Problem 106 - A Complete Resource Book in Chemistry for JEE Main

Question

The decreasing order of acidic nature of \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (I), \(\mathrm{H}_{3} \mathrm{PO}_{4}(\mathrm{II})\), and \(\mathrm{HClO}_{4}(\mathrm{III})\) is (a) \(\mathrm{I}>\mathrm{II}>\mathrm{III}\) (b) \(\mathrm{I}>\mathrm{III}>\mathrm{II}\) (c) \(\mathrm{III}>\mathrm{I}>\mathrm{II}\) (d) \(\mathrm{III}>\mathrm{II}>\mathrm{I}\)

Step-by-Step Solution

Answer

The order is (c) III > I > II.

Step 1: Understand Acid Strength

The strength of an acid depends on its ability to donate protons and the stability of the conjugate base. Generally, the more oxygen atoms around the central atom, the stronger the acid, due to better distribution of negative charge in the conjugate base.

Step 2: Analyze Perchloric Acid (HClO_{4})

Perchloric acid (HClO_{4}) is a very strong acid due to its ability to stabilize the conjugate base with multiple electronegative oxygen atoms. It completely dissociates in water, making it stronger than most other acids.

Step 3: Analyze Sulfuric Acid (H_{2}SO_{4})

Sulfuric acid (H_{2}SO_{4}) is also a strong acid but not as strong as perchloric acid. It is a strong acid due to being able to donate two protons and the stability of its conjugate base, hydrogen sulfate (HSO_{4}^{-}).

Step 4: Analyze Phosphoric Acid (H_{3}PO_{4})

Phosphoric acid (H_{3}PO_{4}) is a weaker acid compared to sulfuric and perchloric acids. It is a triprotic acid, but its conjugate bases (H_{2}PO_{4}^{-}, HPO_{4}^{2-}, and PO_{4}^{3-}) are less stable and less able to efficiently delocalize charge.

Step 5: Rank Acids by Strength

Based on the analysis, perchloric acid (HClO_{4}) is the strongest, followed by sulfuric acid (H_{2}SO_{4}), and phosphoric acid (H_{3}PO_{4}) is the weakest. Thus, the decreasing order of acidic nature is: III > I > II.

Key Concepts

Perchloric Acid

Perchloric acid, denoted as \( \text{HClO}_{4} \), is recognized as one of the strongest acids. Its remarkable strength is due to the complete dissociation of its hydrogen proton in solution, resulting in \( \text{ClO}_{4}^{-} \) ions. The high acidity is facilitated by the presence of multiple electronegative oxygen atoms. These oxygen atoms are strategically positioned around the chlorine central atom.
This strategic placement allows for an even distribution of negative charge when the acid loses a proton, hence stabilizing the conjugate base.

• Complete dissociation in water
• Stabilization through multiple oxygen atoms
• Strong proton donor

This makes perchloric acid not just a strong acid, but a powerful oxidizing agent as well, especially in concentrated forms.

Sulfuric Acid

Sulfuric acid, \( \text{H}_{2}\text{SO}_{4} \), is a notable strong acid, although it is not as potent as perchloric acid. Known for its corrosive nature, sulfuric acid is diprotic, meaning it can donate two protons. Thus, it ionizes in two stages: Initially to form hydrogen sulfate ions \( \text{HSO}_{4}^{-} \), and further ionization results in \( \text{SO}_{4}^{2-} \).

• Two stages of ionization
• Formation and stability of hydrogen sulfate ions \( \text{HSO}_{4}^{-} \)
• Strong oxidative and dehydrating properties

Although sulfuric acid can donate more protons compared to perchloric acid, the presence of fewer oxygen atoms around sulfur compared to chlorine in perchloric acid makes it slightly weaker in terms of acid strength. The negative charge in its conjugate base is less delocalized, affecting its stability.

Phosphoric Acid

Phosphoric acid, \( \text{H}_{3}\text{PO}_{4} \), is a weaker acid in comparison to both perchloric and sulfuric acids. This acid is triprotic, which means it can donate three protons sequentially. While it has multiple opportunities to release protons, each proton is less easily dissociated than those in sulfuric or perchloric acids.

• Three-step ionization process
• Sequential formation of \( \text{H}_{2}\text{PO}_{4}^{-}, \text{HPO}_{4}^{2-}, \text{PO}_{4}^{3-} \)
• Less stable conjugate bases

Phosphoric acid's conjugate bases have less efficient charge delocalization, making each step of ionization less favorable. The decreased stability in the conjugate bases accounts for its lesser overall acid strength.

Conjugate Base Stability

The stability of the conjugate base is a key determinant in the strength of an acid. After an acid donates its proton, the remaining ion is known as the conjugate base. A stable conjugate base results in a stronger acid because it means charge is effectively distributed or delocalized.

• Effective charge distribution increases stability
• The presence of electronegative atoms improves charge delocalization
• More oxygen atoms often correlate with better conjugate base stability

In the case of perchloric, sulfuric, and phosphoric acids, the order of strength follows the ability to stabilize the negative charge through extensive delocalization. Perchloric acid, having the most oxygen atoms, forms the most stable conjugate base, followed by sulfuric acid, then phosphoric acid. This hierarchy confirms the relative acidity order: \( \text{HClO}_{4} > \text{H}_{2}\text{SO}_{4} > \text{H}_{3}\text{PO}_{4} \).