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

Step 1: Understanding Acidic Strength

Acidic strength of an acid is determined by its ability to donate protons (H⁺ ions). The more easily an acid donates a proton, the stronger it is. Factors influencing acidic strength include the stability of the anion formed, the electronegativity of the atoms involved, and the oxidation state of the central atom.

Step 2: Evaluating Perchloric Acid (HClO₄)

HClO₄ is known to be a very strong acid. The ClO₄⁻ ion is very stable due to resonance and the high electronegativity of chlorine. The significant electronegativity of oxygen in ClO₄⁻ also helps delocalize the negative charge, contributing to its strong acidic nature.

Step 3: Evaluating Sulfuric Acid (H₂SO₄)

H₂SO₄ is a strong acid. When it dissociates, it forms HSO₄⁻, which is still a relatively strong acid. However, H₂SO₄ is not as strong as HClO₄, because HSO₄⁻ holds its proton more tightly compared to ClO₄⁻, which doesn't hold its proton at all in solution.

Step 4: Evaluating Phosphoric Acid (H₃PO₄)

H₃PO₄ is a weaker acid compared to both HClO₄ and H₂SO₄. It partially ionizes in solution to form H₂PO₄⁻. The PO₄³⁻ ion is also less stable compared to the sulfate or perchlorate ion, making H₃PO₄ a weaker acid.

Step 5: Ranking the Acids by Strength

Considering the relative strengths calculated, the decreasing order of acidity would be: 1. HClO₄ (strongest due to stable anion and high electronegativity)2. H₂SO₄ (strong but not as strong as HClO₄)3. H₃PO₄ (weakest among the three). Thus, the correct order is \[ \text{III} > \text{I} > \text{II} \]

Key Concepts

Perchloric Acid

Perchloric acid, represented chemically as HClO₄, is renowned for being one of the strongest acids known. This potent acidic strength is due to several key reasons. Firstly, perchloric acid is capable of almost complete dissociation in solution, meaning it readily loses its proton
This characteristic is what defines a strong acid.
A significant factor contributing to this behavior is the stability of the perchlorate anion, ClO₄⁻. This anion benefits from extensive resonance.

• The negative charge is well-distributed over the four oxygen atoms.
• Chlorine is highly electronegative, facilitating the stability of the negative ion.

Additionally, oxygen itself has high electronegativity, which further helps in delocalizing the negative charge. This exceptional stability of ClO₄⁻ distills the need to hold onto the proton, enabling HClO₄ to donate protons more readily and exhibit greater acidic strength compared to other acids, such as sulfuric and phosphoric acids.

Sulfuric Acid

Sulfuric acid, denoted as H₂SO₄, is another well-known strong acid and often used in a variety of industrial applications. Despite its strength, it is slightly less potent than perchloric acid.
When sulfuric acid dissociates in water, it primarily forms the bisulfate anion, HSO₄⁻.

• HSO₄⁻ itself is a moderately strong acid.
• The sulfur atom in sulfuric acid is in a high oxidation state, which affects the overall acidic strength.

While sulfuric acid is powerful, the anion HSO₄⁻ does not relinquish its hydrogen ion as readily compared to the intact release shown by perchlorate ions in HClO₄. This means that sulfuric acid, although strong, holds onto its proton a little more tightly, rendering it slightly weaker than perchloric acid.

Phosphoric Acid

Phosphoric acid, expressed chemically as H₃PO₄, is significantly weaker when compared to both perchloric and sulfuric acids. Phosphoric acid is regarded as a triprotic acid, meaning it can lose up to three protons, but this does not happen readily all at once.

• The anion formed initially when phosphoric acid ionizes is dihydrogen phosphate, H₂PO₄⁻.
• The subsequent ions formed, HPO₄²⁻ and PO₄³⁻, are even weaker.

A critical point in understanding the relative weakness of phosphoric acid lies in the lesser stability of its conjugate base, PO₄³⁻. This instability contrasts sharply with the more stable conjugate bases of stronger acids like HClO₄ and H₂SO₄. Hence, phosphoric acid does not donate its protons as easily, categorizing it as the weakest among these three acids.