Compare the solubility and thermal stability of the following compounds of the alkali metals with those of the alkaline earth metals (a) Nitrates (b) Carbonates (c) Sulphates.

Options:

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Solution:
Solubility :
(a) Alkali metals. Nitrates, carbonates and sulphates of alkali metals are soluble in water. Their, solubility, however, increases as we move down the group since the lattice energies decrease more rapidly than the hydration energies.
(b) Alkaline earth. Nitrates of all alkaline earth metals are soluble in water. Their solubility, however, decreases as we move down the group because their hydration energies decrease more rapidly than the lattice energies. The size of \(\mathrm{CO}_3{ }^{2-}\) and \(\mathrm{SO}_4{ }^{2-}\) anions is much larger than the cations, therefore, within a particular group, lattice energies remain almost constant. Since the hydration energies decrease down the group, therefore, the solubility of alkaline earth carbonates' and sulphates decrease down the group. However, the hydration energy of \(\mathrm{Be}^{2+}\) and \(\mathrm{Mg}^{2+}\) ions overcome the lattice energy factor and therefore, \(\mathrm{BeSO}_4\) and \(\mathrm{MgSO}_4\) are readily soluble in water while the solubility of other sulphates decreases down the group from \(\mathrm{CaSO}_4\) to \(\mathrm{BaSO}_4\). Thermal stability :
(a) Nitrates. Nitrates of both alkali and alkaline earth metals decompose on heating. All alkaline earth metal nitrates decompose to form metal oxide, \(\mathrm{NO}_2\) and \(\mathrm{O}_2\). \(2 \mathrm{M}\left(\mathrm{NO}_3\right)_2 \stackrel{\Delta}{\longrightarrow} 2 \mathrm{MO}+4 \mathrm{NO}_2+\mathrm{O}_2\)
\((\mathrm{M}=\mathrm{Be}, \mathrm{Mg}, \mathrm{Ca}, \mathrm{Sr}\) or \(\mathrm{Ba})\). The nitrates of \(\mathrm{Na}, \mathrm{K}\), \(\mathrm{Rb}\) and \(\mathrm{Cs}\) decompose to form metal nitrites and \(\mathrm{O}_2\).
\(2 \mathrm{MNO}_3 \stackrel{\mathrm{\Delta}}{\longrightarrow} 2 \mathrm{MNO}_2+\mathrm{O}_2\)
\((\mathrm{M}=\mathrm{Na}, \mathrm{K}, \mathrm{Rb}, \mathrm{Cs})\)
However, due to diagonal relationship between \(\mathrm{Li}\) and \(\mathrm{Mg}\), lithium nitrate decomposes like \(\mathrm{Mg}\left(\mathrm{NO}_3\right)_2\) to form metal oxide, \(\mathrm{NO}_2\) and \(\mathrm{O}_2\).
\(4 \mathrm{LiNO}_3 \stackrel{\Delta}{\longrightarrow} 2 \mathrm{Li}_2 \mathrm{O}+4 \mathrm{NO}_2+\mathrm{O}_2\)
(b) Carbonates. Carbonates of alkaline earth metals decompose on heating to form metal oxide and \(\mathrm{CO}_2\)
\(\mathrm{MCO}_3 \underset{\Delta}{\longrightarrow} \mathrm{MO}+\mathrm{CO}_2\)
\((\mathrm{M}=\mathrm{Be}, \mathrm{Mg}, \mathrm{Ca}, \mathrm{Sr}, \mathrm{Ba})\)
Further as the electropositive character of the metal increases down the group, the stability of these metal carbonates increases and hence the temperature of their decomposition increases as shown below:
All other alkali metal carbonates are stable and do not decompose even at high temperatures.
(c) Sulphates. Sulphates of alkaline earth metals decompose on heating giving the oxides and \(\mathrm{SO}_3\). \(\mathrm{MSO}_4 \longrightarrow \mathrm{MO}+\mathrm{SO}_3\)
The temperature of decomposition of these sulphates increases as the electropositive character of the metal or the basicity of the metal hydroxide increases down the group. For example,
\(\begin{array}{lc} \text{Compound: } & \text{Temp. of decomposition:} \\ \mathrm{BeSO}_4 & 773 \mathrm{~K} \\ \mathrm{MgSO}_4 & 1168 \mathrm{~K} \\ \mathrm{CaSO}_4 & 1422 \mathrm{~K} \\ \mathrm{SrSO}_4 & 1644 \mathrm{~K} \\ \text { Among alkali metals due to diagonal relationship, }\end{array}\)
Among alkali metals due to diagonal relationship, \(\mathrm{Li}_2 \mathrm{SO}_4\) decomposes like \(\mathrm{MgSO}_4\) to form the corresponding metal oxide and \(\mathrm{SO}_3\).
Other alkali metals are stable to heat and do not decompose easily.

Topic: Chemistry, s Block Elements