Question

Which alkaline earth metal carbonate decomposes most easily?

• A. \(\text{MgCO}_3\)
• B. \(\text{CaCO}_3\)
• C. \(\text{SrCO}_3\)
• D. \(\text{BaCO}_3\)

Hint:

Thermal stability of group 2 carbonates increases down the group, so MgCO3 decomposes first.

Answer 1

The Correct Option is A

Step 1: Use lattice energy reasoning:
The decomposition product is the oxide $\text{MO}$. The small $\text{Mg}^{2+}$ ion forms an oxide with a very high lattice energy, which pays for the breakdown of the carbonate.

Step 2: Compare the temperatures:
$\text{MgCO}_3$ decomposes at a fairly low temperature. The decomposition temperature rises down the group, and $\text{BaCO}_3$ needs over 1300 degrees C.

Step 3: Pick the answer:
The least stable carbonate is $\text{MgCO}_3$, option A.

Final Answer:
Carbonate stability rises down group 2, so MgCO3 is the least stable. \[ \boxed{\text{(A) }\text{MgCO}_3} \]