Question

Which of the following halides crystallises from its aqueous solution as hydratc? (1) \(\mathrm{LiCl}\) (2) \(\mathrm{KCl}\) (3) \(\mathrm{NaCl}\) (4) \(\mathrm{RbCl}\)

Short answer

\(\mathrm{LiCl}\)

Step-by-step solution

Identify the compounds

List the given halides: (1) \(\mathrm{LiCl}\), (2) \(\mathrm{KCl}\), (3) \(\mathrm{NaCl}\), (4) \(\mathrm{RbCl}\). These are all alkali metal chlorides.

Understand the concept of hydrates

Hydrates form when certain salts crystallize from aqueous solutions with water molecules integrated into their crystal lattice. This typically occurs for salts with high lattice energy and small cations.

Compare the given compounds

Among the alkali metal chlorides, \(\mathrm{LiCoCl}\) is unique. Lithium ion ( \(\mathrm{Li^+}\) ) is the smallest cation with a high charge density, making it more likely to form hydrates than the larger \(\mathrm{Na^+}\), \(\mathrm{K^+}\), and \(\mathrm{Rb^+}\) ions.

Determine which halide crystallizes as hydrate

Lithium chloride ( \(\mathrm{LiCl}\) ) crystallizes from its aqueous solution as a hydrate due to the small size and high charge density of the \(\mathrm{Li^+}\) ion.

Key concepts

Hydrates

When certain salt compounds dissolve in water, they sometimes form structures called hydrates. Hydrates include water molecules that are integrated into their crystalline framework. This water becomes a part of the crystal structure when the salt crystallizes from its aqueous solution.
The formation of hydrates is often favored by salts with specific properties:

• High lattice energy
• Small cations (positively charged ions)

Lattice energy and the size of the cation play a big role in determining whether a salt will form a hydrate. In our exercise, we see that \(\text{LiCl}\) tends to form hydrates while \(\text{NaCl}\), \(\text{KCl}\), and \(\text{RbCl}\) do not. This is primarily due to the smaller ion size and high charge density of the lithium ion.

Lattice Energy

Lattice energy is the measure of the bond strength in ionic compounds. It is the energy required to separate one mole of a solid ionic compound into its gaseous ions.
Ionic compounds with high lattice energy have high melting points and are often more stable. Higher lattice energy also means stronger attraction between the ions in the compound.
In the context of hydrate formation, compounds with higher lattice energy tend to form hydrates more easily if water molecules can fit comfortably in their crystal structure.
For example, \(\text{LiCl}\) has high lattice energy because the lithium ion (\(\text{Li}^{+}\)) is very small and has a high charge density compared to \(\text{Na}^{+}\), \(\text{K}^{+}\), and \(\text{Rb}^{+}\). This makes \(\text{LiCl}\) more likely to incorporate water molecules into its structure, forming hydrates when it crystallizes from a water solution.

Cations in Crystallization

The size and charge of the cations play a crucial role during the crystallization of salts. The cations' properties can influence whether or not water molecules are integrated into the crystal lattice to form hydrates.
Here are the key points:

• Smaller cations with high charge density usually lead to higher lattice energy.
• Higher lattice energy can make it easier for water molecules to be part of the crystal structure.
• Alkali metals display varying tendencies to form hydrates based on their ionic sizes and charge densities.

In our exercise, the lithium ion (\(\text{Li}^{+}\)) is the smallest among the listed alkali metals (sodium (\(\text{Na}^{+}\)), potassium (\(\text{K}^{+}\)), and rubidium (\(\text{Rb}^{+}\))). Due to its small size and high charge density, lithium ion forms hydrates more readily. This is why \(\text{LiCl}\) crystallizes from its aqueous solution as a hydrate, while the others do not.