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: Understanding the Concept:
Group 2 carbonates decompose on heating: \(\text{MCO}_3 \rightarrow \text{MO} + \text{CO}_2\). The ease of decomposition depends on how strongly the metal ion polarises the carbonate ion.

Step 2: Apply the trend:
A small, highly charged cation polarises the large \(\text{CO}_3^{2-}\) anion strongly and weakens a C-O bond, so the carbonate breaks down at a lower temperature. \(\text{Mg}^{2+}\) is the smallest of the four ions here.

Step 3: Rank the carbonates:
Thermal stability: \(\text{MgCO}_3 < \text{CaCO}_3 < \text{SrCO}_3 < \text{BaCO}_3\). So \(\text{MgCO}_3\) decomposes most easily.

Step 4: Why the other options are wrong.
\(\text{CaCO}_3\), \(\text{SrCO}_3\) and \(\text{BaCO}_3\) have larger cations, polarise carbonate less, and need higher temperatures to decompose.

Final Answer:
Magnesium carbonate decomposes most easily. \[ \boxed{\text{(A) }\text{MgCO}_3} \]