Question

The correct sequence of thermal stability of the following carbonates is

• A. ${BaCO3 < CaCO3 < SrCO3 < MgCO3}$
• B. ${MgCO3 < CaCO3 < SrCO3 < BaCO3}$
• C. ${BaCO3 < SrCO3 < CaCO3 < MgCO3}$
• D. ${MgCO3 < SrCO3 < CaCO3 < BaCO3}$

Answer 1

The Correct Option is B

To determine the correct sequence of thermal stability for the given carbonates, we consider the position of the respective elements in the periodic table and their properties.

Thermal Stability of Carbonates: The thermal stability of alkaline earth metal carbonates (group 2 carbonates) increases as we move down the group in the periodic table. This trend is due to the following reasoning:

• The decomposition of carbonate compounds like ${MCO_3}$ occurs according to the reaction: ${MCO_3 \rightarrow MO + CO_2}$.
• As we move down the group, the size of the metal cation increases, weakening the lattice energy of the carbonate. Larger cations stabilize the carbonate ion more effectively due to better accommodation of the charge density.
• The decreasing lattice energy is compensated by increasing polarization of the oxide ion in the product, thereby making thermal decomposition easier for smaller cations.
• Therefore, the order of thermal stability increases from magnesium carbonate (${MgCO_3}$) up to barium carbonate (${BaCO_3}$).

Based on the above understanding, the correct sequence of thermal stability of the carbonates is:

${MgCO_3 < CaCO_3 < SrCO_3 < BaCO_3}$

Therefore, the correct option is B: ${MgCO_3 < CaCO_3 < SrCO_3 < BaCO_3}$.