Question

The ion which shows maximum value of hydration energy is:
\[\begin{align}
  & (A)-L{{i}^{+}} \\
 & (B)-{{K}^{+}} \\
 & (C)-N{{a}^{+}} \\
 & (D)-C{{s}^{+}} \\
\end{align}\]

Answer 1

Hint: Hydration energy is the amount of energy released one mole of ions undergoes hydration. Size of cation determines the degree of hydration. small cations have high charge density and large cations have low charge density. Alkali metals have +1 as charge but only size varies. Lithium is smallest and then across group 1, size goes on increasing.

Complete answer:
-In group 1, as no. of shells increases, size increases. In alkali metals, Lithium has the smallest size and has a charge as+1, so it has a high charge density.
-as it has a high charge density so it attracts water molecules towards it. The amount of hydration energy is determined by the strength of attraction between ion and water molecules.
-smaller the ion, stronger is the attraction between ions and water molecules.
-hydration energy is the amount of energy released when the bond between ion and water is broken.
-As lithium has stronger bonds as compared to other cations because of smaller charge, high charge density, high amount of energy is released.
-sodium is larger than lithium, so charge density is lower than lithium, so hydration energy will be less than lithium.
-potassium is larger than sodium and lithium, so hydration energy will be less than both sodium and lithium.
-Cs is largest in size when compared to lithium, sodium and potassium ions so has the weakest bond and lowest hydration energy.
So, Option (A)- $L{{i}^{+}}$ has the maximum value of hydration energy.

Note: Hydration energy is often confused with solvation energy. Solvation energy refers to solvent molecules whereas hydration refers to water molecules. Hydration energy is inversely proportional to size of cation. The hydration energy of cation is greater than anion as in case of cation, lone pairs of oxygen interact with cation and provide stability to water molecules and cation.