Question

Why is $L{i_2}C{O_3}$ decomposed at a lower temperature whereas $N{a_2}C{O_3}$ at higher temperature?

Answer 1

To solve this question, we need to understand the factors that influence the thermal decomposition of a given substance. Then we need to compare the values of these factors for the given compounds to explain the given observed phenomenon.

Complete Step-by-Step Answer:
Before we move forward with the solution of the given question, let us first understand some important basic concepts.
Polarisation in a molecule can be explained by the difference in electronegativity of the constituent atoms of the given molecule. The carbonate ion contains carbon and oxygen atoms. These atoms have differences in their electronegativity values, as well as in the form of a carbonate ion, this molecule contains extra lone pairs on the two oxygen atoms. This causes the carbonate ion to be polarised and also be unstable in nature.
To counter the instability of the negative character of the carbonate ion, we need an ion which has a relatively higher value of electro positivity. While comparing lithium and sodium, we can observe that the sodium atom is bigger in size as well as has a higher nuclear charge. Because of this, sodium is more electropositive in nature.
Pertaining to this observation, we can say that the sodium ion is more stable than lithium ion when in a compound with a carbonate ion. And we know that breaking the bonds of a highly stable compound requires more energy than breaking the chemical bonds of a significantly less stable compound.
Hence, the energy required to decompose lithium carbonate is lower than the energy required to decompose sodium carbonate. We know that temperature is a direct translation of energy. Hence, $L{i_2}C{O_3}$ decomposed at a lower temperature whereas $N{a_2}C{O_3}$ at higher temperature

Note: The decomposition temperature of a substance is the temperature at which the substance chemically decomposes. The reaction is usually endothermic as heat is required to break chemical bonds in the compound undergoing decomposition.