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.

Short answer

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

Step-by-step solution

Understanding Statements

First, let's take a look at each statement individually to understand what it is saying about the compound \( \mathrm{H}_3\mathrm{BO}_3 \) (boric acid).

Analyzing Statement (a)

Statement (a) claims that boric acid has a layer structure where \( \mathrm{BO}_3 \) units are connected by hydrogen bonds. Boric acid is known for having a planar structure where \( \mathrm{BO}_3 \) units are indeed joined by hydrogen bonds making this statement correct.

Analyzing Statement (b)

Statement (b) states that boric acid can be obtained by treating borax with concentrated \( \mathrm{H}_2\mathrm{SO}_4 \). The reaction of borax with sulfuric acid indeed forms boric acid, so this statement is correct.

Analyzing Statement (c)

Statement (c) claims that boric acid is mainly a monobasic acid and acts as a Lewis acid. Boric acid is known to be a weak monobasic acid as it behaves as a Lewis acid by accepting hydroxide ions rather than donating protons. Thus, this statement is correct.

Analyzing Statement (d)

Statement (d) says boric acid does not donate protons but accepts hydroxyl ions acting as an acid. Boric acid acts as a Lewis acid by accepting hydroxide ions while forming the \( \mathrm{B(OH)_4^-} \) ion, which distinctly means it does not act as a traditional proton donor (Bronsted acid). Hence, this statement is correct.

Final Review

All the given statements - (a), (b), (c), and (d) are consistent with the chemical nature and properties of boric acid.

Key concepts

Layer Structure

Boric acid, or \(\text{H}_3\text{BO}_3\), has a distinctive layer structure that forms due to its molecular arrangement. Its structure consists of \(\text{BO}_3\) units.
These units are flat, consisting of one boron atom and three oxygen atoms that form a triangular configuration.
In boric acid, these \(\text{BO}_3\) units lie on a plane.

As these units align, they create layers, with each layer being held together by hydrogen bonds. The presence of these hydrogen bonds between the layers gives boric acid its unique structural property.
This formation allows for interaction along the plane but keeps the layers distinct, giving rise to its characteristic stability and insolubility properties in certain conditions.

Hydrogen Bonding

Hydrogen bonding is a critical aspect of the structure of boric acid.
In the layer structure, hydrogen bonds form between the \(\text{BO}_3\) units.
The hydrogen atoms from the hydroxyl groups (\(\text{OH}\)) connect with the oxygen atoms of the \(\text{BO}_3\) units in adjacent layers.

This type of bonding is not as strong as covalent bonds but is stronger than van der Waals forces, making it significant in the organization of molecules in boric acid.
These bonds are responsible for the formation of multi-layered arrangements and are crucial for maintaining the planar structure of the acid.
Overall, hydrogen bonding contributes greatly to the behavioral aspects of the compound, such as its ability to dissolve in water.

Monobasic Acid

Boric acid is primarily classified as a monobasic acid.
This means that it can typically donate only one hydrogen ion or proton per molecule.
However, in an aqueous solution, boric acid does not donate a proton in the traditional sense.

Instead, it accepts a hydroxyl ion from water to form \([\text{B(OH)}_4^-]\).
This behavior makes it different from other acids that belong to the monobasic category, as it does not conform to their typical proton-donating characteristic.
Despite this, the nature of the lone hydroxyl ion's involvement still categorizes it as monobasic.

Lewis Acid

Boric acid functions as a Lewis acid, which is essential for understanding its chemical behavior.
A Lewis acid is defined as an electron-pair acceptor.
In boric acid’s case, it acts as a Lewis acid by accepting an electron pair from the hydroxyl ion (\(\text{OH}^-\)).

This interaction results in the formation of the tetrahydroxyborate ion \([\text{B(OH)}_4^-]\).
This behavior distinctly differentiates it from classical Brønsted acids that donate protons.
Therefore, its classification as a Lewis acid is more consistent with its properties, focusing on its ability to integrate electron pairs into its structure.

Proton Donor

Unlike typical acids, boric acid does not generally act as a proton donor.
Proton donors are substances that release \(\text{H}^+\) ions into a solution.
Instead, boric acid participates more actively by accepting hydroxyl ions from the solution.

This distinctive behavior is due to its weak acidic nature and is why it behaves more like a Lewis acid than a Brønsted acid.
In effect, this property explains why boric acid does not behave conventionally as a proton donor despite its capacity to be acidic in certain solutions.
Understanding this helps in differentiating boric acid from more typical acids in various chemical interactions.

Hydroxyl Ion Acceptance

The ability of boric acid to accept hydroxyl ions is a defining feature of its chemical behavior.
In a solution, boric acid interacts with \(\text{OH}^-\) ions rather than releasing \(\text{H}^+\) ions.
This process is characterized by the formation of the ion \([\text{B(OH)}_4^-]\).

This capability underscores its classification as a Lewis acid as it accepts electron pairs from the hydroxyl ions forming a stable anion.
Consequently, this mechanism is essential to explaining its solubility and reactivity in water.
Understanding this is crucial for anyone studying boric acid's role in various chemical reactions and its unique solubility behavior.