Problem 99 - A Complete Resource Book in Chemistry for JEE Main

Question

Solution of sodium metal in liquid ammonia is strongly reducing due to the presence in solution of the following (a) solvated electrons (b) sodium atoms (c) sodium hydride (d) sodium amide

Step-by-Step Solution

Answer

The solution of sodium metal in liquid ammonia is strongly reducing due to solvated electrons.

Step 1: Understand the Exercise

The exercise asks why a solution of sodium metal in liquid ammonia is strongly reducing. We need to consider which species are present in the solution when sodium dissolves in liquid ammonia.

Step 2: Consider Sodium in Liquid Ammonia

When sodium metal is dissolved in liquid ammonia, it forms a blue solution. This is due to the formation of solvated electrons, which are free electrons surrounded by ammonia molecules. They contribute to the color and the reducing nature of the solution.

Step 3: Evaluate Each Option

Let's evaluate each option: (a) Solvated Electrons - These are directly formed when sodium dissolves in ammonia and are known for their reducing power. (b) Sodium Atoms - They do not remain as free atoms in solution. (c) Sodium Hydride - This is not formed in this reaction. (d) Sodium Amide - This is a possible side product but not the primary reducing species.

Step 4: Choose the Correct Answer

The strongly reducing species in the solution is the solvated electrons. Sodium metal loses electrons upon dissolution in ammonia, and these electrons get solvated by ammonia molecules, leading to a highly reducing environment.

Key Concepts

Sodium in Liquid Ammonia

When sodium metal is introduced to liquid ammonia, an interesting transformation occurs. The metal dissolves to form a bright blue solution. This color change is one of the most notable features of this reaction.
As sodium dissolves, it loses electrons. These electrons don't just disappear; they become part of what is known as a "solvated" environment. In this case, they are surrounded by ammonia molecules.

• The positively charged sodium ions separate from these electrons.
• This separation helps create a unique solution with both components existing in an equilibrium.
• The distinct blue color is linked to the presence of these free, solvated electrons in the solution.

This behavior of sodium in liquid ammonia not only gives rise to a striking visual but also underpins the strong reducing nature of the solution. In essence, the resultant blue solution is a vivid indicator of powerful chemical activity.

Solvated Electrons

Solvated electrons are central to the formation of a strongly reducing solution when sodium is dissolved in liquid ammonia. In simple terms, these are free electrons that are temporarily 'trapped' by the solvent, which in this case is ammonia. This might seem complex, but let's break it down.

• When sodium gives up its electrons, these electrons become free to move around.
• Ammonia molecules surround these electrons, creating what we call a 'solvated electron'.
• This environment enables the electrons to behave as powerful reducing agents.

What makes these solvated electrons fascinating is their ability to donate electrons to other substances easily, which is the crux of their reducing ability. These electrons lower the potential energy of the system by stabilizing in the solvent, enabling chemical changes that wouldn't typically occur.

Chemical Solutions

Chemical solutions, like the sodium in liquid ammonia mixture, are intriguing examples of solute-solvent interactions. A chemical solution consists of a solute, which is the dissolved substance, and a solvent, which is the substance doing the dissolving.

• In our example, sodium acts as the solute.
• Liquid ammonia serves as the solvent that facilitates this process.

Understanding how sodium interacts with ammonia gives us insights into not just simple dissolution processes but also into more elaborate chemical reactions that such mixtures can provoke.

The presence of solvated electrons in this chemical solution highlights how different states of matter—solid sodium and liquid ammonia—can combine to produce unexpected and often useful properties. This kind of solution is particularly valued in organic chemistry for its reducing properties, which can be utilized in a variety of synthesis reactions.