Problem 115 - A Complete Resource Book in Chemistry for JEE Main

Question

Borax is converted into B by following steps Borax \(\stackrel{\mathrm{A}}{\longrightarrow} \mathrm{H}_{3} \mathrm{BO}_{3} \stackrel{\Delta}{\longrightarrow} \mathrm{B}_{2} \mathrm{O}_{3} \quad \mathrm{~B} \stackrel{\mathrm{B}}{\longrightarrow}\) Product Reagents \(\mathrm{A}\) and \(\mathrm{B}\) are (a) acid, \(\mathrm{Fe}\) (b) acid, \(\mathrm{Mg}\) (c) acid, \(\mathrm{Sn}\) (d) acid, Al

Step-by-Step Solution

Answer

The reagents are: (b) acid, \(\mathrm{Mg}\).

Step 1: Convert Borax to Boric Acid

Borax, also known as sodium borate, is first converted to boric acid (\(\mathrm{H}_3\mathrm{BO}_3\)) by treating it with an acid. This is a neutralization reaction. The common reagent for this conversion is hydrochloric acid (HCl). Hence, the reagent \(A\) used here is an acid.

Step 2: Decompose Boric Acid to Boron Trioxide

The next step involves heating boric acid (\(\mathrm{H}_3\mathrm{BO}_3\)) to decompose it into boron trioxide (\(\mathrm{B}_2\mathrm{O}_3\)). This process is a thermal decomposition reaction and does not involve any additional reagents. It is indicated by the symbol \(\Delta\) denoting heat.

Step 3: Reduce Boron Trioxide to Elemental Boron

Finally, to obtain elemental boron (\(\mathrm{B}\)), boron trioxide \(\mathrm{B}_2\mathrm{O}_3\) is reduced. A metal is used as the reducing agent to facilitate this reduction. The possible metals given as options in the problem are \( \mathrm{Fe} \), \( \mathrm{Mg} \), \( \mathrm{Sn} \), and \( \mathrm{Al} \). Among these, \( \mathrm{Mg} \) is a common reducing agent for boron trioxide to get elemental boron. Therefore, \(B\) is \( \mathrm{Mg} \).

Key Concepts

Boric Acid Formation

Boric acid formation is a crucial step in the conversion of borax to elemental boron. Borax, or sodium borate, naturally contains boron. To convert borax into boric acid (\( \mathrm{H}_3\mathrm{BO}_3 \)), a neutralization reaction occurs, where borax is treated with an acid.
Hydrochloric acid (\( \mathrm{HCl} \)) is commonly used, facilitating the transformation by reacting with the borate ions to form boric acid and other neutralized products.
This step is represented as:

• Borax + \( \mathrm{HCl} \) \( \rightarrow \) Boric Acid + Other Products

Thermal Decomposition

After forming boric acid, the next objective is to decompose it thermally. Thermal decomposition is a chemical process in which a compound breaks down due to the application of heat. For boric acid (\(\mathrm{H}_3\mathrm{BO}_3\)), heating causes it to convert into boron trioxide (\(\mathrm{B}_2\mathrm{O}_3\)).
This process is critical because boron trioxide is an intermediary step before obtaining pure boron.
The equation for this thermal decomposition is:

• \(\mathrm{H}_3\mathrm{BO}_3 \xrightarrow{\Delta} \mathrm{B}_2\mathrm{O}_3 + \mathrm{H}_2\mathrm{O} \)

Where \(\Delta\) symbolizes the application of heat.

Reduction of Boron Trioxide

In the final conversion step, boron trioxide (\(\mathrm{B}_2\mathrm{O}_3\)) undergoes reduction to achieve elemental boron (\(\mathrm{B}\)). This reduces the oxidation state of boron, by removing oxygen atoms and adding metal atoms.
A metal reducing agent, often magnesium (\(\mathrm{Mg}\)), is used because it has a strong tendency to donate electrons and react with oxygen.

• \(\mathrm{B}_2\mathrm{O}_3 + 3\mathrm{Mg} \rightarrow 2\mathrm{B} + 3\mathrm{MgO} \)

This reaction emphasizes the removal of oxygen, liberating the pure boron.

Neutralization Reaction

Neutralization reactions provide a foundational understanding of converting borax to boric acid.
These reactions occur between an acid and a base, resulting in a salt and water, often releasing heat.
In the context of borax conversion, it means combining borax with an acid like hydrochloric acid, leading to the formation of boric acid and neutral salts from the base and acid interaction.

• Example: Borax + \( \mathrm{HCl} \) \( \rightarrow \) Boric Acid + Neutralized Salts

Understanding this principle helps in recognizing how neutralization fits into larger transformation processes in chemistry.

Reducing Agents in Chemistry

Reducing agents are substances that can donate electrons to another substance, causing their own oxidation while reducing the other substance.
In the borax to boron conversion, a reducing agent is essential in turning boron trioxide (\(\mathrm{B}_2\mathrm{O}_3\)) to elemental boron. Metals like magnesium (\(\mathrm{Mg}\)) serve this purpose.
They effectively strip away oxygen atoms from boron compounds by undergoing an oxidation reaction themselves:

• \(\mathrm{B}_2\mathrm{O}_3 + 3\mathrm{Mg} \rightarrow 2\mathrm{B} + 3\mathrm{MgO} \)

By understanding reducing agents, you can appreciate their role in various chemical transformations, not just in this specific context.