sblock Elements 2 Question 11
11. The alkaline earth metal nitrate that does not crystallise with water molecules, is
(2019 Main, 9 Jan I)
(a) $\mathrm{Ca}\left(\mathrm{NO}{3}\right){2}$
(b) $\mathrm{Sr}\left(\mathrm{NO}{3}\right){2}$
(c) $\mathrm{Ba}\left(\mathrm{NO}{3}\right){2}$
(d) $\mathrm{Mg}\left(\mathrm{NO}{3}\right){2}$
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Answer:
Correct Answer: 11. (b)
Solution:
- A saturated aqueous solution of $M\left(\mathrm{NO}{3}\right){2}$ on crystallisation will produce hydrated crystal, ${ }{M}^{\mathrm{II}}\left(\mathrm{NO}{3}\right){2} \cdot n \mathrm{H}{2} \mathrm{O}$ only when hydration enthalpy ( $\Delta H^{\circ}{ }_{\text {hyd }}$ ) of $M^{2+}$ ion will be appreciably more negative.
Hydration of an ion depends on its size. Smaller the size of an ion, higher will be its charge density and as a result it will remain more solvated (hydrated) through ion dipole interaction.
Size of group-2 metal ions increases on going down the group. So, their ability to form hydrated crystals follows the order:
$$ \mathrm{Be}^{2+}»\mathrm{Mg}^{2+}»\mathrm{Ca}^{2+}»\mathrm{Sr}^{2+}»\mathrm{Ba}^{2+} $$
Thus, $\mathrm{Ba}\left(\mathrm{NO}{3}\right){2}$ is slightly or almost insoluble in water.
