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) \(\mathrm{A}\) 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

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

Step 1: Analyze the Assertion (A)

The assertion claims that lithium salts are mostly hydrated among alkali metal salts. This can be verified by considering the small size and high charge density of lithium ions. These characteristics lead to a strong attraction for water molecules, making lithium salts highly hydrated.

Step 2: Evaluate the Reason (R)

The reason given states that the hydration enthalpies of alkali metal ions increase with decreasing ionic size. As ionic size decreases, ions can attract water molecules more strongly, leading to higher hydration enthalpy, which is indeed true.

Step 3: Check if Reason Explains the Assertion

Given that lithium ions are the smallest among alkali metals, their high hydration enthalpy due to their small size explains why lithium salts are highly hydrated. Thus, the reason is not just true but also explains the assertion effectively.

Step 4: Conclusion Based on Analysis

Both the assertion and the reason are correct, and the reason supports and explains the assertion well. Therefore, option (a) is the correct choice.

Key Concepts

Assertion and Reason questions

When tackling Assertion and Reason questions, it is important to clearly understand both statements before drawing a conclusion. These questions present two linked statements, an assertion and a reason, and test the ability to evaluate their truth and interconnections. There are five possible outcomes:

• Both assertion and reason are correct, and the reason explains the assertion.
• Both are correct, but the reason doesn't explain the assertion.
• The assertion is true, but the reason is false.
• The assertion is false, but the reason is true.
• Both are false.

Approaching these questions with a strategic mindset is crucial. Start by individually assessing the truth of both statements. Then, consider the logical relationship between them. The key challenge is not just deciding whether both statements are true, but understanding if the reason provides the appropriate explanation for the assertion. Practicing this type of question can improve critical thinking skills in examinations.

Alkali metal salts

Alkali metal salts are a fascinating group in chemistry, encompassing compounds formed from the alkali metal elements like lithium, sodium, potassium, rubidium, and cesium. These metals belong to Group 1 in the periodic table and have distinct physical and chemical properties due to their single valence electron. Alkali metal salts often display unique behaviors especially in aqueous solutions. The size of the metals influences properties like solubility, lattice energy, and particularly hydration, which plays a significant role in solution chemistry. The small ionic size of lithium allows its salts to have a strong interaction with water molecules. This leads to high degrees of hydration, making lithium salts special compared to other alkali metal salts. This character is vital in various applications, including batteries and pharmaceuticals. Understanding alkali metal salts involves recognizing how their chemical behavior is strongly tied to their position in the periodic table, thus affecting their interaction with other substances like water.

Hydration enthalpies

Hydration enthalpy refers to the amount of energy released when ions interact with water molecules. This is a critical concept in the study of solutes in aqueous solutions, affecting properties such as solubility, stability, and reactivity. The key factor influencing hydration enthalpies is the size of the ion. Generally, smaller ions with a higher charge density will have stronger attractions to water molecules, thus exhibiting higher hydration enthalpies. For alkali metals, lithium has the smallest ionic radius, resulting in the highest hydration enthalpy among this group. A high hydration enthalpy means the ion can strongly attract and hold onto water molecules. This explains why lithium salts are more hydrated compared to other alkali metals. In chemical processes, considering hydration enthalpies is crucial as it affects how ions dissolve and interact in water-borne reactions.