Question

Why is li2co3 decomposed at a lower temperature whereas na2co3 at higher temperature?

Answer 1

Sodium is a much more reactive metal than Lithium, hence sodium has more affinity for C and O than lithium. Thus you cannot separate carbonate radical from sodium at low temperatures. In fact, it is not possible to decompose sodium carbonate to sodium oxide and carbon dioxide because sodium carbonate is stable to heat.

Answer 2

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.

Li2CO3  ------  LiO2 +  CO2

Therefore, lithium carbonate decomposes at a low temperature while a stable sodium carbonate decomposes at a high temperature.