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) Nat 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 remains diamagnetic does not occur; option (b) is incorrect.

Step 1: Understanding the reaction

When sodium metal is dissolved in liquid ammonia at low temperatures, it undergoes a reaction where sodium (Na) donates one electron to form sodium ions (40 ions) and solvated electrons are produced. The solvated electrons give the solution a characteristic deep blue color, indicating reactions have occurred.

Step 2: Evaluating the Options

Let's evaluate the given options based on the reaction process: - (a) Sodium ions (40 ) are indeed formed in the solution since sodium donates its electron. - (b) The presence of solvated electrons makes the solution paramagnetic, not diamagnetic, due to unpaired electrons. - (c) The presence of these free, solvated electrons also means the solution becomes a good conductor of electricity. - (d) The solution takes on a blue color due to the free electrons.

Step 3: Identifying the Incorrect Statement

Compare the eligible occurrences in the dissolution process of sodium in liquid 1H as mentioned in Step 2: - Options (a), (c), and (d) are consistent with the described reaction. - Option (b) claims the solution remains diamagnetic, which does not happen because of the solvated electrons.

Key Concepts

Sodium in Liquid Ammonia

Dissolving sodium in liquid ammonia is a fascinating process with a couple of unique characteristics. When you place sodium metal in liquid ammonia at low temperatures, an interesting reaction occurs. Sodium, a metal known for being highly reactive, loses an electron. This electron is then taken up by the surrounding ammonia molecules. The result of this interaction is the formation of sodium ions and 'solvated electrons'. These electrons, while in a free state, interact with the ammonia molecules. They are essentially swimming around, lending the solution peculiar and unexpected properties. One noticeable characteristic is the striking blue color of the solution. This occurs due to the free electrons absorbing visible light and thus changing the way the solution appears.

Solvated Electrons

Solvated electrons play a crucial role in the dissociation of sodium in liquid ammonia. These are electrons that have been solubilized by their interaction with the solvent, in this case, ammonia. When sodium loses its electron, it doesn't just vanish into the ether; it becomes part of the solvated electron cloud. The presence of these free electrons provides the solution with quite distinct properties:

• The aforementioned deep blue color arises as the electrons absorb specific wavelengths of light.
• These electrons contribute to the magnetic properties of the solution.

Indeed, solvated electrons make the solution paramagnetic, which occurs due to unpaired electron spins. The entire phenomenon is not just academically intriguing, but it is also a spectacle to witness several changes that are not typical of regular solutions.

Conductivity in Solutions

Conductivity is a common topic in electrochemistry, and understanding it in the context of sodium in liquid ammonia offers some valuable insights. Conductivity refers to a solution's ability to conduct electricity, which depends heavily on the presence of ions or charged particles. In the case of sodium dissolved in ammonia:

• Both sodium ions and solvated electrons act as charge carriers.
• The free electrons effectively flow through the solution, mimicking the function of an electrical circuit.

This results in the solution becoming an excellent conductor of electricity. The quick movement of these charges means that electrons can travel easily through the solution compared to most other liquid media.

Paramagnetism vs Diamagnetism

Understanding the difference between paramagnetism and diamagnetism is vital when studying chemical reactions like sodium in liquid ammonia. Paramagnetism and diamagnetism are different types of magnetism observed as a result of electron arrangements in substances. Sodium in liquid ammonia demonstrates:

• Paramagnetism: This type arises from the presence of unpaired electrons. These electrons spin in such a way that the solution becomes magnetic in an external magnetic field, briefly aligning with it.
• Diamagnetism: Substances with all paired electrons fall under this category, indicating a weak repulsion from a magnetic field.

The solvated electrons in the sodium-ammonia solution cause the mixture to exhibit paramagnetism due to their unpaired positions. This quality is opposite to what we would expect in a diamagnetic substance, where there would be no unpaired electrons contributing to magnetic properties. Hence, understanding these properties clarifies much about the fundamental interactions at play in such rigorous experiments.