(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 .
