Why is Li2CO3 decomposed at a lower temperature whereas Na2CO3 at a higher temperature?
Li2CO3 is a covalent compound whereas Na2CO3 is an ionic compound. Therefore, Lattice energy of Na2CO3 is higher than that of Li2CO3. Thus, LiCO3 is decomposed at a lower temperature.
As we move down the alkali metal group, the electropositive character increases. This causes an increase in the stability of alkali carbonates. However, lithium carbonate is not so stable to heat. This is because lithium carbonate is covalent. Lithium-ion, being very small in size, polarizes a large carbonate ion, leading to the formation of more stable lithium oxide.
Therefore, lithium carbonate decomposes at a low temperature while a stable sodium carbonate decomposes at a high temperature.