Question

\(\mathrm{H}_{3} \mathrm{BO}_{j}\), Boric acid is a. Aprotic acid b. dibasic acid c. tribasic acid d. monobasic acid

Short answer

Boric acid is an aprotic acid.

Step-by-step solution

Identify the Nature of Boric Acid

Boric acid, with the formula \( \text{H}_3\text{BO}_3 \), is commonly discussed in chemistry as an acid. However, to determine its specific properties, we must understand how it behaves in water.

Understand Boric Acid's Behavior in Water

Boric acid does not donate protons directly in aqueous solutions. Instead, it acts as a Lewis acid by accepting hydroxide ions \( \text{OH}^- \) from water, forming \( \text{B(OH)}_4^- \). This process decreases the relative concentration of \( \text{H}^+ \) ions, indirectly releasing \( \text{H}^+ \) ions.

Evaluate Acidic Properties

Since boric acid accepts hydroxide ions rather than releasing hydrogen ions, it cannot be classified as a monobasic, dibasic, or tribasic acid, which typically donate one, two, or three protons respectively.

Key concepts

Lewis Acid

Boric acid is an interesting molecule that serves as a classic example of a Lewis acid. Unlike traditional acidic compounds that donate protons, a Lewis acid has the hallmark property of accepting electron pairs. In the case of boric acid, it specifically acts by accepting hydroxide ions from water.

• Lewis acids are electrons acceptors, making them distinct from the Bronsted-Lowry acid definition which focuses on proton donation.
• This behavior is crucial in understanding the nature of boric acid's reaction in water.

When it accepts a hydroxide ion, the reaction facilitates the release of hydrogen ions into the solution, but through a different pathway than donating hydrogen ions directly. This unique mechanism helps define why boric acid is classified as a Lewis acid rather than a traditional proton-donating acid.

Proton Acceptor

It might seem counterintuitive, but boric acid is not a typical acid in the sense of donating protons (or hydrogen ions, \( ext{H}^+ \)). Instead, it behaves in the aquatic environment as a proton acceptor indirectly through its interaction with hydroxide ions. Here’s how it works:

• Upon interaction with water, boric acid accepts hydroxide ions \( ext{OH}^- \).
• This interaction forms the compound \( ext{B(OH)}_4^- \).

Since boric acid accepts the base component (hydroxide ion) of water, it essentially reduces the availability of hydroxide ions which shifts the balance, leading to the indirect release of hydrogen ions. This nature makes it function as a proton acceptor in a broader chemical context.

Acidic Properties

The examination of boric acid’s acidic properties provides insight into why it cannot be classified alongside the typical monobasic, dibasic, or tribasic acids. Traditional acids in these categories donate one, two, or three protons respectively.

• A monobasic acid releases one proton.
• A dibasic acid releases two protons.
• A tribasic acid releases three protons.

Boric acid does neither. Instead of releasing protons directly, its mechanism always involves the acceptance of hydroxide ions, shifting the equilibrium to affect the proton levels indirectly. Therefore, its method of interacting with water sets it apart from proton-releasing acids, marking its place as a Lewis acid instead.

Aqueous Solutions

The behavior of boric acid in aqueous solutions highlights a significant characteristic of its chemical activity. In these solutions, boric acid undergoes a reaction that does not directly correlate with the typical acid behavior of releasing hydrogen ions. The presence of water is essential to understand this pattern.

• In aqueous solutions, boric acid reacts with water to accept \( ext{OH}^- \) ions.
• This creates \( ext{B(OH)}_4^- \), reducing hydroxide and subsequently changing the concentration of \( ext{H}^+ \).

Therefore, while boric acid affects the hydrogen ion concentration indirectly, its action occurs through a different pathway than what would be expected from classic proton donating acids. This unique response in aqueous environments further underscores why boric acid is characterized as a Lewis acid.