Problem 15 - A Complete Resource Book in Chemistry for JEE Main

Question

When strongly heated, orthoboric acid gives (a) \(\mathrm{H}_{2} \mathrm{~B}_{4} \mathrm{O}_{7}\) (b) \(\mathrm{B}_{2} \mathrm{O}_{3}\) (c) \(\mathrm{HBO}_{2}\) (d) \(\mathrm{NaBO}_{2}+\mathrm{B}_{2} \mathrm{O}_{3}\)

Step-by-Step Solution

Answer

(b) \( \mathrm{B}_{2} \mathrm{O}_{3} \) is formed when orthoboric acid is strongly heated.

Step 1: Understand the Reaction

Orthoboric acid, chemically represented as \( \text{H}_3\text{BO}_3 \), is a hydrate of boric oxide and decomposes upon heating. We need to identify what it yields when strongly heated.

Step 2: Consider the Thermal Decomposition Process

When orthoboric acid \( \text{H}_3\text{BO}_3 \) is heated above 100°C, it releases water molecules and starts to decompose into various compounds. Further heating leads to the formation of specific products.

Step 3: Product Formation from Orthoboric Acid

On strong heating, orthoboric acid \( \text{H}_3\text{BO}_3 \) first loses water to eventually form boric anhydride, \( \text{B}_2\text{O}_3 \). The overall reaction can be represented as \( 2 \text{H}_3\text{BO}_3 \rightarrow \text{B}_2\text{O}_3 + 3 \text{H}_2\text{O} \).

Step 4: Identify the Correct Answer

From the options given:- (a) \( \text{H}_2 \text{B}_4 \text{O}_7 \) is not formed by this process- (b) \( \text{B}_2\text{O}_3 \) is indeed the product of strongly heating orthoboric acid- (c) \( \text{HBO}_2 \) is not the final product, though it is an intermediate at lower temperatures- (d) \( \text{NaBO}_2 + \text{B}_2\text{O}_3 \) involves sodium, which is irrelevant here Thus, the correct product of strong heating of orthoboric acid is \( \text{B}_2\text{O}_3 \).

Key Concepts

Boric Anhydride Formation

When orthoboric acid, denoted as \( \text{H}_3\text{BO}_3 \), is subjected to high temperatures, it undergoes a thermal decomposition process, ultimately leading to the formation of boric anhydride. This compound is represented by the chemical formula \( \text{B}_2\text{O}_3 \).

The decomposition involves a loss of water molecules as the temperature increases. Specifically, the reaction proceeds as follows:
\[ 2 \text{H}_3\text{BO}_3 \rightarrow \text{B}_2\text{O}_3 + 3 \text{H}_2\text{O} \]

This reaction produces boric anhydride as the final product, accompanied by the release of three water molecules for every two molecules of orthoboric acid. Thus, the correct answer to the given problem is that \( \text{B}_2\text{O}_3 \) is formed under conditions of strong heating.

Orthoboric Acid Chemistry

Orthoboric acid, \( \text{H}_3\text{BO}_3 \), is a weak acid often used in various chemical applications. Its chemistry is centered around its hydrate nature and its ability to form different compounds upon heating.

Some key points about orthoboric acid include:

• It acts as a Lewis acid, meaning it can accept electron pairs.
• It typically exists as a white crystalline solid.
• Orthoboric acid dissolves in water and is mainly used for antiseptic purposes or as an electronics component.

When heated, its chemical nature allows it to lose water incrementally. At temperatures above 100°C, this dehydration process initiates, transitioning through intermediate stages before finally forming boric anhydride. This capacity to transform through temperature-induced reactions underlies its versatility in industrial and laboratory settings.

High Temperature Reactions

Reactions that occur at high temperatures, such as the decomposition of orthoboric acid, are pivotal in the field of chemistry. These reactions often involve breaking bonds and forming new ones, leading to different products than what you'd find under normal conditions.

Some features of high-temperature reactions include:

• They typically require careful control of temperature to manage the specific phases and products formed.
• Such reactions may not be reversible, as new bonds formed are often more stable.
• They can lead to the creation of compounds that do not form naturally.
  

For orthoboric acid, the high-temperature reaction decomposes it into boric anhydride, exposing the importance of understanding these conditions to manipulate chemical processes effectively. This showcases how chemists can create desired substances by applying heat, resulting in transformations that would not occur at room temperature.