Problem 38 - Chemistry An Atoms First Approach

Question

Many more anhydrous lithium salts are hygroscopic (readily absorb water) than are those of the other alkali metals. Explain.

Step-by-Step Solution

Answer

In conclusion, the hygroscopic nature of lithium salts can be explained by the unique properties of lithium ion (Li+), particularly its small size and high charge density. This leads to stronger interactions with water molecules, resulting in a higher propensity for water absorption from the environment and making lithium salts more hygroscopic compared to other alkali metal salts.

Step 1: Understand the concept of hygroscopic materials

Hygroscopic materials are those that readily absorb water from the environment. In the context of this exercise, the hygroscopic nature of lithium salts refers to their ability to absorb water molecules and form hydrates more readily than other alkali metal salts.

Step 2: Compare the size and charge of lithium ion with other alkali metals

Lithium ion (Li+) has the smallest size and highest charge density among the alkali metal ions. This is because it has the smallest atomic radius, and since both lithium and other alkali metals have the same charge (+1), lithium ion's charge is concentrated over a smaller volume. Other alkali metals in respective order: sodium (Na+), potassium (K+), rubidium (Rb+), and cesium (Cs+), all have larger atomic radii and lower charge densities.

Step 3: Understand the interaction of lithium ion with water molecules

Due to its high charge density, lithium ion (Li+) attracts the negatively charged oxygen atoms of water molecules more strongly, resulting in a stronger interaction. This strong interaction causes lithium salts to absorb water more readily than other alkali metal salts.

Step 4: Relate the properties of lithium ion to the hygroscopic nature of its salts

The stronger interaction between lithium ion (Li+) and water molecules, due to its high charge density and small size, is what causes lithium salts to be more hygroscopic than other alkali metal salts. Since lithium salts have a stronger affinity for water molecules, they will absorb water more readily from the environment to form hydrates. In conclusion, the hygroscopic nature of lithium salts can be explained by the unique properties of lithium ion, particularly its small size and high charge density, which result in stronger interactions with water molecules and thus a higher propensity for water absorption from the environment.