The solubility of fluorides of alkali metals in water is

Options

A. `LiF gt NaF gt KF gt RbF gt CsF`
B. `CsF gt RbF gt KF gt NaF gt LiF`
C. `LiF gt KF gt NaF gt CsF gt RbF`
D. `CsF gt KF gt NaF gt RbF gt LiF`

Answer

To determine the solubility trend of alkali metal fluorides in water, we need to analyze the factors affecting solubility, namely lattice energy and hydration energy.

### Step-by-Step Solution:

1. **Understanding Lattice Energy and Hydration Energy**:
- **Lattice Energy**: This is the energy required to separate one mole of a solid ionic compound into its gaseous ions. It is influenced by the size of the ions; smaller ions generally lead to higher lattice energy due to stronger electrostatic forces between them.
- **Hydration Energy**: This is the energy released when ions are surrounded by water molecules. Smaller ions can be hydrated more effectively, leading to higher hydration energy.

2. **Analyzing the Alkali Metal Fluorides**:
- The alkali metal fluorides we are considering are: Lithium Fluoride (LiF), Sodium Fluoride (NaF), Potassium Fluoride (KF), Rubidium Fluoride (RbF), and Cesium Fluoride (CsF).
- Fluoride ion (F⁻) is relatively small, and thus the size of the cation (alkali metal ion) will significantly influence the lattice energy.

3. **Trend in Lattice Energy**:
- As we move down the group from Lithium to Cesium, the size of the alkali metal cations increases. This leads to a decrease in lattice energy because the distance between the cations and anions increases, weakening the electrostatic attraction.
- Therefore, the trend in lattice energy is: LiF > NaF > KF > RbF > CsF (decreasing).

4. **Trend in Hydration Energy**:
- Hydration energy also decreases as we move down the group, but the decrease is not as pronounced as the decrease in lattice energy. Smaller cations (like Li⁺) have higher hydration energy due to their ability to attract water molecules more effectively.

5. **Comparing Lattice Energy and Hydration Energy**:
- For a compound to be soluble, its hydration energy must be greater than its lattice energy.
- As we move down the group, the lattice energy decreases more significantly than the hydration energy. This means that the solubility of these fluorides increases down the group.

6. **Solubility Trend**:
- Based on the above analysis, the solubility trend in water for alkali metal fluorides is:
- Most soluble: CsF > RbF > KF > NaF > LiF: Least soluble.

### Final Answer:
The solubility of fluorides of alkali metals in water increases down the group, with cesium fluoride (CsF) being the most soluble and lithium fluoride (LiF) being the least soluble.