. The interactions of ions with solvent is one factor that influences salt dissolving. Water interacts with ions and produces a strong bond that releases electrons, as previously mentioned.

Application of Hydration Enthalpy

The process in which a solute in a liquid, solid or gaseous phase completely dissolves in a solvent to produce a solution is known as the dissolution process. The termination process could be known as a combination of two actions.

In the first condition of Hydration,

M+(s) → M+(g)                △HLatt =Lattice enthalpy

In the second condition of Hydration,

M + (g) + H2O → M+(aq)  △HHyd = Hydration enthalpy

Examples of the Hydration Enthalpy 

One of the most suitable examples of the application of hydration enthalpy is the reaction of the cement with water. The reaction between these compounds is exothermic and releases a large amount of heat. 

What are the trends of solubility?

• Increasing configuration inconsistency between the cation and anion leads to higher solubility, so LiI and CsF can be the soluble alkali halides.
• Rising in the covalent bonds leads to lower the salt’s solubility because of the larger EL. For example, AgCl, AgI, AgBr and AGF show lower solubility due to the increasing covalency.

AgF>AgCl>AgBr>AgI

• With the increase in the charge, the solubility of an anion decreases as the rise in EL is highly relative to the increase in the EH.
• Small, polyvalent cations, such as ClO4–, NO3–, I–, acetate, and PF6–form soluble salts with big, univalent anions.

Factors Affecting the Size of Hydration Enthalpy

• The charge of the ion is directly proportional to the force of attraction between the amount of attraction between the ions and the water molecules, 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 (Na+).
• 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 little 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 ions increase bigger, the hydration enthalpy decreases in both groups.
• Ionic radius and Charge affect the enthalpy of Hydration. The factors that affect the enthalpy are radiation and ionic charge. There might be rigorous coordinate covalent connections between oxygen atoms, positive ions, and water molecules, or merely be loose ion-dipole attractions.
• Another crucial factor that affects the hydration enthalpy of elements is the Ionic radius. As we reduce the radius of the ion, ΔHhydθ gets more exothermic. The charge density is exceptionally high in small components, making the ion-dipole attraction stronger between ions and water molecules in the solution. The system becomes hydrated, due to which it releases more energy. As a result, it became more exothermic. 

Conclusion

In the above chapter, we have read the concept, definition, principle, objectives and the application of Hydration Enthalpy and Solubility. Lattice energies could be used to determine the compound solubilities. The solubilities of the atom depend upon various factors such as atomic configuration, covalent bonds, and charges of the atom. Hydration enthalpy and solubility are closely associated with each other.