Problem 107 - 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

(c)  om{III} >  om{I} >  om{II}

Step 1: Understanding Acid Strength

The strength of an acid is determined by its ability to donate protons (hydrogen ions, H"). Stronger acids dissociate more completely in water, releasing more H+ ions.

Step 2: Examining  om{HClO}_4

Perchloric acid,  om{HClO}_4, is a very strong acid. It is known to be one of the strongest Bronsted-Lowry acids, fully dissociating in solution to release hydrogen ions.

Step 3: Examining  om{H}_2 om{SO}_4

Sulfuric acid,  om{H}_2 om{SO}_4, is a strong acid and fully dissociates in its first ionization step, but it is weaker than  om{HClO}_4 due to its partial dissociation in secondary ionization.

Step 4: Examining  om{H}_3 om{PO}_4

Phosphoric acid,  om{H}_3 om{PO}_4, is a weak acid. Only its first ionization is significant, and it is much weaker compared to  om{HClO}_4 and  om{H}_2 om{SO}_4.

Step 5: Arranging in Decreasing Order of Acidic Strength

Based on the above, the order from strongest to weakest acid is  om{HClO}_4 >  om{H}_2 om{SO}_4 >  om{H}_3 om{PO}_4.

Key Concepts

Acid Dissociation

Acid dissociation is a key concept in understanding how acids behave in solution. It refers to the process whereby an acid releases hydrogen ions, represented as \(H^+\), into a solution. The extent of dissociation determines the strength of the acid. - **Complete Dissociation**: Strong acids, such as perchloric acid \(\text{HClO}_4\), undergo nearly complete dissociation. This means that almost all acid molecules release their \(H^+\) ions into the solution, leading to a high concentration of hydrogen ions.- **Partial Dissociation**: Weak acids do not dissociate completely. For example, phosphoric acid \(\text{H}_3\text{PO}_4\) partially dissociates, meaning not all of its molecules release \(H^+\) ions.The degree of dissociation directly impacts the acidic strength. A higher level of dissociation corresponds to a stronger acid.

Bronsted-Lowry Acids

The Bronsted-Lowry theory is a fundamental concept in chemistry that classifies acids based on their ability to donate protons. According to this concept, an acid is any substance that can donate a proton, or \(H^+\) ion.- **Perchloric Acid \(\text{HClO}_4\)**: As a strong Bronsted-Lowry acid, perchloric acid readily donates protons, making it one of the strongest acids.- **Sulfuric Acid \(\text{H}_2\text{SO}_4\)**: Known for its strong acidic nature, it donates protons readily in its first ionization step but is less potent than \(\text{HClO}_4\) due to its partial secondary dissociation.Understanding Bronsted-Lowry theory helps in recognizing the ability of different acids to donate protons and thus explains their relative acidic strengths.

Comparison of Acids

When comparing the strength of acids, it is crucial to evaluate their dissociation tendencies and proton donation capabilities. In our exercise, we're comparing the acidic nature of sulfuric acid, phosphoric acid, and perchloric acid.- **Perchloric Acid \(\text{HClO}_4\)**: As highlighted, it is one of the strongest acids known due to its complete dissociation and high ability to donate protons. It stands at the top of the acidic strength for this list.- **Sulfuric Acid \(\text{H}_2\text{SO}_4\)**: Although a strong acid, it is not as powerful as \(\text{HClO}_4\) because its secondary dissociation step is partial. However, the primary dissociation is complete, making it stronger than many other acids.- **Phosphoric Acid \(\text{H}_3\text{PO}_4\)**: It is weaker than both \(\text{HClO}_4\) and \(\text{H}_2\text{SO}_4\) due to limited dissociation chiefly in its first step.By aligning these acids in descending order of strength, we confirm that \(\text{HClO}_4 > \text{H}_2\text{SO}_4 > \text{H}_3\text{PO}_4\). This comparison helps understand their varied uses and behaviours in chemical reactions.