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

(i) Solubility ,(a) Alkali metals . Nitrates , carbonates and sulphates of alkali metal are soluble in water . Their , solubility , however, increases as we move down the group since the lattice enthalpies decrease more rapidly than the hydrogen enthalpies .
(b) Alkaline earth metals. Nitrates of all alkaline earth metals are soluble in water . Their solubility , however , decreases we move down the group because their hydration enthalpies decrease more rapidly than the lattice enthalpies .
The size of `CO_(3)^(2-)` and `SO_(4)^(2-)` anions is much larger than the cations , therefore , within a particular group lattice enthalpies remain almost constant . Since the hydration enthalpies decrease down the group , therefore, the solubility of alkaline earth carbonates and sulphates decrease down the group . However , the hydration enthalphy of `Be^(2+)` and `Mg^(2+)` ions overcome the lattice enthalpy factor and therefore , `BeSO_(4) and MgSO_(4)` are readily soluble in water while the solubility of other sulphates decreases down the group from `CaSO_(4) and BaSO_(4)` .
(ii) 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 , `NO_(2) and O_(2)^(-)` .
`2 M (NO_(3))_(2) overset(Delta)(to) 2 MO + 4 NO_(2) + O_(2) (M = Be , Mg , Ca , Sr` or Ba)
The nitrates of Na , K , Rb and Cs decompose to form metals nitrites and `O_(2)`.
`2 M NO_(3) overset(Delta)(to) 2 MNO_(2) + O_(2) ( M = Na , K , Rb , Cs)`
However , due to diagonal relationship between Li and Mg , lithium nitrate decomposes like `Mg(NO_(3))_(2)` to form metal oxide , `NO_(2)` and `O_(2)` .
`4 Li NO_(3) overset(Delta)(to) 2 Li_(2)O + 4 NO_(2) + O_(2)`
(b) Carbonates . Carbonates of alkaline earth metals decompose on heating to form metal oxide and `CO_(2)`
`MCO_(3) overset(Delta) (to) MO + CO_(2) (M = Be` , Mg, Ca , Sr , Ba)
Further , as the electropositive character of metal increases down the group , the stability of these metal carbonates increases or the temperature of the their decomposition increases as shown below :
`{:(BeCO_(3) ,, MgCO_(3) ,, CaCO_(3) ,, SrCO_(3) ,, BaCO_(3)), ( lt 373 K ,, 813 K ,, 1173 K ,, 1563 K ,, 1633 K):}`
Due to diagonal relationship between Li and Mg `Li_(2)CO_(3)` decomposes in the same way as `MgCO_(3)` .
`Li_(2)CO_(3) overset(Delta) (to) Li_(2)O + CO_(2)`
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 `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 ,
`{:("Compound :" ,, BeSO_(4) ,, MgSO_(4) ,, CaSO_(4) ,, SrSO_(4)) , ("Temp. of decomposition" : ,, 773 K ,, 1168 K ,, 1422 K ,, 1644 K):}`
Among alkali metals due to diagonal relationship , `Li_(2)SO_(4)` decompose like `MgSO_(4)`to from the corresponding metal oxide and `SO_(3)`.
`Li_(2)SO_(4) overset(Delta)(to) Li_(2)O + SO_(3) , MgSO_(4) overset(Delta) (to) MgO + SO_(3)`
Other alkali metal sulphates are stable to heat and do not decompose easily .