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

Solubility. (a) Nitrates, carbonates and sulphates of alkali metals are soluble in water. Their solubility in water increases on moving down the group because their lattice enthalpies decrease more rapidly than the hydration enthalpies.
(b) On the other hand, nitrates of alkaline earth metals are soluble in water. However, their solubility decreases down the group because their hydration enthalpies decrease more rapidly than the lattice enthalpies.
The carbonates and sulphates of alkaline earth metals are less soluble than corresponding alkali metals. Their solubility decreases down the group due to decrease in hydration enthalpy.
Thermal stability. (a) Nitrates of both alkali metals and alkaline earth metals decompose on heating. Alkaline earth metals nitrates on heating give metal oxide, `NO_2` and oxygen. The nitrates of Na, K, Rb and Cs decompose to give metal nitrites and oxygen.
`2M(NO_3)_2 overset(Delta)to 2MO + 4NO_2 + O_2` (M=Be, MG,Ca,Sr,Ba)
`2MNO_3 overset(Delta)to 2MNO_2 + O_2` (M=Na,K,Rb,Cs)
Lithium nitrate decomposes like `Mg(NO_3)_2` to form metal oxide , `NO_2 and O_2` .
`4LiNO_3 overset(Delta)to 2Li_2O + 4NO_2 + O_2`
This is because `Li^(+)` being small in size cannot stabilize large `NO_3^(-)` ion.
(b) The carbonate of alkaline earth metals are relatively less stable towards heat and decompose to `CO_2`.
`MCO_3 to MO + CO_3`
(M=Be, Mg, Ca, Sr, Ba )
`{:(bar(BeCO_3" " MgCO_" "CaCO_" "SrCO_3" "BaCO_3)),(lt 373K" "813K" "1173K" "1563K" "1635K),(ul" Thermal stability increases "to" " ):}`
`Li_2CO_3` decmposes similar to `MgCO_3` because of diagonal relationship .
`Li_2CO_3 overset(Delta) to Li_2O +CO_2`
All other alkali metal carbonates are stable and donot decompose readily .
(c) Sulphates of alkaline earth metals decomposes on heating and give `SO_3`.
`MSO_4 overset(Delta)to MO +SO_3`
The stability of sulphates of alkaline earth metals increases as the electropositive characater of the metals increases .
`{:(BeSO_4, MgSO_4, CasO_4, SrSO_4),(773K,1168K, 1422K,1644K):}`
Among alkali metal sulphates `Li_2SO_4` decomposes like `MgSO_4` to give `SO_3` .
`Li_2SO_4 overset(Delta)to Li_2O + SO_3`
`MgSO_4 overset(Delta) to MgO + SO_3`
Other alkali metal sulphates are stable to heat and donot decompose easily.