Question

Which one of the following statements is NOT correct about the compounds of alkaline earth metals?

• A. Basic nature increases from Mg(OH)₂ to Ba(OH)₂
• B. Thermal stability decreases from BeCO₃ to BaCO₃
• C. Solubility of sulphates in water decreases from BeSO₄ to BaSO₄
• D. Nitrates of these on heating give oxides

Hint:

Thermal stability of carbonates \textbf{increases} down the group due to the larger cation stabilizing the carbonate ion.

Answer 1

The Correct Option is B

Step 1: Analyze Each Statement Statement 1: Basic Nature of Hydroxides - The basicity of hydroxides increases down the group due to decreasing ionization energy and increasing lattice solubility.
- The correct order: \[ \text{Mg(OH)}_2 < \text{Ca(OH)}_2 < \text{Sr(OH)}_2 < \text{Ba(OH)}_2 \] - This statement is correct. Statement 2: Thermal Stability of Carbonates
- Incorrect because the thermal stability of carbonates increases down the group.
- BeCO₃ is the least thermally stable, while BaCO₃ is the most stable.
- Correct order: \[ \text{BeCO}_3 < \text{MgCO}_3 < \text{CaCO}_3 < \text{SrCO}_3 < \text{BaCO}_3 \] - This statement is incorrect, making it the correct answer to the question. Statement 3: Solubility of Sulphates
- Solubility decreases down the group as hydration energy decreases.
- Correct order: \[ \text{BeSO}_4 > \text{MgSO}_4 > \text{CaSO}_4 > \text{SrSO}_4 > \text{BaSO}_4 \] - This statement is correct. Statement 4: Nitrate Decomposition - All alkaline earth metal nitrates decompose on heating to form metal oxides, NO₂, and O₂.
- Example reaction: \[ \text{Ca(NO}_3)_2 \rightarrow \text{CaO} + 2\text{NO}_2 + \frac{1}{2} O_2 \] - This statement is correct. Final Answer: Statement 2 is incorrect. \bigskip

Answer 2

Alkaline earth metals consist of elements in Group 2 of the periodic table: beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). These elements form various compounds, including carbonates (MCO₃), where M represents the metal cation.

One important property of these carbonates is their thermal stability, i.e., their resistance to decomposition upon heating. The general trend of thermal stability of alkaline earth metal carbonates is as follows:

1. Thermal stability increases down the group from BeCO₃ to BaCO₃.
2. This means BaCO₃ is more thermally stable than SrCO₃, which is more stable than CaCO₃, and so on, with BeCO₃ being the least stable.

Reason for the trend:
- The stability of metal carbonates depends on the polarization of the carbonate ion (CO₃²⁻) by the metal cation (M²⁺).
- Smaller cations with higher charge density (like Be²⁺) strongly polarize the carbonate ion, distorting its structure and making it easier to decompose.
- Larger cations (like Ba²⁺) have lower charge density and cause less polarization, resulting in greater stability of the carbonate.

Implications:
- BeCO₃ decomposes easily on heating to give BeO and CO₂.
- BaCO₃ requires much higher temperatures to decompose.

Therefore, the statement "Thermal stability decreases from BeCO₃ to BaCO₃" is incorrect because the actual trend is the opposite:
Thermal stability increases down the group from BeCO₃ to BaCO₃.

Summary:
| Compound | Thermal Stability (Low to High) |
|----------|-------------------------------|
| BeCO₃ | Least stable |
| MgCO₃ | More stable |
| CaCO₃ | More stable |
| SrCO₃ | More stable |
| BaCO₃ | Most stable |

Hence, the correct understanding is:
The thermal stability of alkaline earth metal carbonates increases from BeCO₃ to BaCO₃.
The given statement claiming the opposite is therefore NOT correct.