Problem 159
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
Verified Answer
All statements (a), (b), (c), and (d) are correct.
1Step 1: Understand the Structure of Boric Acid
Boric acid,
H
3
BO
3
, is known to have a layer structure. Within these layers, the
BO
3
units are connected by hydrogen bonds. This supports statement (a): "It has a layer structure in which
BO
3
units are joined by hydrogen bonds."
2Step 2: Formation of Boric Acid
Boric acid is typically formed by reacting borax (
Na
2
B
4
O
7
·10
H
2
O) with concentrated
H
2
SO
4
. This reaction prevents the loss of water and results in the formation of
H
3
BO
3
, validating statement (b): "It is obtained by treating borax with conc.
H
2
SO
4
."
3Step 3: Acidic Nature of Boric Acid
Boric acid is considered a weak monobasic acid because it does not dissociate in water to release protons but instead accepts hydroxide ions from water, thus functioning as a Lewis acid. This confirms part of statement (c): "It is mainly monobasic acid and a Lewis acid."
4Step 4: Boric Acid as a Lewis Acid
As mentioned, boric acid acts as a Lewis acid by accepting hydroxyl ions. It does not donate protons, aligning with statement (d): "It does not act as a proton donor but acts as an acid by accepting hydroxyl ions." This further describes its Lewis acid behavior.
Key Concepts
Layer StructureHydrogen BondsLewis AcidMonobasic Acid
Layer Structure
Boric acid, with the chemical formula \( \text{H}_3\text{BO}_3 \), is a compound that has a unique and interesting layer structure. These layers are not ordinary; they consist of planar units known as \( \text{BO}_3 \) triangles. Each of these triangles is connected to its neighbors, forming a two-dimensional structure.
The connections between these \( \text{BO}_3 \) units are established through hydrogen bonds, which are a special type of bond that occurs when hydrogen is attracted to an electronegative element nearby. In the case of boric acid, these hydrogen bonds occur between the layers, holding them together like a stack of sheets.
This layer structure with its hydrogen bonding network is crucial for its physical properties and influences how boric acid interacts with other substances.
The connections between these \( \text{BO}_3 \) units are established through hydrogen bonds, which are a special type of bond that occurs when hydrogen is attracted to an electronegative element nearby. In the case of boric acid, these hydrogen bonds occur between the layers, holding them together like a stack of sheets.
This layer structure with its hydrogen bonding network is crucial for its physical properties and influences how boric acid interacts with other substances.
Hydrogen Bonds
Hydrogen bonds are an essential feature of many molecules, including boric acid. In boric acid, hydrogen bonds help create and maintain the unique layering formation. But what exactly are hydrogen bonds?
A hydrogen bond is a force that occurs when a hydrogen atom bonded to a highly electronegative atom like oxygen or nitrogen is attracted to another electronegative atom in a nearby molecule. This is essentially an attraction between a hydrogen atom and an electronegative atom.
In boric acid, these bonds play a key role in holding the layers of \( \text{BO}_3 \) units together, which influences its solubility and overall stability when combined with other compounds. Hydrogen bonds are important for the structural integrity of the entire compound, much like rivets in a metal structure.
A hydrogen bond is a force that occurs when a hydrogen atom bonded to a highly electronegative atom like oxygen or nitrogen is attracted to another electronegative atom in a nearby molecule. This is essentially an attraction between a hydrogen atom and an electronegative atom.
In boric acid, these bonds play a key role in holding the layers of \( \text{BO}_3 \) units together, which influences its solubility and overall stability when combined with other compounds. Hydrogen bonds are important for the structural integrity of the entire compound, much like rivets in a metal structure.
Lewis Acid
Boric acid is classified as a Lewis acid. The term "Lewis acid" might sound complex, but it's a simple concept. Unlike the Bronsted-Lowry definition of acids as proton donors, a Lewis acid is any species that can accept a pair of electrons.
In the case of boric acid, it does not release a proton when dissolved in water as one might expect from an acid. Instead, it interacts with water by accepting an electron pair from the hydroxide ion \( \text{OH}^- \), subsequently forming \( \text{B(OH)}_4^- \).
This ability to accept electron pairs without donating protons is why boric acid is a Lewis acid. This makes it quite different from typical acids that students might be familiar with, such as hydrochloric acid (\( \text{HCl} \)). Understanding this behavior is crucial to understanding why boric acid behaves the way it does in chemical reactions.
In the case of boric acid, it does not release a proton when dissolved in water as one might expect from an acid. Instead, it interacts with water by accepting an electron pair from the hydroxide ion \( \text{OH}^- \), subsequently forming \( \text{B(OH)}_4^- \).
This ability to accept electron pairs without donating protons is why boric acid is a Lewis acid. This makes it quite different from typical acids that students might be familiar with, such as hydrochloric acid (\( \text{HCl} \)). Understanding this behavior is crucial to understanding why boric acid behaves the way it does in chemical reactions.
Monobasic Acid
Boric acid is termed a monobasic acid, and this term describes its capacity to react with bases. A monobasic acid is one that can furnish only one hydrogen ion (\( \text{H}^+ \)) per molecule when it reacts with a base.
Despite being identified as monobasic, boric acid behaves differently from typical monobasic acids. Instead of directly supplying a proton, it functions by accepting a hydroxide ion. When boric acid is dissolved in water, it forms the tetrahydroxyborate ion \( \text{B(OH)}_4^- \), by gaining a hydroxide ion.
This distinctive method of interaction aligns with its classification as both a monobasic and Lewis acid. It's a fascinating aspect of boric acid that provides insight into the diversity of acid types and their interaction with bases, adding depth to our understanding of chemical reactions.
Despite being identified as monobasic, boric acid behaves differently from typical monobasic acids. Instead of directly supplying a proton, it functions by accepting a hydroxide ion. When boric acid is dissolved in water, it forms the tetrahydroxyborate ion \( \text{B(OH)}_4^- \), by gaining a hydroxide ion.
This distinctive method of interaction aligns with its classification as both a monobasic and Lewis acid. It's a fascinating aspect of boric acid that provides insight into the diversity of acid types and their interaction with bases, adding depth to our understanding of chemical reactions.
Other exercises in this chapter
Problem 157
Which one of the following is/are the incorrect statement(s)? (a) Boric acid is a protonic acid (b) Beryllium exhibits coordination number of six. (c) Chlorides
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Which of the following statement(s) is correct here: (a) Hypo phosphorous acid shows reducing properties (b) Pyrophosphoric acid has although \(4-\mathrm{OH}\)
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