• # question_answer 20) Compare the solubility and thermal stability of the following compounds of the alkali metals with those of alkaline earth metals: (a) nitrates (b) carbonates (c) sulphates.

Solubility: (i) Nitrates, carbonates and sulphates of alkali metals are soluble in water. The solubility increases as we move down the group as lattice energies decrease more rapidly than hydration energies due to increase in the sizes of metal ions. (ii) Nitrates of alkaline earth metals are also soluble in water. Their solubility decreases as we move down the group as hydration energies decrease more rapidly than the lattice energies. The sulphates of Be and Mg are soluble in water. The solubility of carbonates and sulphates of alkaline earth metals decrease down the group as hydration energies decrease and lattice energies remain nearly the same down the group as the sizes of$CO_{3}^{2-}$and $SO_{4}^{2-}$anions are much larger than cations. Thermal stability: (i) Except$LiN{{O}_{3}}$, all other alkali metal nitrates decompose into corresponding nitrites and ${{O}_{2}}$. $2MN{{O}_{3}}\to 2MN{{O}_{2}}+{{O}_{2}}(M=Na,K,Rb,Cs)$ $LiN{{O}_{3}}$and all the alkaline earth metal nitrates decompose to form corresponding oxides, $N{{O}_{2}}$and${{O}_{2}}.$ $2M{{(N{{O}_{3}})}_{2}}\to 2MO+2N{{O}_{2}}+{{O}_{2}}$$(M=Be,Mg,Ca,Sr,orBa)$ $4LiN{{O}_{3}}\to 2L{{i}_{2}}O+2N{{O}_{2}}+{{O}_{2}}$ (ii) Except$L{{i}_{2}}C{{O}_{3}}$, all other alkali metal carbonates are thermally stable. $L{{i}_{2}}C{{O}_{3}}$decomposes on heating. $L{{i}_{2}}C{{O}_{3}}\to L{{i}_{2}}O+C{{O}_{2}}.$ Carbonates of alkaline earth metals decompose on heating into corresponding oxides and$C{{O}_{2}}$. Their decomposition temperatures increase gradually from $BeC{{O}_{3}}$ to$BeC{{O}_{3}}$. $MC{{O}_{3}}\to MO+C{{O}_{2}}$ Except $L{{i}_{2}}S{{O}_{4}}$, all other alkali metal sulphates are thermally stable. $L{{i}_{2}}S{{O}_{4}}$decomposes on heating. $L{{i}_{2}}S{{O}_{4}}\to L{{i}_{2}}O+S{{O}_{3}}$ Sulphates of alkaline earth metals decompose on heating giving the corresponding oxide and $S{{O}_{3}}$ $MS{{O}_{4}}\to MO+S{{O}_{3}}$ The decomposition temperatures increase gradually on moving down the group.