Page 19 - index
P. 19

2. s-Block Elements      eLearn.Punjab

  iii) Carbonates

The carbonates of alkali metals are all soluble in water and are
stable towards heat except Li2CO3 which is not only insoluble but
also decompose on heating to lithium oxide. The decomposition is made
easy because the electrostatic attraction in converting from carbonate to
oxide is considerable. In case of large cation like K+ in K2CO3, the gain in
electrostatic attraction is relatively much less and the decomposition
is difficult. Sodium carbonate is very important industrial chemical.
At temperature below 35.2°C, Na2CO3 crystallizes out from water as
Na2CO3.10H2O, which is called washing soda. Above this temperature
it crystallizes as Na2CO3. H2O. On standing in air, Na2CO3.10H2O
slowly loses water and converted to a white powder Na2CO3.H20. The
solution of Na2CO3 in water is basic due to hydrolysis of carbonate ion.

      Na2CO3 (s) + 2H2O() → 2NaOH(aq) + H2CO3 (aq)

Unlike the alkali metal carbonates, the alkaline earth metal
carbonates are only very slightly soluble in water, with the solublity
decreasing down the group. They also decompose on heating
and the ease of decomposition decreases down the group.

              CaCO3(s) → CaO(s) + CO2 (g)

The ease of decomposition can be related to the size of the
metal ion, the smaller the ion, the more is the lattice energy of
the resulting oxide and hence higher the stability of the product.

iv) Nitrates

  Nitrates of both alkali and alkaline-earth metals are soluble in water. Nitrates
  of Li, Mg , Ca and Ba decompose on heating to give O2, NO2and the metallic
  oxide whereas nitrates of Na and K decompose to give different products.

                     19
   14   15   16   17   18   19   20   21   22   23   24