Question

Many lithium salts are hygroscopic (absorb water), but the corresponding salts of the other alkali metals are not. Why are lithium salts different from the others?

Short answer

Lithium salts are different from other alkali metal salts in that they are hygroscopic because of their high hydration energy. The small size and high charge density of lithium ions result in strong electrostatic attraction with anions and water molecules. This leads to a higher lattice energy and significantly greater hydration energy compared to other alkali metal ions, making lithium salts more prone to absorb water from their surroundings.

Step-by-step solution

Understand hygroscopic nature

Hygroscopic substances absorb water from their surroundings. Many lithium salts exhibit this behavior because of their specific chemical and physical properties.

Study the periodic trends of alkali metals

In the periodic table, alkali metals belong to Group 1. They include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). As we move down the group, the atomic and ionic size increases. This trend affects the properties of these metals and their salts.

Analyze the ionic size of lithium

Lithium (Li) is the smallest among alkali metals. Its small size leads to a high charge density on its ion, creating a strong electrostatic attraction between the lithium ion and anions in its salts.

Examine lattice energy

Lattice energy is the energy released when ions in a crystalline solid are brought together to form a lattice structure. For alkali metal salts, the lattice energy decreases down the group. This means that lithium salts have a higher lattice energy than other alkali metals salts, mainly due to the small size of the lithium ion and strong electrostatic attraction with anions.

Investigate hydration energy and water molecules

When salts dissolve in water, they release hydration energy - the energy released when ions interact with water molecules. The small size and high charge density of lithium ion results in significantly greater hydration energy compared to other alkali metal ions.

Understand the role of hydration energy in hygroscopic nature

The high hydration energy of lithium ions induces strong attraction between lithium salts and water molecules, making lithium salts more prone to absorb water from their surroundings. This strong attraction results in the hygroscopic nature of lithium salts, while the corresponding salts of other alkali metals, with lower hydration energies, do not exhibit the same behavior.