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

Question

Borax is converted into \(\mathrm{B}\) 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, Sn (d) acid, Al

Step-by-Step Solution

Answer

The reagents are (d) acid, Al.

Step 1: Identify the Conversion Process

The problem describes a conversion starting with Borax and resulting in elemental silicon, \( \mathrm{B} \). The conversion sequence includes intermediate steps producing \(\mathrm{H}_{3}\mathrm{BO}_{3}\) and \(\mathrm{B}_{2}\mathrm{O}_{3}\). Reagents \( \mathrm{A} \) and \( \mathrm{B} \) are used in these steps.

Step 2: Analyze Reagent A

In the reaction Borax \(\rightarrow \mathrm{H}_{3}\mathrm{BO}_{3}\), reagent \( \mathrm{A} \) must convert Borax to orthoboric acid. Typically, Borax is treated with a mineral acid like hydrochloric acid (HCl) for this conversion, indicating \( \mathrm{A} \) is an acid.

Step 3: Analyze the Dehydration Step

The compound \( \mathrm{H}_{3}\mathrm{BO}_{3}\) is thermally dehydrated to form \( \mathrm{B}_{2}\mathrm{O}_{3} \) through heating (\(\Delta\)). This step doesn't require reagent \( \mathrm{B} \), but prepares for its use.

Step 4: Identify Reagent B

In the final stage, \( \mathrm{B}_{2}\mathrm{O}_{3} \) is reduced to yield elemental boron \( \mathrm{B} \). This reduction is typically achieved using a reducing metal. Common reducing agents for such reductions include aluminum (\( \mathrm{Al} \)) or magnesium (\( \mathrm{Mg} \)). According to the options given, \( \mathrm{B} \) is most likely aluminum (\(\mathrm{Al}\)).

Step 5: Match with the Options Provided

Considering \( \mathrm{A} \) is an acid and \( \mathrm{B} \) is aluminum, the correct answer is option \((\mathrm{d}) \) acid, Al.

Key Concepts

Borax conversion

The conversion of borax into its elemental form, boron, involves several intricate chemical reactions. Borax is a complex compound, chemically represented as \( ext{{Na}}_2[ ext{{B}}_4 ext{{O}}_5( ext{{OH}})_4] \cdot 8 ext{H}_2 ext{O}\). When it undergoes conversion to boron, the first step involves treating borax with an acid. This treatment leads to the production of orthoboric acid (\( ext{H}_3 ext{BO}_3\)).
Reagent A, the acid used, typically hydrochloric acid (HCl), plays a crucial role in this process. When HCl is added to borax, it effectively transforms borax into orthoboric acid through a simple acid-base reaction.
Following this, orthoboric acid undergoes a dehydration process upon heating where it loses water molecules. This dehydration is crucial as it results in the formation of boron trioxide \( ext{B}_2 ext{O}_3\), a pivotal intermediate in this conversion sequence.
Understanding these steps highlights the importance of chemical reactions and their sequential nature in achieving borax conversion.

Reagents in chemical reactions

Reagents are substances or compounds added to a system to bring about a chemical reaction or are involved in a chemical reaction. In our borax conversion, two primary reagents play significant roles: Reagent A and Reagent B.
Reagent A is selected to expertly set off the initial conversion from borax to orthoboric acid. In the given problem, this reagent is an acid, commonly hydrochloric acid (HCl). This choice of reagent ensures a proper reaction setup and is vital for a successful conversion process.
Reagent B is involved in the reduction process that follows the formation of boron trioxide \( ext{B}_2 ext{O}_3\). For this reduction, a reducing metal such as aluminum (Al) steps in. The use of aluminum is advantageous due to its ability to donate electrons, thereby transforming \( ext{B}_2 ext{O}_3\) into elemental boron \( ext{B}\).
Understanding the purpose and selection criteria for these reagents is essential in inorganic chemistry as it provides a foundational explanation for how reactions proceed and how products are ultimately formed.

Reduction of B2O3

The reduction of boron trioxide \( ext{B}_2 ext{O}_3\) to achieve elemental boron is a critical step in the borax conversion process. This reduction process involves a chemical reaction where \( ext{B}_2 ext{O}_3\) is converted into pure boron \( ext{B}\) through the use of a reducing agent.
A reducing agent, as chosen in this conversion sequence, is typically a reactive metal. In many reactions of this nature, aluminum \( ext{Al}\) is used due to its strong reducing properties. Aluminum engages in a process known as electron donation, which is essential for breaking the oxides present in \( ext{B}_2 ext{O}_3\). This leads to the liberation of boron as a pure element.
This reduction is crucial as it represents the final conversion phase from borax to pure boron. Without it, the chemical pathway would end prematurely at the oxide stage. Thus, the choice of reducing agent significantly influences the efficiency and success of the overall chemical transformation.