Definition of solvation energy:
When a gaseous cation or a gaseous anion interacts with the solvents forming a solvated cation or a solvated anion and in this process energy is liberated, which is called solvation energy. If water is used as a solvent, then gaseous cation or gaseous anion forms hydrated ion, and the liberated energy associated with this process is termed as a hydration energy.
M + (g) + n sol = [M(sol) n] + + Solvation Energy ( released)
X– (g) + n sol = [X(sol) n] – + Solvation Energy ( released)
The ion formed in this process is called a solvated ion ( in the case of H2O, it is called hydrated ion). Here, n is called solvation number. n depends on the size of cation.
When H2O is used as a solvent, we can write the above reactions in the following way:
M + (g) + n H2O = [M(H2O) n] + + Hydration Energy ( released)
X – (g) + n H2O = [X(H2O) n] – + Hydration Energy ( released)
From the above reactions, it can be concluded that the solvation/hydration enthalpy is the energy liberated due to ion-dipole interaction.
Factors affecting the magnitude of Solvation or Hydration energy:
- Charge of ion
- Size of ion
- Dipole moment of solvent
Hydration energy increases with the increasing of charge of cation.
Na+ (422.6 kJ/mole)< Mg2+ (1953.9 kJ/mole) < Al3+ (4694.5 kJ/mole)
In general, when we move left to right in a period of periodic table, this value gradually increases.
Size of ion: Energy decreases with the increasing of cationic size
Li+ (0.68 Angstrom)< Na+ (0.95 Angstrom)<K+ (1.33 Angstrom)<Rb+ (1.48 Angstrom)< Cs+ (1.69 Angstrom)
Trend of hydration energy:
Li+ (531.4 kJ/mole)> Na+ (422.6 kJ/mole)> K+ 9338.9 kJ/mole)> Rb+ 9318.8 kJ/mole)> Cs+ (280.3 kJ/mole)
In case of IIA elements:
Be2+ (0.31 Angstrom)< Mg2+ (0.65 Angstrom)<Ca2+ (0.99 Angstrom)<Sr2+ (1.13 Angstrom)< Ba2+ (1.35 Angstrom)
Be2+ (2494 kJ/mole)> Mg2+ (1953.9 kJ/mole)>Ca2+ (1577 kJ/mole)>Sr2+ (1443 kJ/mole)>Ba2+ (1352 kJ/mole)
For halogens, same trend is also observed:
F– (515 kJ/mole)> Cl– (381 kJ/mole)>Br– (347kJ/mole)>I– (305 kJ/mole)
Solvation or hydration energy also depends on the dipole moments of solvents. In general, the greater the dipole moment the stronger the attraction by an ion and usually, the greater the solvation energy.