Question 36

Ion having highest hydration enthalpy among the given alkaline earth metal ions is:

Solution

Hydration enthalpy depends mainly on:

$$\text{Charge density} = \frac{\text{charge}}{\text{ionic size}}$$

For alkaline earth metal ions: $$Be^{2+},\ Mg^{2+},\ Ca^{2+},\ Sr^{2+},\ Ba^{2+}$$

All ions have the same charge: $$+2$$

As ionic size increases down the group: $$Be^{2+} < Mg^{2+} < Ca^{2+} < Sr^{2+} < Ba^{2+}$$

Smaller ion has greater attraction for water molecules and hence larger hydration enthalpy.

Therefore: $${Be^{2+}}$$ has the highest hydration enthalpy.

Hence, Option A is correct.