On heating to `400-500^@C`, relatively unstable hydrides and carbonates decompose. Which of the following will decompose when heated to `400-500^@`C

Options

A. LiH
B. NaH
C. `Li_2CO_3`
D. `Na_2CO_3`

Answer

To determine which compounds will decompose when heated to 400-500°C, we need to analyze the thermal stability of the given hydrides and carbonates.

### Step-by-Step Solution:

1. **Identify the Compounds**: The question provides us with four options, which include hydrides and carbonates of Group 1 elements (alkali metals). Let's denote them as:
- Option 1: Lithium Hydride (LiH)
- Option 2: Sodium Hydride (NaH)
- Option 3: Lithium Carbonate (Li2CO3)
- Option 4: Sodium Carbonate (Na2CO3)

2. **Understand Thermal Stability Trends**:
- **Hydrides**: The thermal stability of Group 1 hydrides decreases down the group. This means that as we move from lithium to sodium to potassium, the hydrides become less stable and are more likely to decompose upon heating.
- **Carbonates**: Conversely, the thermal stability of Group 1 carbonates increases down the group. Lithium carbonate is less stable than sodium carbonate, meaning it is more likely to decompose when heated.

3. **Analyze Hydrides**:
- **Lithium Hydride (LiH)**: More stable compared to sodium hydride.
- **Sodium Hydride (NaH)**: Less stable and will decompose at lower temperatures compared to LiH. Therefore, NaH is likely to decompose when heated to 400-500°C.

4. **Analyze Carbonates**:
- **Lithium Carbonate (Li2CO3)**: Least stable among the carbonates listed and will decompose at lower temperatures.
- **Sodium Carbonate (Na2CO3)**: More stable than lithium carbonate and will not decompose at 400-500°C.

5. **Conclusion**: Based on the analysis, the compounds that will decompose when heated to 400-500°C are:
- Sodium Hydride (NaH) from the hydrides.
- Lithium Carbonate (Li2CO3) from the carbonates.

### Final Answer:
- **Sodium Hydride (NaH)** and **Lithium Carbonate (Li2CO3)** will decompose when heated to 400-500°C.