Question

The correct order of hydration energy of alkali is:

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

Answer 1

Answer: (A)

${Li}^{+}>{Na}^{+}>K^{+}>{Rb}^{+}$

Hydration energy $\propto \frac{Charge}{Size}$

$\therefore L{i}^{+}>N{a}^{+}>K^{+}>R{b}^{+}>C{s}^{+}$

With the increase in the size of the alkali metal, it is difficult for the metal to be surrounded by water molecules. Hence less energy is released when the size increases,

In a group size increases on going down the group from top to bottom $i.e.$,

${Rb}^{+}>K^{+}>{Na}^{+}>{Li}^{+}$

$\therefore$ Increasing order of hydration energy

${Li}^{+}>{Na}^{+}>K^{+}>{Rb}^{+}$

Hence, the correct option is $\text{A}$