Question

Which of the following metal carbonates decomposes on heating? (This question has multiple correct options.)
(a)- $L{{i}_{2}}C{{O}_{3}}$
(b)- $N{{a}_{2}}C{{O}_{3}}$
(c)- ${{K}_{2}}C{{O}_{3}}$
(d)- $MgC{{O}_{3}}$

Answer 1

Hint: The carbonates of group 1 (alkali metals) and group 2 (alkaline earth metals) are quite stable. But some having very small size and high lattice enthalpy decomposes on heating. All metal carbonates of group 2 decompose on heating.

Complete answer:
The metal carbonates of groups 1 and 2 are quite stable.
The carbonates of alkali metals are very stable towards heat. $L{{i}_{2}}C{{O}_{3}}$, however, is considerably less stable and decomposes readily on heating. This is because of the fact that small lithium cation easily polarizes the oxygen atom of the nearby larger carbonate ion. As a result, the C-O bond weakens and the Li-O bond strengthens thereby, favoring the formation of $L{{i}_{2}}O$ and $C{{O}_{2}}$ from $L{{i}_{2}}C{{O}_{3}}$. In other words, it is the higher lattice energy of $L{{i}_{2}}O$over that of $L{{i}_{2}}C{{O}_{3}}$which favors the decomposition of $L{{i}_{2}}C{{O}_{3}}$. Further, as the size of the cation increases from sodium ion to cesium ion, the lattice energy of their corresponding oxides decreases and hence the stability of their carbonates increases from $N{{a}_{2}}C{{O}_{3}}\text{ to }C{{s}_{2}}C{{O}_{3}}$
$L{{i}_{2}}C{{O}_{3}}\to L{{i}_{2}}O+C{{O}_{2}}$
The carbonates of all alkaline earth metals decompose on heating to form the corresponding metal oxide and carbon dioxide.
The stability of the metal carbonates of alkaline earth metals increases down the group.
So, the magnesium of this group will decompose to:
$MgC{{O}_{3}}\to MgO+C{{O}_{2}}$
Hence, from the above options (a)- $L{{i}_{2}}C{{O}_{3}}$and (d)- $MgC{{O}_{3}}$ will decompose on heating.

So, the correct options are (a) and (d).

Note: Beryllium element of the second group has a very small size and is higher lattice enthalpy hence beryllium carbonate is very unstable and kept under carbon dioxide.