Question

\[\text{ZnC}{{\text{O}}_{3}}\] is thermally more stable than $\text{MgC}{{\text{O}}_{3}}$ because:
A. $\text{Mg(OH}{{\text{)}}_{2}}$ exhibits only basic properties while $\text{Zn(OH}{{\text{)}}_{2}}$ is atmospheric.
B. polarising action of $\text{Z}{{\text{n}}^{2+}}$ with 18-electron configuration on the anion is larger than that of $\text{M}{{\text{g}}^{2+}}$ with a noble gas electron configuration and of same size and charge.
C. both (a) and (b) are correct.
D. none of the above is correct.

Answer 1

Hint: For this problem, we have to study the concept of the effect of polarising power on the lattice energy as well as on the thermal stability of the molecule. After which we will be able to choose the correct option for the thermal stability of zinc carbonate.

Complete Step-by-Step answer:
- In the given question we have to explain the reason behind more thermal stability of the zinc carbonate than the magnesium carbonate.
- As we know that thermal stability of a molecule is defined as the ability of the molecule to resist the heat and remain stable.
- Now, polarising power is the ability of a cation molecule to attract an anion or an electron cloud and it depends on the size of the cation as well as the size of the anion.
- The concept of polarising power was given by Fajan and commonly known as Fajan's rule.
- Now, as we know that the size of zinc is more than the magnesium due to the presence of extra outermost shell.
- So, the size of the cation $\text{Z}{{\text{n}}^{2+}}$ is greater than the cation $\text{M}{{\text{g}}^{2+}}$, which means that the polarising power of zinc ion will be greater.
- Due to the greater polarising power, zinc will attract the electron cloud more strongly than the magnesium due to which the bond formed by zinc will be stronger.
- Due to the strong bond formation, the lattice energy of the molecule will also increase, and which increases the stability of the molecule.
Therefore, option B is the correct answer.

Note: Lattice energy is defined as the energy that is required to break the bond between solids. More will be lattice energy required, more will be the bond strength.