Problem 17 - Chemistry An Atoms First Approach

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?

Step-by-Step Solution

Answer

Lithium salts are hygroscopic due to the small size and high charge density of lithium cations (\(Li^+\)), which leads to strong interactions with water molecules and cause them to easily absorb water from their surroundings. On the other hand, other alkali metal salts do not readily absorb water because their larger ions have lower charge densities and weaker interactions with water molecules.

Step 1: Understand the properties of alkali metals and their salts

Alkali metals are found in Group 1 of the periodic table and include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). They all have similar properties due to having one electron in their outermost energy level, which they can easily lose to form positive ions (cations). The salts of alkali metals are composed of these cations and anions from various acids.

Step 2: Investigate the ionic radii of alkali metal cations

The ionic radii of alkali metal cations increases down the group, with lithium being the smallest and francium being the largest. Smaller ions have a greater charge density due to having their charge concentrated over a smaller area. Ionic radii: \(Li^+\) < \(Na^+\) < \(K^+\) < \(Rb^+\) < \(Cs^+\).

Step 3: Compare interactions between water molecules and alkali metal cations

Water is a polar molecule due to the electronegativity difference between oxygen and hydrogen atoms. Water molecules can easily be attracted to and surround ions in a process called hydration or solvation. The strength of the hydration depends on the charge density of the ion: the higher the charge density of an ion, the stronger the interaction with water molecules.

Step 4: Explain the hygroscopic nature of lithium salts

Lithium salts are hygroscopic because of the high charge density of the smaller lithium cation (\(Li^+\)). The \(Li^+\) cation has a strong interaction with the polar water molecules, which leads to a greater hydration enthalpy and a tendency to draw water from the surroundings. This results in lithium salts readily absorbing water and forming hydrated salts.

Step 5: Discuss the hygroscopic nature of other alkali metal salts

In comparison to lithium salts, other alkali metal salts have larger cations with lower charge densities. This means that their hydration enthalpies are less negative, and the interactions between the cations and water molecules are weaker. As a result, the corresponding salts of other alkali metals are not hygroscopic. _In summary, lithium salts are hygroscopic due to the small size and high charge density of lithium cations (\(Li^+\)), which leads to strong interactions with water molecules, causing them to easily absorb water from their surroundings. On the other hand, other alkali metal salts do not readily absorb water because their larger ions have lower charge densities and weaker interactions with water molecules._