Problem 152 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following $$ \begin{array}{ll} \text { List-I } & \text { List-II } \\ \hline \text { a. } \operatorname{Borax} \stackrel{\Delta}{\longrightarrow} & \text { (p) } \mathrm{BN} \\ \text { b. } \text { Borax }+\mathrm{NH}_{4} \mathrm{Cl} \stackrel{\Delta}{\longrightarrow} & \text { (q) } \mathrm{H}_{2} \mathrm{~B}_{4} \mathrm{O}_{7} \end{array} $$ c. Borax \(\stackrel{\Delta}{\longrightarrow}\) (r) \(\mathrm{H}_{3} \mathrm{BO}_{3}\) d. Borax \(+\mathrm{H}_{2} \mathrm{O} \longrightarrow\) (s) \(\mathrm{NaBO}_{2}+\mathrm{B}_{2} \mathrm{O}_{3}\)

Step-by-Step Solution

Answer

a-s, b-p, c-r, d-r

Step 1: Understanding Borax Decomposition

Firstly, recognize that when Borax (Na2B4O7) is decomposed by heat (Δ), it forms products. The main decomposition reaction for Borax under heat is:\[\text{Na}_2\text{B}_4\text{O}_7 \stackrel{\Delta}{\longrightarrow} \text{NaBO}_2 + \text{B}_2\text{O}_3\]Thus, the match for Borax decomposing by heat is option (s).

Step 2: Borax with NH4Cl

The reaction of Borax with NH4Cl under heat results in the formation of Boron Nitride (BN). This is not a direct or common reaction in basic chemistry but typically involves a two-step synthesis process involving decomposition and subsequent reaction to yield BN. Hence, option (p) is a match with Borax + NH4Cl.

Step 3: Reaction of Borax with Water

When Borax is dissolved in water, it typically forms a boric acid solution. The balanced reaction under neutral conditions is:\[\text{Na}_2\text{B}_4\text{O}_7 + 7\text{H}_2\text{O} \longrightarrow 4\text{H}_3\text{BO}_3 + 2\text{NaOH}\]Thus, Borax with water matches with option (r) \(\text{H}_3\text{BO}_3\).

Step 4: Check Remaining Match

The given choices should naturally pair the remaining process (Borax alone) with its outlet option. Therefore, Borax decomposing naturally from List-I without further specifics tends to match standard decomposition, which was recognized with boric acid \(\text{H}_3\text{BO}_3\) in standard processes. Thus List-II's option (s) aligns here on refinement.

Key Concepts

Borax decomposition

Borax, scientifically known as sodium tetraborate and having the formula \( \text{Na}_2\text{B}_4\text{O}_7 \), undergoes a decomposition reaction when heated. This process is commonly represented by the equation:\[\text{Na}_2\text{B}_4\text{O}_7 \stackrel{\Delta}{\longrightarrow} \text{NaBO}_2 + \text{B}_2\text{O}_3\]
This reaction involves breaking down the Borax molecule into two simpler substances: sodium metaborate (\( \text{NaBO}_2 \)) and boron trioxide (\( \text{B}_2\text{O}_3 \)).

• Sodium Metaborate (\(\text{NaBO}_2 \)): A compound that is often used in various industrial applications due to its basic nature.
• Boron Trioxide (\(\text{B}_2\text{O}_3\)): It acts as a precursor in several chemical reactions, including the preparation of borosilicate glass.

Heating Borax initiates this decomposition, making it an important step in producing these valuable compounds.

reaction with ammonium chloride

When Borax is combined with ammonium chloride (\( \text{NH}_4\text{Cl} \)), especially under heat, it forms a product known as boron nitride (BN). This synthesis usually requires a multi-step process that involves both decomposition and reaction steps.

Understanding Boron Nitride

Boron nitride is a unique compound with properties similar to graphite. It’s widely used due to its:

• Lubricating ability: Ideal for applications needing heat resistance.
• Thermal conductivity: Used in electronic and engineering materials.

The reaction itself can be complex and isn't typically part of standard undergraduate chemistry due to its multiple stages. However, the final synthesized BN is crucial in advanced technological and industrial applications.

boric acid formation

Boric acid (\( \text{H}_3\text{BO}_3 \)) readily forms when Borax is mixed with water. This reaction occurs in a neutral solution resulting in the equation:\[\text{Na}_2\text{B}_4\text{O}_7 + 7\text{H}_2\text{O} \longrightarrow 4\text{H}_3\text{BO}_3 + 2\text{NaOH}\]This transforms Borax into boric acid and sodium hydroxide, both significant in different uses:

• Boric Acid: Known for its antiseptic properties, and used in products like roach killer and as a pH buffer in pools.
• Sodium Hydroxide: A classic alkali, used for cleaning and manufacturing various materials.

This reaction is essential as boric acid acts as a precursor to many boron-containing compounds utilized in daily chemistry-related processes.

boron nitride synthesis

Boron nitride synthesis is a complex process requiring specific starting materials and conditions. This compound (BN) is praised for its remarkable properties similar to those of carbon-based materials like graphite, but with enhanced thermal and chemical stability.

Steps in Synthesis

To synthesize boron nitride, a primary reaction involves Borax and ammonium chloride, following a heat-induced pathway. The challenge is in the careful control of reaction conditions, since:

• High temperature: Necessary to drive the reaction to completion.
• Multi-step process: Requires intermediate steps of decomposition and reaction.

Eventually, the process results in boron nitride, renowned for its uses in:

• High-performance insulators.
• Heat-resistant materials.
• Hafnium/hexagonal BN ceramics.

Boron nitride thus finds applications in advanced technology sectors, with its synthesis being a nuanced area of chemistry requiring precision and an understanding of high-temperature processes.