The Question is from neet-chemistry the-s-block-elements , and it was created by the author Mr. S M HUSSAIN

Question

Which of the following statements is INCORRECT regarding the thermal stability of carbonates of alkaline earth metals?

Mr. S M HUSSAIN · Accepted Answer

## CORE INFORMATION
✓ **Answer**: B
## CONCEPT FRAMEWORK
 **Primary Concept**: Thermal stability of alkaline earth metal carbonates
 **Related Topics**: Polarizing power, ionic radius, lattice energy
 **Prerequisites**: Understanding of periodic trends, ionic bonding, and decomposition reactions.
## SOLUTION PATHWAY
**Given Information**:
The question asks to identify the incorrect statement about the thermal stability of alkaline earth metal carbonates ($MCO_3$, where M = Be, Mg, Ca, Sr, Ba).
**Method & Steps**:
1. Analyze each statement individually.
 Reasoning: We need to examine the factors affecting the thermal stability of these carbonates.  Thermal stability is related to the strength of the metal-oxygen bond in the carbonate.  This is influenced by the size and charge of the metal cation.
2. Evaluate the effect of cation size and polarizing power.
 Working: Down the group, the ionic radius of the alkaline earth metal increases.  Larger cations have weaker polarizing power, meaning they less effectively distort the electron cloud of the carbonate anion ($CO_3^{2-}$). Weaker polarization leads to weaker metal-oxygen bonds and thus lower thermal stability.  Conversely, smaller cations like $Be^{2+}$ have high polarizing power, leading to stronger metal-oxygen bonds and higher thermal stability (although $BeCO_3$ is unstable in the usual sense, as it readily decomposes to $BeO$ and $CO_2$).
3. Determine the incorrect statement.
 Result: Option B is incorrect. While $Be^{2+}$ does have high polarizing power, $BeCO_3$ is not more thermally stable than $MgCO_3$.  Both decompose readily, but $BeCO_3$ is much less stable due to its tendency to form basic carbonate.  The other options are correct. Option A reflects the trend of decreasing thermal stability down the group. Option C accurately describes the higher decomposition temperature of $BaCO_3$ compared to $MgCO_3$. Option D correctly states the products of decomposition.
## OPTION ANALYSIS
A ✓:  The thermal stability increases up the group, not down the group. Smaller cations have higher polarizing power leading to higher stability.
B :x: This statement is incorrect.  While $Be^{2+}$ has high polarizing power, this is counteracted by the low stability of $BeCO_3$, making it less stable than $MgCO_3$.
C ✓: This statement is correct. $BaCO_3$ is significantly more thermally stable than $MgCO_3$ due to the larger size and weaker polarizing power of $Ba^{2+}$.
D ✓: This statement is correct.  The general decomposition reaction is $MCO_3(s) ightarrow MO(s) + CO_2(g)$.
## LEARNING AIDS
 **Common Mistakes**:
  1.Confusing polarizing power with ionic radius:  High polarizing power is associated with small cations, not large ones.
  2.Ignoring the overall stability of the carbonate: $BeCO_3$ is an exception due to its inherent instability, not just its polarizing power.
 **Quick Tips**:
  1.Remember the trend of decreasing thermal stability down the group for alkaline earth metal carbonates.
  2.Consider both the polarizing power and the overall stability of the compound when analyzing thermal stability.

Mr. S M HUSSAIN · Answer

The thermal stability increases down the group.

Mr. S M HUSSAIN · Answer

The correct Option is "BeCO$_3$ is more thermally stable than MgCO$_3$ due to the high polarizing power of Be$^{2+}$"

Mr. S M HUSSAIN · Answer

BaCO$_3$ decomposes at a much higher temperature than MgCO$_3$.

Mr. S M HUSSAIN · Answer

The decomposition of these carbonates yields the corresponding metal oxide and CO$_2$.

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