Question

The correct order of increasing thermal stability of $K_{2}C{O}_3,MgC{O}_3,CaC{O}_3$ and $BeC{O}_3$ is:

• A. $K_{2}C{O}_3<MgC{O}_3<CaC{O}_3<BeC{O}_3$
• B. $BeC{O}_3<MgC{O}_3<K_{2}C{O}_3<CaC{O}_3$
• C. $BeC{O}_3<MgC{O}_3<CaC{O}_3<K_{2}C{O}_3$
• D. $MgC{O}_3<BeC{O}_3<CaC{O}_3<K_{2}C{O}_3$

Answer 1

The correct option is C

$BeC{O}_3<MgC{O}_3<CaC{O}_3<K_{2}C{O}_3$

Between $K_{2}C{O}_3$ and $CaC{O}_3$, the former is much more stable thermally. This is because the potassium cation is 'softer' owing to the larger size plus lower charge density. According to the Hard-Soft Acid-Base or HSAB theory, similar anions and cations - a soft cation paired with a soft anion OR a hard cation paired with a hard anion - come together to produce more stable chemical substances. Hence, the correct order is $BeC{O}_3<MgC{O}_3<CaC{O}_3<K_{2}C{O}_3$

As we go down the group, the thermal stability of alkaline earth carbonates increases. This can also be explained using the HSAB theory.