Problem 20 - Chemistry: Introducing Inorganic, Organic and Physical Chemistry

Question

\(\mathrm{H}_{3} \mathrm{BO}_{3}\) is a monobasic acid whereas \(\mathrm{H}_{3} \mathrm{PO}_{3}\) is a dibasic acid. Account for this difference. (Sections 27.2,27.4 )

Step-by-Step Solution

Answer

\(\mathrm{H}_{3} \mathrm{BO}_{3}\) acts as a Lewis acid, while \(\mathrm{H}_{3} \mathrm{PO}_{3}\) donates two protons, making them monobasic and dibasic respectively.

Step 1: Understand the Basicity of Acids

Basicity refers to the number of hydrogen ions (H+) an acid can donate in a chemical reaction. A monobasic acid donates one proton per molecule, while a dibasic acid donates two.

Step 2: Structure and Behavior of \(\mathrm{H}_{3} \mathrm{BO}_{3}\)

Step 3: Structure and Behavior of \(\mathrm{H}_{3} \mathrm{PO}_{3}\)

Analyze the chemical structure of \(\mathrm{H}_{3} \mathrm{PO}_{3}\). It has one hydrogen atom directly bonded to oxygen, and two bonded to phosphorus as part of P-OH groups. The H atoms in the P-OH groups can be ionized, making it a dibasic acid.

Step 4: Compare the Molecular Structures

When comparing, note that \(\mathrm{H}_{3} \mathrm{BO}_{3}\) acts as a Lewis acid by accepting OH\(^{-}\) ions rather than donating H\(^{+}\) ions in solution. Meanwhile, \(\mathrm{H}_{3} \mathrm{PO}_{3}\) acts as a Brønsted acid, where H\(^{+}\) ions from the P–OH groups can be donated, leading to dibasic properties.

Key Concepts

Monobasic Acid

A monobasic acid is an acid that can donate only one hydrogen ion (H\(^+\)) per molecule during a chemical reaction. In the context of the provided exercise, boric acid, or \( \mathrm{H}_{3} \mathrm{BO}_{3} \), behaves as a monobasic acid. This might seem confusing at first because its formula suggests three hydrogen atoms, but upon closer inspection, it becomes clear why it is monobasic.
Boric acid, \( \mathrm{H}_{3} \mathrm{BO}_{3} \), doesn’t directly release H\(^+\) ions. Instead, it acts as a Lewis acid, which means it accepts hydroxide ions (OH\(^-\)). When dissolved in water, \( \mathrm{H}_{3} \mathrm{BO}_{3} \) forms tetraborate ions by accepting OH\(^-\) ions and in turn, releases H\(^+\) indirectly through water molecules. Thus, its contribution of only one H\(^+\) makes it monobasic.
Key points to remember about monobasic acids like boric acid:

• It involves indirect release of H\(^+\) through hydroxide ion acceptance.
• Behaves as a Lewis acid rather than a traditional Brønsted acid.
• Despite multiple hydrogen atoms in the structure, only one H\(^+\) is effectively donated in reactions.

Dibasic Acid

A dibasic acid is characterized by its ability to donate two hydrogen ions (H\(^+\)) per molecule. This feature is evident in phosphorous acid, or \( \mathrm{H}_{3} \mathrm{PO}_{3} \). Understanding its chemical structure will clarify this behavior.
In \( \mathrm{H}_{3} \mathrm{PO}_{3} \), there is one hydrogen atom directly bonded to an oxygen atom, forming a hydroxyl (OH) group. There are also two hydrogen atoms connected directly to the phosphorus atom as part of two P-OH groups. These hydrogen atoms in the P-OH groups can be ionized, which provides the acid with its dibasic nature because each group can donate an H\(^+\) ion.
Here are some characteristics of dibasic acids like phosphorous acid:

• Contains at least two ionizable hydrogen atoms.
• The acid's donation of two H\(^+\) ions results from its structure, particularly from OH groups attached via P-OH linkage.
• Behaves according to Brønsted acid theory by donating H\(^+\).

Chemical Structure Analysis

Chemical structure plays a crucial role in determining whether an acid is monobasic or dibasic. By analyzing the molecular structure of \( \mathrm{H}_{3} \mathrm{BO}_{3} \) and \( \mathrm{H}_{3} \mathrm{PO}_{3} \), we gain insights into their different acidic behaviors.
For \( \mathrm{H}_{3} \mathrm{BO}_{3} \), its structure reveals that it cannot donate H\(^+\) in the typical manner. As a Lewis acid, the boron atom prefers to accept an OH\(^-\) ion, forming a stable tetrahedral structure in solution, highlighting its unique behavior as a monobasic acid despite having three hydrogen atoms.
Meanwhile, \( \mathrm{H}_{3} \mathrm{PO}_{3} \) illustrates a classic example of a Brønsted acid wherein its P-OH bonds allow the sequential release of two H\(^+\) ions, underlining its classification as a dibasic acid.
Why chemical structure matters:

• The connectivity and types of bonds dictate the acid’s ability to donate hydrogen ions.
• Lewis versus Brønsted acid behavior is rooted in the structural arrangement of atoms.
• Synonymous formulas may disguise diverse functional behaviors depending on structural details.