Problem 77 - A Complete Resource Book in Chemistry for JEE Main

Question

On dissolving moderate amount of sodium metal in liquid \(\mathrm{NH}_{3}\) at low temperature, which one of the following does not occur? (a) Na+ ions are formed in the solution (b) liquid \(\mathrm{NH}_{3}\) solution remains diamagnetic (c) liquid \(\mathrm{NH}_{3}\) solution becomes a good conductor of electricity (d) blue coloured solution is obtained

Step-by-Step Solution

Answer

The solution does not remain diamagnetic; it becomes paramagnetic.

Step 1: Understand the problem

We need to identify the phenomenon that does not occur when sodium metal is dissolved in liquid ammonia at low temperatures. Each provided option describes a potential outcome of this process.

Step 2: Analyze each option

Let's evaluate each statement: - (a) **Na+ ions are formed in the solution:** When sodium is dissolved in liquid ammonia, it dissociates into Na+ ions and electrons, forming a solvated electron. This statement is true. - (b) **Liquid ammonia solution remains diamagnetic:** The presence of solvated electrons provides unpaired electrons in the solution, making it paramagnetic rather than diamagnetic. This statement is incorrect. - (c) **Liquid ammonia solution becomes a good conductor of electricity:** The free solvated electrons allow the solution to conduct electricity. This statement is true. - (d) **Blue coloured solution is obtained:** The presence of solvated electrons also gives the solution a characteristic blue color. This statement is true.

Step 3: Identify the incorrect statement

By analyzing each option, we determine that option (b) is the statement that does not occur. The liquid ammonia solution does not remain diamagnetic; it becomes paramagnetic due to the unpaired electrons.

Key Concepts

Solvated Electrons

When sodium metal dissolves in liquid ammonia, an interesting phenomenon occurs. The sodium atoms become ions, but that's not the end of the story. The lone electrons leftover from this ionic separation are not left to drift away into empty space. Instead, they become surrounded by ammonia molecules. This is what we call **solvated electrons**. These electrons are literally swimming in a sea of ammonia molecules.
The presence of these solvated electrons affects the properties of the solution:

• The solution begins to exhibit a beautiful blue color. This color change is a direct result of the interactions between the free electrons and the light they absorb.
• These free electrons also significantly increase the ability of the solution to conduct electricity, as they can move freely through the solution.

Solvated electrons are a fundamental component in understanding why certain properties, like electrical conductivity and color, change when substances like sodium dissolve in liquid ammonia.

Paramagnetism

Paramagnetism arises when a substance contains unpaired electrons. In the case of sodium dissolved in liquid ammonia, the presence of **solvated electrons** causes this phenomenon. These free, unpaired electrons in the solution's sea affect its magnetic properties, resulting in paramagnetism.
In paramagnetic materials:

• There is a tendency for the unpaired electrons to align themselves with an external magnetic field. This is because unpaired electrons have magnetic moments due to their spin, unlike paired electrons which cancel each other's magnetic properties.
• This alignment enables the solution to be attracted slightly by a magnet, a characteristic not observed if the solution remained diamagnetic, where all electrons are paired and no net magnetic moment exists.

It's important to note that the introduction of unpaired electrons is the key factor altering the solution's magnetic characteristic from diamagnetic to paramagnetic.

Conductivity in Solutions

The electrical conductivity of a solution refers to its ability to conduct electricity. In the context of sodium in liquid ammonia, the presence of **solvated electrons** is crucial. These solvated electrons act similarly to ions in a typical ionic solution:

• They move freely throughout the solution, enabling the transfer of electric charge. This free movement of electrons or ions is central to a material's conductivity.
• Unlike many other solutions which rely solely on ionic movement for conductivity, the free electrons here greatly enhance the solution's ability to conduct electricity. Thus, sodium dissolved in liquid ammonia forms a solution much more conductive than pure ammonia alone.

Therefore, the formation of *solvated electrons* is key to the enhanced electrical conductive properties of the solution, marking a significant shift from a non-conducting state to one with excellent conductivity.