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

The correct answer is (b) \(\mathrm{B}_{2}\mathrm{O}_{3}\).

Step 1: Identify the chemical reaction

When orthoboric acid (\(\text{H}_3\text{BO}_3\)) is heated, a dehydration reaction occurs. This involves the loss of water molecules.

Step 2: Explore the dehydration process

Initially, heating orthoboric acid results in the formation of metaboric acid, \(\text{HBO}_2\), by the loss of one water molecule.

Step 3: Dehydration to form dense boron oxide

Upon further heating, metaboric acid (\(\text{HBO}_2\)) loses additional water molecules to form boric oxide, \(\text{B}_2\text{O}_3\), which is a common glass-forming oxide.

Step 4: Conclusion

The most stable compound formed during the complete dehydration of orthoboric acid is \(\text{B}_2\text{O}_3\), indicating that option (b) is the correct answer.

Key Concepts

Dehydration Reaction

A dehydration reaction is a chemical reaction where water (H_2O) is removed from a molecule. It usually happens when a substance, especially one containing hydrogen and oxygen, is heated, causing these components to form water vapor that escapes the structure of the original compound. In the case of orthoboric acid (H_3BO_3), when it is subjected to heat, it undergoes successive dehydration reactions.

Initially, orthoboric acid loses a water molecule through heating, leading to the formation of metaboric acid (HBO_2). With further heating, metaboric acid continues to lose water, eventually forming boric oxide (B_2O_3). These dehydration steps are crucial for transforming and obtaining new chemical compounds from the original substance. Understanding dehydration reactions helps in predicting the behavior of substances when heated, aiding in their practical applications, such as in glass making and industrial chemical processes.

Boric Oxide

Boric oxide, or boron trioxide (B_2O_3), is a significant compound formed from the heating of orthoboric acid through dehydration. It is a white, glassy solid and is known for being a common precursor in the production of borosilicate glass, which is widely used in laboratory glassware and ovenware due to its durability and resistance to thermal shock.

• Properties: Boric oxide is hard, has a high melting point, and is known for its chemical stability. This makes it valuable in products that require high heat resistance.
• Applications: It is also used in the production of ceramics and as a starting material for synthesizing other boron compounds.
• Chemical Behavior: In humid environments or upon contact with water, it can revert back to form boric acid, demonstrating its versatile nature.

Thus, boric oxide serves a valuable role in both industrial applications and academic research to synthesize or recreate essential chemical compounds.

Metaboric Acid

Metaboric acid (HBO_2) is an intermediate compound formed during the dehydration of orthoboric acid. This acid emerges after the initial dehydration step, where orthoboric acid (H_3BO_3) loses one molecule of water through heat. It appears as a white crystalline solid and is less commonly encountered in nature compared to orthoboric acid.

Metaboric acid plays a crucial role as a transitional form before further dehydration leads to boric oxide (B_2O_3). Its unique capacity to undergo further dehydration makes it a key compound in understanding the chemical behavior of boron-containing acids. Although not as well-known, metaboric acid showcases the typical properties of boron compounds, such as forming complex anions and reacting under specific conditions to yield different products. These characteristics make metaboric acid an intriguing subject in both chemical research and industrial application contexts.