Question

Question 10.15 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.

Class XI The s-Block Elements Page 306

Answer 1

1. solubility of 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 decreases more rapidly than the hydration energies.

(b) solubility of Alkaline earth metals : Nitrates, of alkaline earth metals are soluble in water.thier solubility, however decreases as we move down the group because their hydration energies decreases more rapidly than the lattice energies.

2. Thermal stability:(a) Nitrates of alkali and Alkaline earth metals decompose on heating.
All alkaline earth metals Nitrates decompose to form metal oxide, NO2 and O2.
2M(NO3)2 --->2MO + 4NO + O2 (M = Be,Mg,Ca,Sr,or Ba)

The nitrates of Na,K,Rb and Cs decompose to form metal nitrates and O2. e.g., 2MNO3--->2MNO2 + O2 {where M = Na, K, Rb,Cs}

But 4LiNO3 --->2Li2O + 4NO2 + O2
(b) carbonate of Alkaline earth metals decompose 9n heating to form metal oxides and CO2.
Further electropositive character of metal increases down the group, the stability of these metal carbonates increases .
except LiCO3 ,all the other alkali metals carbonates are stable and do not decompose even at high temperature.
Lithium carbonate decompose in Li2O and CO2.

(c) sulphates :- Sulphates of Alkaline earth metals decompose on heating gives the oxides and SO3.
MSO4 --->MO + SO3 {where M=Be,Mg......}
the temperature of decomposition of these sulphates increases as the electropositive character of the metal.

Among alkali metals, except Li(Li2SO4 --->Li2O+SO3),all the other alkali metals are stable and do not decompose easily.