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) \(\mathrm{Na}^{+}\)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 correct answer is (b) liquid \( \mathrm{NH}_{3} \) solution remains diamagnetic.

Step 1: Understanding Sodium Dissolution in Liquid Ammonia

When a moderate amount of sodium metal dissolves in liquid ammonia at low temperatures, the sodium atoms release electrons. This results in the formation of sodium ions \( \mathrm{Na}^{+} \) and free electrons in the solution.

Step 2: Formation of Sodium Ions (Na⁺)

The sodium metal dissociates to form\( \mathrm{Na}^{+} \) ions. This is a known and expected outcome of sodium dissolving in liquid ammonia, answering option (a).

Step 3: Color of the Solution

The free electrons in the solution caused by the dissolution of sodium impart a blue color to the liquid ammonia solution. Thus, a blue colored solution is observed, affirming option (d).

Step 4: Conductivity of Solution

The presence of free electrons in the solution significantly increases its electrical conductivity, making the solution a good conductor of electricity. This means option (c) is correct.

Step 5: Assess the Magnetic Property of the Solution

The free electrons in the solution render the solution paramagnetic, as opposed to diamagnetic, due to the presence of unpaired electrons. Thus, option (b) does not occur, meaning liquid ammonia does not remain diamagnetic after sodium dissolves.

Key Concepts

Sodium Ion Formation

When sodium metal is introduced into liquid ammonia at low temperatures, a fascinating chemical reaction takes place. Sodium atoms lose their outermost electron to become positively charged sodium ions, denoted as \( \mathrm{Na}^{+} \). This process is known as ionization.

• Sodium atoms release electrons.
• \( \mathrm{Na} \rightarrow \mathrm{Na}^{+} + e^- \)
• As electrons are released, sodium ions are formed.

This ion formation is crucial because it contributes to various properties of the solution, including electrical conductivity and color change. The positively charged sodium ions are stabilized in the solution, while the free electrons roam around. They play a significant role in the unique traits of this chemical mixture.

Color Change in Chemistry

Dissolving sodium in liquid ammonia provides a visual treat as the solution turns a distinctive blue. This color shift occurs due to the presence of free electrons dispersed throughout the ammonia.

• Free electrons absorb and reflect light, creating a blue hue.
• This phenomenon is termed solvated electron formation.

The blue color is a key indicator of the presence of such electrons, signifying that the dissolution process has successfully taken place. This captivating blue color offers more than aesthetic appeal; it's a direct result of chemical interactions at the molecular level, showcasing the dynamic nature of electrons in the system.

Electrical Conductivity in Solutions

Liquid ammonia solutions containing dissolved sodium metal exhibit enhanced electrical conductivity. The free electrons introduced during the ionization of sodium atoms carry electrical current through the solution.

• Conductivity is due to the mobility of free electrons.
• Electrons move freely, transmitting electrical charge.

This property makes the solution an effective conductor of electricity. The increased conductivity underscores the fundamental role of electrons in transporting electrical signals within liquid mediums. It highlights a critical aspect of how dissolved substances can alter the electrical traits of a solution.

Magnetic Properties of Solutions

When sodium dissolves in liquid ammonia, it impacts the magnetic properties of the resulting solution. Normally, substances can be classified as diamagnetic, paramagnetic, or ferromagnetic based on their magnetic characteristics.

• Diamagnetic: Substances repel magnetic fields because of paired electrons.
• Paramagnetic: Materials are attracted to magnetic fields due to unpaired electrons.

Upon the dissolution of sodium, free unpaired electrons are introduced into the ammonia solution. These unpaired electrons make the solution paramagnetic, allowing it to be attracted by magnetic fields rather than repelling them. This change provides insight into how chemical reactions can transform the inherent properties of a solution, especially in relation to magnetic behavior.