Question

On heating which of the following,$C{{O}_{2}}$ releases most easily?
A. $MgC{{O}_{3}}$
B. $CaC{{O}_{3}}$
C. ${{K}_{2}}C{{O}_{3}}$
D. $N{{a}_{2}}C{{O}_{3}}$

Answer 1

There are some compounds which break down into smaller compounds by dissociation process. The metal carbonates break down into the metal oxides and the carbon-dioxide gas by dissociation process. The formed products are more stable and so the reaction is favourable.

Complete answer:
-The metal carbonates are less stable and they dissociate themselves in the form of metal oxides releasing the carbon-dioxide gas. This process is also used in metallurgy for the extraction of metals as the metal oxides formed are highly stable and also non-reactive.
-The carbonate which is least stable will lead to the dissociation in the shortest span. Also the energy has to be released during the process and only then can the reaction be said to be feasible.
-There are two different types of energies involved during this reaction. One is the lattice energy which is generated when the oxides are formed. The other is the bond energy of dissociation of the carbonates.
-The overall energy should be released to make the process feasible. It means that the reaction should be exothermic. For that, energy absorbed in making the bonds of the oxides should be much more than the dissociation energy and the electron affinity of the molecules.
-The rate also depends on the stability of the oxides formed. The stability also depends on the size of the atoms along with the energies of bond dissociation and lattice energies.
-Oxygen belongs to period 2 of the periodic table. The metals which have the size similar to that of oxygen will form a better bond with it due to similarity in the size of the orbitals as it will lead to the stronger bond as the orbitals will be at the least distance from one another.
-We know that the size increases down the group and decreases across the period. Among the given options, potassium and calcium belong to period 3 and so they can be directly eliminated as their size will be larger than oxygen.
-Now we are left with two more options which are sodium and magnesium. Both of them belong to the same period as that of oxygen. We know that the size of the atom decreases across the period. So magnesium is smaller than sodium due to effective charge and oxygen is smaller than both of them.
-So the most appropriate choice of the atom is magnesium. It has a similar radius compared to oxygen and forms oxide very easily thus resulting in the formation of carbon-dioxide very easily. Its reaction can be given as
$MgC{{O}_{3}}\xrightarrow{\Delta }MgO+C{{O}_{2}}$

Thus the correct option is A.

Note:
Energy is required to break the carbonate into the oxide and the carbon-dioxide gas. But the required energy in the form of heat is much less than the energy released in the formation of the oxide of magnesium and the formation of carbon-dioxide. So the net energy is released in the process and process is feasible.