Question

Orthoboric acid \(\left(\mathrm{H}_{3} \mathrm{BO}_{3}\right)\) and metaboric acid \(\left(\mathrm{HBO}_{2}\right)\) differ in respect of (a) Structure (b) Basicity (c) Melting point (d) All of these

Short answer

(d) All of these

Step-by-step solution

Analyze the Structure

Orthoboric acid (H₃BO₃) forms a layer structure with BO₃ units and intermolecular hydrogen bonds, whereas metaboric acid (HBO₂) has a polymeric chain structure. Therefore, they have different structural arrangements.

Assess the Basicity

Orthoboric acid is often considered a monobasic Lewis acid, accepting hydroxide ions rather than dissociating into protons, which reflects different acid behavior compared to metaboric acid.

Determine the Melting Points

Orthoboric acid has a melting point of about 170°C, while metaboric acid typically melts at higher temperatures around 236°C, indicating variation in melting points.

Conclusion

Considering the differences in structure, basicity, and melting point, orthoboric acid and metaboric acid differ in all the mentioned aspects.

Key concepts

Orthoboric Acid

Orthoboric acid, with the formula \( ext{H}_3 ext{BO}_3\), is a gentle but significantly important compound in the world of chemistry. It is often represented by the structure consisting of a central boron atom attached to three hydroxyl groups \( ext{OH}\). This forms a layer structure, interconnected by hydrogen bonds. These hydrogen bonds are crucial as they contribute to its solid form and mild acidic properties. Known in practical applications as boric acid, it sees popular use as an antiseptic, insecticide, and in laboratory settings for buffering solutions.

• Formula: \( ext{H}_3 ext{BO}_3\)
• Structure: Layered, consisting of \( ext{BO}_3\) units with hydrogen bonds
• Uses: Antiseptic, insecticide, laboratory buffer

Metaboric Acid

Unlike orthoboric acid, metaboric acid \( ext{HBO}_2\) differs significantly in its structural formation. This compound adopts a polymeric chain-like structure. The difference in structure is because metaboric acid can be obtained by heating orthoboric acid, leading to a dehydration process that condenses the molecule into its chain form. Its uses include being a synthetic chemical intermediate and having unique applications in some fibrous materials and specialty glasses.

• Formula: \( ext{HBO}_2\)
• Structure: Polymeric chain
• Uses: Synthetic intermediate, specialty glasses

Structure and Properties

Both orthoboric and metaboric acids exhibit distinct structural dissimilarities that affect their properties. Orthoboric acid, forming a planar trigonal structure, allows for intermolecular hydrogen bonding, leading to a layered assembly. This setup not only affects its solidity but also plays a role in how it interacts with other compounds as a weak acid. On the other hand, metaboric acid's chain polymeric structure is more complex, resulting in different physical properties, such as solubility and reactivity.

• Orthoboric: Planar and hydrogen-bonded layers
• Metaboric: Chain polymerization

These structural variations directly influence their behavior in chemical reactions and physical settings.

Basicity

In the realm of acids, basicity refers to the acid's ability to donate protons or accept electron pairs. Orthoboric acid is somewhat unconventional when it comes to acid classification. It behaves more like a Lewis acid, meaning it prefers to accept hydroxide ions \( ext{OH}^-\) to form complexes such as \([ ext{B(OH)}_4]^–\). This behavior limits the typical 'proton donation' associated with more common stronger acids. Metaboric acid, though similar, shows variant proton-exchanging characteristics due to its polymeric nature.
This leads to unique reaction paths depending on the environment where these acids are present.

• Orthoboric: Lewis acid, prefers forming complexes
• Metaboric: Varies due to polymeric form

Melting Point

The melting point is a fundamental property that indicates the temperature at which a substance changes from solid to liquid. Orthoboric acid melts at approximately 170°C. This relatively low melting point reflects its weak layer structure held together by hydrogen bonds. In contrast, metaboric acid melts at a much higher temperature of around 236°C. This is because its polymeric chain structure provides additional stability, resisting thermal breakdown more effectively than orthoboric acid's layered structure.
Understanding melting points is key in applications where thermal stability and reactivity matter, such as in material sciences or chemical syntheses.

• Orthoboric acid: Melts at 170°C
• Metaboric acid: Melts at 236°C

This comparison highlights the significance of structural integrity in thermal properties.