s-Block Elements

The correct order of increasing thermal stability of K2CO3, MgCO3, CaCO3 and BeCO3 is

1. BeCO3 < MgCO3 < CaCO3 < K2CO3
2. MgCO3 < BeCO3 < CaCO3 < K2CO3
3. K2CO3 < MgCO3 < CaCO3 < BeCO3
4. BeCO3 < MgCO3 < K2CO3 < CaCO3

Answer

(1) BeCO3 < MgCO3 < CaCO3 < K2CO3

Solution

As per NCERT, the carbonates of alkaline earth metals decompose on heating, MCO3 MO + CO2, and their thermal stability increases down the group with increasing cationic size.

Be2+ is the smallest cation, it polarises CO32- the most, so BeCO3 is the least stable and the order is BeCO3 < MgCO3 < CaCO3.

K+ is large and singly charged (very low polarising power), so K2CO3 does not decompose even at high temperature; alkali metal carbonates are more stable than alkaline earth metal carbonates.

∴ BeCO3 < MgCO3 < CaCO3 < K2CO3

Answer: (1)