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.

Explanation

(a) Alkali metals: Nitrates, carbonates and sulphate 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 metals: Nitrates of all alkaline earth metals are soluble in water. Their solubility, however, we go down the group because their hydration Energies decrease mora rapidly than the lattice energies.
The size of `CO_(3)^(2-)` and`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 `Be^(2+)` and `Mg^(2+)` ons overcome the lattice energy factor and therefore, `BaSO_(4)` and `MgSO_(4)` are readily soluble in weer while the solubility of other sulphates decreases down the group from `CaSO_(4)` to `BeSC_(4)` Thermal stability:
(a) Nitrates: Nitrates of both alkali and alkaline earth metals decompose on heating AB alkaline earth metal nitrates decompose to form metal oxide, `NO_(2)` and `O_( 2)`
`2M (NO_(3))_(2)to2MO+ 4 NO_(2)+O_(2)` (M = Na, K, Rb, Cs)
However, due to diagonal relationship between U and Mg, lithium nitrate decomposes like `Mg(NO_(3))_(2)` to form metal oxide, `NO_(2)` and `O_(2-)`
`4 LINO_(3)overset(Delta)to2LI_(2)O+4 NO_(2)+ O_(2)`
(b) Carbonates: Carbonates of alkaline earth metals decomposes on heating to form metal oxide and `CO_(2+)`
` MCO_(3)toMO+CO_(2)`  ( M Be, Mg, Ca, Sr, Ba)
Further as the electropositive character of the metal increases down the group the stability of these metal carbonates increases or the temperature of their decomposition increases as shown below
`BeCO_(3)`   `MgCO_(3)`   `CaCO_(3)`   `SrCO_(3)`  `BaCO_(3)`
373K   813K  1173K  1563K  1633K  

Due to diagonal relationship between Li and Mg, `Li_(2)CO_(3)` decomposes in the same way as, `MgCO_(3)`
`LiCO_(3)overset(Delta)toLi_(2)O+CO_(2)`
Al other alkali metal carbonates are stable and do not decompose, even al high temperature
(c) Sulphates: Sulphates of alkaline earth metals decompose on heating giving the oxides and `SO_(3)`
` MSO_(4)overset(Delta)toMO+SO_(3)`
The temperature of decomposition of these sulphates increases as the electropositive character of the metal of the basicity of the metal hydroxide increases down the group. For example,
Compound `BeSO_(4)`   `MgSO_(4)`.`CaSO_(4)`  `SrSO_(4)`
  Tomb of decomposition:773K    1168K   1422K  1344K 
Among alkali metals, due to diagonal relationship. LISO, decomposes like `MgSO_(4)` ta form corresponding metal oxide and `SO_(3)`
`LiSO_(4)overset(Delta)toLiO+SO_(3)`,   `MgSO_(4)overset(Delta)toMgO+SO_(3)`
Other skall metal sulphates are stable to heat and do not decomposers easily.