Question

STATEMENT-1: Alkali metals dissolve in liquid ammonia to give blue solutions. because STATEMENT-2: Alkali metals in liquid ammonia give solvated species of the type \(\left[\mathrm{M}\left(\mathrm{NH}_{3}\right)_{\mathrm{n}}\right]^{+}(\mathrm{M}=\) alkali metals \()\). (A) Statement- 1 is True, Statement- 2 is True; Statement- 2 is a correct explanation for Statement-1 (B) Statement-1 is True; Statement-2 is True; Statement- 2 is NOT a correct explanation for Statement-1 (C) Statement-1 is True, Statement-2 is False (D) Statement-1 is False, Statement-2 is True

Short answer

(B) Statement-1 is True; Statement-2 is True; Statement- 2 is NOT a correct explanation for Statement-1.

Step-by-step solution

Identify the Statements and Understand the Concepts

Firstly, identify the two statements provided. Statement-1 states that alkali metals dissolve in liquid ammonia to give blue solutions. This is a well-known observation in chemistry. Statement-2 provides a specific detail on the nature of the species formed when alkali metals dissolve in ammonia, stating that solvated species of the type \(\left[\mathrm{M}\left(\mathrm{NH}_{3}\right)_{\mathrm{n}}\right]^{+}\) (where M is an alkali metal) are produced.

Examine the Truth Values of the Statements

Next, we determine if each statement is true or false. Statement-1 is true: alkali metals do indeed dissolve in liquid ammonia to give a blue solution. Statement-2 is also true: when alkali metals dissolve in liquid ammonia, they form solvated electrons alongside the solvated cations described in the statement.

Determine the Correct Explanation

Finally, we need to determine if Statement-2 is a correct explanation for Statement-1. The blue color actually arises from the solvated electrons that are present along with the solvated cations. Therefore, while Statement-2 correctly describes a species formed when alkali metals are dissolved in ammonia, it is not by itself a complete explanation for the blue color observed. The blue color is due to the solvated electrons, not just the solvated cations.

Key concepts

Solvated Species

When we talk about solvated species, particularly in the context of alkali metals in liquid ammonia, we're referring to the ionic and molecular entities that result from dissolving a solute in a solvent where the atoms, ions, or molecules of the solute become surrounded by molecules of the solvent. In our specific case, the alkali metal atom donates an electron to the ammonia molecules, resulting in the formation of a positively charged ion, known as a cation, which is solvated by the ammonia molecules.

The general formula for these solvated cations is \(\left[\mathrm{M}\left(\mathrm{NH}_{3}\right)_{\mathrm{n}}\right]^{+}\), where \(\mathrm{M}\) represents an alkali metal and \(\mathrm{n}\) is the number of ammonia molecules surrounding the metal ion. It's important to acknowledge that the solvation process involves an intricate interaction between the solute and solvent molecules, stabilizing the ions in solution through solvation or hydration energy.

Blue Solutions

The characteristic blue color observed when alkali metals are dissolved in liquid ammonia is a fascinating phenomenon. This blue color arises due to the presence of solvated electrons which are free to move through the liquid. These solvated electrons absorb energy in the visible range of the spectrum, thus imparting a blue hue to the solution.

The solvated electrons and cations coexist in the solution, contributing to its overall stability. This process illustrates the unique ability of liquid ammonia to act as a solvent that not only dissociates the alkali metals but also stabilizes the electrons. As a result, these electrons are not immediately attracted to the cations due to the shielding effect of the ammonia molecules, maintaining the solution's distinct color.

It is this remarkable property of solvated electrons being able to absorb light at wavelengths responsible for the blue color, that makes this topic fascinating to students studying JEE Advanced chemistry.

JEE Advanced Chemistry

In preparation for competitive examinations like the JEE Advanced, which is an entry point into some of India's top engineering institutions, understanding the behaviors of chemical species in various environments is critical. The topic of alkali metals in liquid ammonia poses a good example of the kind of advanced conceptual understanding students are expected to have.

Questions related to the dissolution of metals, the formation of solvated species, and the resultant color changes are not just to test rote memory but to assess a student's ability to apply concepts in explaining observed chemical phenomena. For JEE aspirants, it is not enough to know that alkali metals create blue solutions in ammonia; they must understand how solvated electrons and cations interact, and how this interaction affects solution properties.

Hence, a deep dive into this topic enhances their analytical skills and fosters a better grasp of physical chemistry principles, equipping them with the knowledge needed to tackle complex problems and succeed in competitive exams.