Question

The correct order of hydration enthalpies of alkali metal ions is

• A. ${\mathrm{Li}}^{+}>{\mathrm{Na}}^{+}>{\mathrm{K}}^{+}>{\mathrm{Cs}}^{+}>{\mathrm{Rb}}^{+}$
• B. ${\mathrm{Na}}^{+}>{\mathrm{Li}}^{+}>{\mathrm{K}}^{+}>{\mathrm{Rb}}^{+}>{\mathrm{Cs}}^{+}$
• C. ${\mathrm{Li}}^{+}>{\mathrm{Na}}^{+}>{\mathrm{K}}^{+}>{\mathrm{Rb}}^{+}>{\mathrm{Cs}}^{+}$
• D. ${\mathrm{Na}}^{+}>{\mathrm{Li}}^{+}>{\mathrm{K}}^{+}>{\mathrm{Cs}}^{+}>{\mathrm{Rb}}^{+}$

Answer 1

The correct option is C

${\mathrm{Li}}^{+}>{\mathrm{Na}}^{+}>{\mathrm{K}}^{+}>{\mathrm{Rb}}^{+}>{\mathrm{Cs}}^{+}$

The explanation for the correct option:

Option (C): ${\mathrm{Li}}^{+}>{\mathrm{Na}}^{+}>{\mathrm{K}}^{+}>{\mathrm{Rb}}^{+}>{\mathrm{Cs}}^{+}$

1. Hydration energy is the amount of energy produced when a mole of an ion dissolves in a large volume of water, generating an infinitely dilute solution.
2. The higher the hydration enthalpy, the smaller the ion. This is because smaller atoms can contain a large number of water molecules and get hydrated. The size of the atom increases and the hydration enthalpy lowers down the group due to the inclusion of extra valence shells.
3. In the group $2$elements of the periodic table, Lithium has the smallest ion hence, it will have the highest hydration enthalpy.
4. Caesium has the largest size in group $2$ and will have minimum hydration energy.
5. Hence, it is the correct option.

The explanation for the incorrect option:

Option (A): ${\mathrm{Li}}^{+}>{\mathrm{Na}}^{+}>{\mathrm{K}}^{+}>{\mathrm{Cs}}^{+}>{\mathrm{Rb}}^{+}$

1. Hydration energy is inversely proportional to the size of the ion.
2. Caesium is the largest element among Lithium, Sodium, Potassium, and Rubidium will have the minimum hydration enthalpy.
3. Hence, it is an incorrect option.

Option (B): ${\mathrm{Na}}^{+}>{\mathrm{Li}}^{+}>{\mathrm{K}}^{+}>{\mathrm{Rb}}^{+}>{\mathrm{Cs}}^{+}$

1. Hydration enthalpy is inversely proportional to the size of the ion.
2. Lithium being the smallest ion in the group will have the highest hydration enthalpy.
3. Hence it is an incorrect option.

Option (D): ${\mathrm{Na}}^{+}>{\mathrm{Li}}^{+}>{\mathrm{K}}^{+}>{\mathrm{Cs}}^{+}>{\mathrm{Rb}}^{+}$

1. Hydration enthalpy is inversely proportional to the size of the ion.
2. Lithium being the smallest ion will have the highest hydration enthalpy, followed by the Sodium ion.
3. Hence it is an incorrect option.

Therefore, the correct order of hydration enthalpies of alkali metal ions is ${\mathrm{Li}}^{+}>{\mathrm{Na}}^{+}>{\mathrm{K}}^{+}>{\mathrm{Rb}}^{+}>{\mathrm{Cs}}^{+}$, option (C) is the correct answer.