The low solubility of LiF and that of CsI in water are respectively due to which of the properties of the alkali metal ions?

Options

A. Higher hydration enthalpy of `Li^(+)`, higher lattice enthalpy of `Cs^(+)`
B. Smaller hydration enthalpy of `Li^(+)`, higher latttice enthalpy of `Cs^(+)`
C. Smaller lattice enthalpy of `Li^(+)`, higher hydration enthalpy of `Cs^(+)`
D. Higher lattice enthalpy of `Li^(+)`, smaller hydration enthalpy of `Cs^(+)`.

Answer

To solve the question regarding the low solubility of LiF (lithium fluoride) and CsI (cesium iodide) in water and the properties of alkali metal ions responsible for this, we can follow these steps:

### Step 1: Understand the Concept of Solubility
- Solubility of ionic compounds in water depends on two main energies: lattice energy and hydration energy.
- **Lattice Energy**: The energy required to separate one mole of a solid ionic compound into its gaseous ions. Higher lattice energy means stronger ionic bonds.
- **Hydration Energy**: The energy released when gaseous ions are surrounded by water molecules. Higher hydration energy means better solvation of ions.

### Step 2: Analyze Lithium Fluoride (LiF)
- LiF has a high lattice energy because both lithium (Li) and fluoride (F) ions are small in size.
- The small size of these ions leads to a strong electrostatic attraction between them, resulting in high lattice energy.
- The hydration energy of Li+ is relatively low compared to the lattice energy, making the overall energy balance unfavorable for solubility. Thus, LiF is almost insoluble in water.

### Step 3: Analyze Cesium Iodide (CsI)
- CsI has larger ions (Cs and I), which results in lower lattice energy compared to LiF.
- However, the larger size of cesium (Cs) and iodide (I) ions leads to less effective hydration because the ions are less tightly surrounded by water molecules.
- The hydration energy in this case is also lower due to the larger ionic sizes, which means that the energy released during hydration is not sufficient to overcome the lattice energy. Therefore, CsI is also insoluble in water.

### Step 4: Conclusion
- The low solubility of LiF is primarily due to its high lattice energy compared to its hydration energy.
- The low solubility of CsI is due to the larger size of its ions leading to lower hydration energy, which is not enough to overcome its lattice energy.

### Final Answer
- The low solubility of LiF is due to high lattice energy, while the low solubility of CsI is due to low hydration energy.

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