Which of the following arranges the ions in decreasing order of magnitude of enthalpy of hydration?

A $Na X^{+} > Mg X^{2 +} > K X^{+} > Ca X^{2 +}$
B $Ca X^{2 +} > Mg X^{2 +} > Na X^{+} > K X^{+}$
C $Mg X^{2 +} > Na X^{+} > Ca X^{2 +} > K X^{+}$
D $Mg X^{2 +} > Ca X^{2 +} > Na X^{+} > K X^{+}$

Answer: D

Hydration enthalpy rises with charge density. Doubly charged ions are far more strongly hydrated than singly charged ones, and within each charge the smaller ion wins. The order is

Mg X^{2 +}

(about

- 1920

) >

Ca X^{2 +}

(about

- 1580

) >

Na X^{+}

(about

- 406

) >

K X^{+}

(about

- 322

) in

kJ mo l^{- 1}

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Which of the following arranges the ions in decreasing order of magnitude of enthalpy of hydration?

Answer: D

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