Problem 6 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following compounds is not a protonic acid? (a) \(\mathrm{SO}_{2}(\mathrm{OH})_{2}\) (b) \(\mathrm{B}(\mathrm{OH})_{3}\) (c) \(\mathrm{PO}(\mathrm{OH})_{3}\) (d) \(\mathrm{SO}(\mathrm{OH})_{2}\)

Step-by-Step Solution

Answer

(b) \( \mathrm{B}(\mathrm{OH})_{3} \) is not a protonic acid; it's a Lewis acid.

Step 1: Understand Protonic Acids

Identify what a protonic acid is. A protonic acid is a substance that can donate a proton (H⁺) in an aqueous solution.

Step 2: Analyze Compound (a)

Consider \( \mathrm{SO}_{2}(\mathrm{OH})_{2} \). This compound is sulfuric acid, \( \mathrm{H}_2\mathrm{SO}_3 \), which can donate protons, making it a protonic acid.

Step 3: Analyze Compound (b)

Consider \( \mathrm{B}(\mathrm{OH})_{3} \). This is boric acid, which is a Lewis acid, not a protonic acid because it does not readily donate protons. Instead, it accepts OH⁻ ions.

Step 4: Analyze Compound (c)

Consider \( \mathrm{PO}(\mathrm{OH})_{3} \). This represents phosphoric acid, \( \mathrm{H}_3\mathrm{PO}_4 \), which can donate protons, making it a protonic acid.

Step 5: Analyze Compound (d)

Consider \( \mathrm{SO}(\mathrm{OH})_{2} \). This is sulfurous acid, \( \mathrm{H}_2\mathrm{SO}_3 \), and can donate protons as well, hence it is a protonic acid.

Key Concepts

Lewis Acids

Lewis acids are substances that accept an electron pair. This concept is different from the Bronsted-Lowry definition of acids, which involves the donation of a proton (H⁺). Lewis acids do not need to donate protons. Instead, they focus on the acceptance of electrons from another atom or molecule.

For example, boric acid, \( \mathrm{B}(\mathrm{OH})_{3}\) , is a classical Lewis acid. Although it doesn't donate a proton, it forms bonds by accepting electrons. When boric acid interacts with water, it accepts an OH⁻ ion, making it an acid in the Lewis sense.

Understanding Lewis acids is important for grasping how different substances can act as acids in various chemical reactions even if they don't release protons.

Sulfuric Acid

Sulfuric acid is a strong and widely used chemical in many industrial processes. Its chemical formula is \( \mathrm{H}_{2}\mathrm{SO}_{4}\), and it is known as a strong acid because it completely dissociates in water, releasing two protons (H⁺ ions) into the solution.

This ability to donate protons easily makes sulfuric acid a classical example of a protonic acid.

In industrial applications, sulfuric acid is used in the production of fertilizers, in petroleum refining, and in wastewater processing. Its strong acidic nature is due to the presence of the highly electronegative sulfate group \( \mathrm{SO}_{4}^{2-}\), which encourages the release of protons.

Boric Acid

Boric acid is an interesting compound due to its unique acid behavior. Unlike typical acids, boric acid does not readily donate protons. Instead, it behaves as a Lewis acid by accepting an electron pair.

The molecular structure of boric acid is \( \mathrm{B}(\mathrm{OH})_{3}\), and it interacts with water by accepting a hydroxide ion (OH⁻) rather than releasing a proton. This property sets it apart from other more common protonic acids.

Boric acid finds use in various applications such as antiseptics, insecticides, and as a precursor in the production of other boron compounds. Its multifaceted nature illustrates the diversity of acid behaviors beyond just proton donation.

Phosphoric Acid

Phosphoric acid is another classic protonic acid. Its molecular formula is \( \mathrm{H}_{3}\mathrm{PO}_{4}\), and it can release up to three protons in a stepwise manner when dissolved in water. This ability to donate multiple protons makes phosphoric acid quite versatile.

It is widely used in the food and beverage industry, particularly as an additive in soft drinks for acidulating and providing a tangy flavor. It is also employed in the production of fertilizers and in rust removal products.

The stepwise donation of protons from phosphoric acid illustrates how protonic acids can have different strengths of acidity depending on the number of available hydrogen ions for donation.