sblock Elements 1 Question 10
6. The correct order of hydration enthalpies of alkali metal ions is
(2019 Main, 8 April I)
(a) $\mathrm{Li}^{+}>\mathrm{Na}^{+}>\mathrm{K}^{+}>\mathrm{Cs}^{+}>\mathrm{Rb}^{+}$
(b) $\mathrm{Na}^{+}>\mathrm{Li}^{+}>\mathrm{K}^{+}>\mathrm{Rb}^{+}>\mathrm{Cs}^{+}$
(c) $\mathrm{Na}^{+}>\mathrm{Li}^{+}>\mathrm{K}^{+}>\mathrm{Cs}^{+}>\mathrm{Rb}^{+}$
(d) $\mathrm{Li}^{+}>\mathrm{Na}^{+}>\mathrm{K}^{+}>\mathrm{Rb}^{+}>\mathrm{Cs}^{+}$
Show Answer
Solution:
Key Idea The amount of energy released when one mole of gaseous ions combine with water to form hydrated ions is called hydration enthalpy.
The correct order of hydration enthalpies of alkali metal ions is
$$ \mathrm{Li}^{+}>\mathrm{Na}^{+}>\mathrm{K}^{+}>\mathrm{Rb}^{+}>\mathrm{Cs}^{+} $$
$\mathrm{Li}^{+}$possesses the maximum degree of hydration due to its small size. As a consequence of hydration enthalpy, their mobility also get affected. $\mathrm{Cs}^{+}$has highest and $\mathrm{Li}^{+}$has lowest mobility in aqueous solution.
