Problem 126 - A Complete Resource Book in Chemistry for JEE Main

Question

In the following questions two statements (Assertion) (A) and Reason (R) are given. Mark (a) If both \(\mathrm{A}\) and \(\mathrm{R}\) are correct and \(\mathrm{R}\) is the correct explanation of \(\mathrm{A}\). (b) If both \(\mathrm{A}\) and \(\mathrm{R}\) are correct but \(\mathrm{R}\) is not the correct expalnation of \(\mathrm{A}\). (c) A is true but \(\mathrm{R}\) is false. (d) A is false but \(R\) is true. (e) \(\mathrm{A}\) and \(\mathrm{R}\) both are false. Assertion: Lithium slats are mostly hydrated amongst alkalimetal salts Reason: The hydration enthalpies of alkali metal ions increases with decrease in ionic sizes

Step-by-Step Solution

Answer

Both A and R are correct, and R is the correct explanation of A.

Step 1: Understanding Assertion and Reason

The assertion states that lithium salts are mostly hydrated when compared with other alkalimetal salts. The reason provided suggests that this is due to the hydration enthalpies of alkali metal ions increasing as ionic size decreases.

Step 2: Analyzing Assertion A

Lithium ions ( ext{Li}^+) have a very small ionic size compared to other alkali metals. Due to this small size, lithium ions have a high charge density, which leads to strong interactions with water molecules, resulting in high hydration levels. Therefore, Assertion A is correct.

Step 3: Analyzing Reason R

As the ionic size decreases, the charge density increases, causing stronger interactions with water, and thus the hydration enthalpy is indeed higher. Therefore, Reason R is also correct.

Step 4: Deciding Relationship between A and R

Since Reason R correctly explains that the smaller size of the lithium ion, which results in higher hydration enthalpies, causes lithium salts to be more hydrated, Reason R is a correct explanation of Assertion A.

Key Concepts

Hydration Enthalpy

Hydration enthalpy is an important concept in chemistry that refers to the energy change when water molecules surround and interact with ions. This energy change is due to the interaction between water molecules and ions and plays a critical role in determining the solubility and stability of ionic compounds. When an ion dissolves in water, it's surrounded by water molecules—a process called hydration. During this, there is an attraction between the ion and the dipole nature of water molecules. This leads to a release or absorption of energy, known as hydration enthalpy. Generally, smaller ions with higher charge densities have stronger attractions to water molecules, resulting in higher hydration enthalpies.

• Smaller ionic radius leads to higher charge density.
• Higher charge density increases attraction to water, raising hydration enthalpy.

Hydration enthalpy impacts an ion's stability in a solution and influences how hydrated a salt becomes. In the case of alkali metals like lithium, this strongly affects their chemical behavior in an aqueous environment.

Ionic Sizes

Ionic size is a key factor in understanding how ions interact with other ions and molecules, especially in solutions. Ionic sizes are not constant and can change depending on the charge of the ions, as well as their environment, but they do have general trends within the periodic table. When the size of an ion decreases, its charge density increases. This means the ion can exert a greater attractive force over other charged species or polar molecules like water. Essentially, the smaller the ion, the stronger its interaction with other particles, like water molecules when it becomes hydrated.

• As you move across a period, ionic sizes typically decrease due to increased nuclear charge.
• Smaller ions have higher charge density, impacting interactions in solutions.

These interactions are crucial in determining properties like solubility and reactivity of ionic compounds. This is why lithium ions, being very small, interact strongly with water molecules, influencing the hydration of lithium salts as mentioned in the assertion from the exercise.

Lithium Salts

Lithium salts are a class of compounds where lithium, an alkali metal element, forms ionic bonds with various anions. These salts are noteworthy in chemistry for their high hydration levels, which make them distinct among alkali metal salts. Lithium ions ( ext{Li}^+) are characteristically small with a high charge density. This unique combination allows lithium salts to form strong interactions with water molecules, leading to high levels of hydration. Such properties make lithium salts quite soluble and crucial in several chemical applications including medications and industrial processes.

• Lithium's small ionic size increases its salts' hydration.
• High hydration makes lithium salts quite soluble in water.

In the context of the exercise, the assertion that lithium salts are highly hydrated confirms that this characteristic principally arises from the small ionic size of lithium. This results in high hydration enthalpy, driving their unique behavior in aqueous environments.