Hydration enthalpy of which of the following ions is highest?
Hydration enthalpy of which of the following ions is highest?
$\mathrm{Mg}^{2+}$
$\mathrm{Na}^{+}$
$\mathrm{Ca}^{2+}$
$\mathrm{K}^{+}$
Solution
Hydration enthalpy is the amount of energy released when one mole of ions are dissolve in excess of water. Hydration enthalpy increases as the ionic size decreases and its charge increases. $\mathrm{Na}^{+}$and $\mathrm{Mg}^{2+}$ belong to third period and therefore have smaller size than $\mathrm{K}^{+}$and $\mathrm{Ca}^{2+}$ of fourth period. $\mathrm{Mg}^{2+}$ is smaller than $\mathrm{Na}^{+}$and also has higher charge than $\mathrm{Na}^{+}$, therefore $\mathrm{Mg}^{2+}$ will have highest hydration enthalpy.