The trioxocarbonate(IV) of Group-1 metal are known to be relatively more stable to heat as compared to those of group-2 metals which are Jess Stable.
CARBONATES AND HYDROGEN CARBONATES
The trioxocarbonate(IV) of alkali metals except lithium trioxocarbonate(IV) are stable to heat. The trioxocarbonate( IV) of group-2 metals i.e., magnesium and calcium and that of lithium decompose on heating, forming an oxide along with the evolution of carbon dioxide. For example,
Reason. The stability of carbonate towards heat depends upon the relative stability of the resulting metal oxide. More is the stability of the resulting metal oxide lesser is the stability of the carbonate towards heat and vice versa. Now the stability of resulting metal oxides decreases down the group due to decrease in lattice enthalpy, (because of bigger size) therefore the stability of carbonates towards heat increases.
Li2CO3 decomposes on heating because it gives lithium oxide. The small O2- anion is very strongly attracted to Li+ ion resulting in an ionic compound with high lattice energy and therefore it is more stable than Li2CO3 Greater is the stability of resulting oxide more is its tendency of formation and hence lower is the thermal stability of the carbonate.
In period ·3, magnesium trioxocarbonate(IV) decomposes more readily than sodium trioxocarbonate(IV). It is because the resulting Mg O is more stable than MgCO3 because of strong ionic bond between Mg2+ and O2- ions. Thus its lattice energy is high and hence the stability. The stabilities of carbonates of group-2 metals increase on moving down the group. For example, BeCO3 decomposes at 373 K, MgCO3 at 813 K, CaCO3 at 1173 K, SrCO3 at 1563 K and BeCO3 at 1633 K.
Lithium and group-2 metals do not form solid hydrogencarbonates, although they exist in solution. On heating these solutions, the hydrogencarbonates decompose to form carbonates and CO2 gas is liberated. The solid hydrogencarbonates of alkali metals decompose between 375 and 575 K.
2NaHCO3(s) à NaCO3(s) + H2O(g) + CO2(g)
Trioxonitrate(V) of alkali metals (Na, K, etc.), except LiNO3, decompose on strong heating forming nitrites and oxygen. For example,
Trioxonitrate(V) of Mg and Ca metals and LiNO3 decompose on heating to form oxides, nitrogen dioxide and oxygen.