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) \(\mathrm{Na}^{+}\) ions are formed in the solution. (b) Liquid \(\mathrm{NH}_{3}\) solution remains diamagnetic. (c) Liquid \(\mathrm{NH}_{3}\) solution becomes good conductor of electricity. (d) Blue coloured solution is obtained.

Short answer

The solution does not remain diamagnetic.

Step-by-step solution

Understanding the Reaction Context

When sodium (Na) is dissolved in liquid ammonia ( NH_3 ) at low temperatures, a well-known phenomenon occurs. The dissociation of sodium in liquid ammonia leads to the formation of solvated electrons and ions. This sets the stage for various observable outcomes in the solution.

Formation of Ions

During the dissolution process, sodium metal reacts with liquid ammonia to form sodium ions ( Na^+ ) and solvated electrons ( e^- ). This process can be represented by the equation: Na(s) + (x+y) NH_3(l) → Na^+(NH_3)_x + e^-(NH_3)_y. These ions are expected in the solution.

Observing Physical Properties

The solution containing Na^+ ions and solvated electrons appears deep blue due to the presence of solvated electrons that absorb specific wavelengths of light. Therefore, a blue-colored solution is a characteristic outcome.

Electrical Conductivity

The solvated electrons in the solution allow it to conduct electricity, making the solution a good conductor. This is because the mobility of the charges (electrons and ions) facilitates electric current flow through the liquid ammonia.

Evaluating Magnetic Properties

In general, the presence of unpaired electrons (as solvated electrons are) imparts paramagnetic properties to a solution. A diamagnetic material has no unpaired electrons, which means that a solution with solvated electrons would not be diamagnetic.

Conclusion

Among the given options, the observation that 'Liquid NH_3 solution remains diamagnetic' is incorrect. With solvated electrons present, the solution actually exhibits paramagnetic behavior.

Key concepts

Solvated Electrons

When sodium is dissolved in liquid ammonia at low temperatures, an interesting chemical reaction occurs. The sodium metal dissociates in the liquid ammonia, resulting in the formation of sodium ions \( (\text{Na}^+) \) and solvated electrons \( (\text{e}^-) \). These solvated electrons are electrons that become "trapped" in the solvent milieu.

In more detail, the electrons are surrounded by ammonia molecules, creating a unique environment where they are stabilized and can move freely. This phenomenon is a key factor contributing to several distinctive properties of the solution.

The appearance of these solvated electrons gives the solution a deep blue color. This is because they absorb particular wavelengths of light, leading to this visual transformation. It is a hallmark of sodium-ammonia solutions and is often the first observable property signaling the presence of solvated electrons.

Paramagnetic Properties

The presence of solvated electrons in a sodium-ammonia solution also imparts unique magnetic properties. Magnetic properties depend on the presence of unpaired electrons. Solvated electrons indeed contain unpaired electrons, which leads to the solution exhibiting paramagnetic properties.

Paramagnetism is characterized by a weak attraction to magnetic fields, suggesting that the solution’s net magnetic moment is nonzero due to these unpaired electrons. Paramagnetic substances, unlike diamagnetic ones, are influenced by external magnetic fields.

This is why a solution of sodium dissolved in liquid ammonia cannot exhibit diamagnetism, as diamagnetism requires all electrons to be paired. In this setting, the solvated electrons maintain their unpaired state, confirming the paramagnetic nature of the solution.

Electrical Conductivity of Solutions

The ability of a solution to conduct electricity hinges significantly on the presence and mobility of charge carriers, such as ions and electrons. In the sodium-liquid ammonia system, both sodium ions \( (\text{Na}^+) \) and solvated electrons \( (\text{e}^-) \) act as these charge carriers.

The solvated electrons, in particular, are quite mobile within the ammonia medium, facilitating the flow of electric current. This movement of electrons and ions renders the solution a good conductor of electricity.

Therefore, this solution showcases its electrical conductivity as a direct result of having these mobile charges. It's a fascinating example where a typical liquid, liquid ammonia, becomes significantly more electrically conductive due to the presence of dissolved metal.