Problem 159 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statement is/are correct for \(\mathrm{H}_{3} \mathrm{BO}_{3} ?\) (a) It has a layer structure in which \(\mathrm{BO}_{3}\) units are joined by hydrogen bonds (b) It is obtained by treating borax with conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (c) It is mainly monobasic acid and a Lewis acid (d) It does not act as a proton donor but acts as an acid by accepting hydroxyl ions.

Step-by-Step Solution

Answer

All statements (a), (b), (c), and (d) are correct.

Step 1: Analyze Statement (a)

Boric acid, \( \mathrm{H}_3\mathrm{BO}_3 \), has a layered structure where \( \mathrm{BO}_3 \) units are indeed connected by hydrogen bonds. Thus, statement (a) is correct.

Step 2: Evaluate Statement (b)

Boric acid can be obtained by reacting borax (\( \mathrm{Na}_2\mathrm{B}_4\mathrm{O}_7 \cdot 10 \mathrm{H}_2\mathrm{O} \)) with concentrated sulfuric acid (\( \mathrm{H}_2\mathrm{SO}_4 \)). Therefore, statement (b) is correct.

Step 3: Check Statement (c)

Boric acid is indeed a monobasic acid, meaning it can release only one hydrogen ion in an aqueous solution. Additionally, it acts as a Lewis acid because it accepts electron pairs. Hence, statement (c) is correct.

Step 4: Assess Statement (d)

Although boric acid doesn't donate protons, it does act as an acid by accepting hydroxyl ions from water (from \( \mathrm{H_2O} \)) to form tetrahydroxyborate ion \( \mathrm{[B(OH)_4]^-} \). So, statement (d) is correct.

Key Concepts

Layer Structure

Boric Acid, known as \( \mathrm{H}_{3} \mathrm{BO}_{3} \), exhibits a fascinating layer structure. This structure is primarily formed by the arrangement of \( \mathrm{BO}_{3} \) units. Within these layers, the \( \mathrm{BO}_{3} \) groups are flat, triangular units.

One of the appealing aspects of these layers is the presence of hydrogen bonds. These bonds are crucial as they connect the different layers together. A hydrogen bond forms when a hydrogen atom bonds between two electronegative atoms, such as oxygen.

• The hydrogen bonds make the structure stable.
• They contribute to the physical properties of the boric acid, like melting point and solubility.
This layered and hydrogen-bonded structure allows boric acid to have a solid and slightly soluble nature in water.

Hydrogen Bonds

Hydrogen bonds play a vital role in the composition and properties of boric acid. These are special types of attractions that occur when hydrogen meets very electronegative elements like oxygen.

In boric acid, each hydrogen atom is involved in hydrogen bonding with oxygen atoms from adjacent \( \mathrm{BO}_{3} \) units.

• This interaction between hydrogen atoms and oxygen gives rise to a repeated stable layer configuration.
• These bonds are not as strong as covalent bonds but are essential for maintaining the integrity of the molecular structure.

These bonds are partly responsible for the somewhat weak acidic nature of boric acid compared to other stronger acids.

Monobasic Acid

Boric acid is referred to as a monobasic acid. A monobasic acid is one which can donate only one hydrogen ion (proton) per molecule in an aqueous solution.

However, boric acid behaves a bit differently from typical monobasic acids.

• Instead of donating a proton, it acts by accepting a hydroxide ion (\( \mathrm{OH}^- \)).
• This forms \( \mathrm{B(OH)_4^-} \), a tetrahydroxyborate ion.

This unorthodox method of acting as an acid aligns with its properties as a Lewis acid, making it unique among other acids.

Lewis Acid

A Lewis acid is defined as a chemical compound that can accept an electron pair. Boric acid fits well into this category.

Unlike typical acids that donate protons, boric acid does not release hydrogen ions in water directly. Instead, it accepts electrons which results in its acidic behavior.

• This electron-accepting ability is showcased when boric acid interacts with hydroxide ions (\( \mathrm{OH}^- \)).
• Through this interaction, boric acid forms the \( \mathrm{B(OH)_4^-} \) ion, which is a stable complex.

Boric acid's function as a Lewis acid explains its behavior in various chemical reactions and emphasizes its importance in organic and inorganic chemistry.