Which of the following carbonate is thermally most unstable?

Options

A. `BeCO_(3)`
B. `CaCO_(3)`
C. `K_(2)CO_(3)`
D. `Rb_(2)CO_(3)`

Answer

To determine which carbonate is thermally most unstable among the given options, we need to analyze the thermal stability of the carbonates based on the basic character of the corresponding hydroxides.

### Step-by-Step Solution:

1. **Identify the Carbonates**: The options provided are:
- Option 1: BeCO3 (Beryllium Carbonate)
- Option 2: CaCO3 (Calcium Carbonate)
- Option 3: K2CO3 (Potassium Carbonate)
- Option 4: Rb2CO3 (Rubidium Carbonate)

2. **Understand the Groups**:
- Beryllium (Be) and Calcium (Ca) belong to the alkaline earth metals (Group 2).
- Potassium (K) and Rubidium (Rb) belong to the alkali metals (Group 1).

3. **Basicity and Thermal Stability**:
- The thermal stability of carbonates is inversely related to the basicity of the corresponding metal hydroxides. This means that the more basic the hydroxide, the more thermally stable the carbonate.
- Basicity increases down the group in both alkali and alkaline earth metals.

4. **Analyze the Basicity**:
- For alkaline earth metals:
- Be(OH)2 is less basic than Ca(OH)2.
- For alkali metals:
- K(OH) is less basic than Rb(OH).
- Therefore, the order of basicity is:
- Rb(OH) > K(OH) > Ca(OH) > Be(OH).

5. **Determine Thermal Stability**:
- Since basicity is directly proportional to thermal stability, we can conclude:
- BeCO3 is the least stable (most unstable) because beryllium is at the top of the alkaline earth metals group, making its hydroxide less basic.
- CaCO3 is more stable than BeCO3.
- K2CO3 is more stable than CaCO3.
- Rb2CO3 is the most stable among the alkali metal carbonates.

6. **Conclusion**:
- Therefore, the carbonate that is thermally most unstable is **BeCO3**.

### Final Answer:
**BeCO3 (Beryllium Carbonate) is thermally the most unstable carbonate.**

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