Question

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

Short answer

Anhydrous lithium salts are more hygroscopic than those of other alkali metals because their smaller ionic size leads to higher hydration energy and lattice energy. However, the hydration energy of lithium salts increases more dramatically than their lattice energy, making the net energy change favorable for dissolution in water. This high hydration energy results in a strong attraction between Li⁺ ions and water molecules, making lithium salts readily absorb water from their surroundings.

Step-by-step solution

Understand the structure of alkali metal salts

All alkali metal salts are ionic in nature, meaning they are formed by the transfer of electrons between a positively charged metal cation (such as Li^+, Na^+, or K^+) and a negatively charged anion (such as Cl^-, NO_3^-, or SO_4^2-).

Compare the size of lithium ion to other alkali metal ions

Lithium has the smallest atomic size among the alkali metals, which results in a smaller ionic size for Li^+ compared to other alkali metal ions such as Na^+ or K^+. A smaller ionic size results in a higher charge density for Li^+ ions, meaning there is more charge per unit volume.

Understand hydration energy and its relation to ion size

Hydration energy is the energy released when a hydrated ion is formed from a gaseous ion and a certain number of water molecules. The smaller the ion, the more strongly it attracts water molecules because of its high charge density. The attraction between the ion and water molecules results in the release of hydration energy, which is a negative quantity. The greater the charge density (smaller ionic size), the higher the hydration energy.

Understand lattice energy and its relation to ion size

Lattice energy is the energy required to separate one mole of an ionic compound into its gaseous ions. In the context of alkali metal salts, it is a measure of the strength of the electrostatic forces between the alkali metal cations and the anions in the salt. Smaller ions, such as Li^+, have stronger electrostatic interactions with the anions due to their higher charge density. Therefore, lithium salts have higher lattice energies than those of other alkali metals.

Explain the hygroscopic nature of lithium salts

The hydration energy, being a negative quantity, favors the dissolving process of a salt in water. On the other hand, lattice energy, which is a positive quantity, opposes this process, as it needs to be supplied for the salt to dissolve. An ionic salt is more likely to dissolve in water and be hygroscopic if its hydration energy outweighs its lattice energy. In the case of lithium salts, their smaller ionic size leads to both higher hydration energy and higher lattice energy compared to salts of other alkali metals. However, the hydration energy of lithium salts increases more dramatically than their lattice energy, making the net energy change favorable for dissolution in water. This is why anhydrous lithium salts are more hygroscopic than those of the other alkali metals.

Key concepts

Alkali Metal Salts Ionic Nature

When we talk about alkali metal salts, we're referring to compounds formed by alkali metals, such as lithium (Li), sodium (Na), and potassium (K), that have donated their outermost electrons to non-metals, creating salts with an iconic ionic charge. For instance, lithium will lose an electron to become Li⁺, while a chlorine atom gains an electron to become Cl^-, resulting in lithium chloride (LiCl), a type of alkali metal salt.

For students understanding the basic nature of these salts is crucial. They are held together by the strong electrostatic forces of attraction between oppositely charged ions. The electric force not only dictates the structure but also the properties of these salts, such as melting point, solubility, and in this case, hygroscopic behavior.

In the case of hygroscopic lithium salts, their ability to absorb water is attributed to the ionic nature of lithium which, when combined with its small ionic size, leads to a stronger attraction for water molecules. This heightened attraction causes the salts to readily absorb moisture from their environment.

Hydration Energy

Diving deeper into the concept, hydration energy is the energy that's released when salts dissolve in water. Specifically, it's the process of water molecules completely surrounding the ion resulting from a salt compound, which stabilizes the ion in solution. This energy is especially significant for smaller ions like lithium because of their high charge density—that is, the amount of charge per unit volume of the ion can be seen as more concentrated.

Why is this important for students to grasp?

The higher the charge density, the stronger the electrostatic pull towards the polar water molecules, leading to a larger release of hydration energy. So, lithium ions, being much smaller, will have a greater affinity for the surrounding water molecules compared to larger alkali metal ions, such as those of sodium or potassium. This means that when lithium salts are exposed to the atmosphere, they release more energy by reacting with water vapor, and thus, they more readily absorb that moisture, exhibiting hygroscopic characteristics.

Lattice Energy

Lattice energy might sound a bit intimidating, but it’s just the term used to describe the amount of energy that would be needed to break apart the ions in a salt compound. This concept plays an essential role in understanding why lithium salts are particularly hygroscopic. It’s related to the strength of the ionic bonds in the crystal lattice of the salt, crystalline grids which hold the positive and negative ions in place.

For smaller ions, like those of lithium, the ions are closer together due to their small size, resulting in a more substantial electrostatic attraction between them. This means that lithium salts have quite high lattice energies. However, interestingly, despite the high lattice energy, the even higher hydration energy leads to an overall energy release when lithium salts interact with water. As a result, these salts are able to overcome the forces holding them together in their solid state and absorb water more easily, explaining their hygroscopic nature.