Problem 48 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The total number of diprotic acids among the following is: \(\mathrm{H}_{3} \mathrm{PO}_{4}, \mathrm{H}_{2} \mathrm{SO}_{4}, \mathrm{H}_{3} \mathrm{PO}_{3}, \mathrm{H}_{2} \mathrm{CO}_{3}, \mathrm{H}_{2} \mathrm{~S}_{2} \mathrm{O}_{7}, \mathrm{H}_{3} \mathrm{BO}_{3}, \mathrm{H}_{3} \mathrm{PO}_{2}, \mathrm{H}_{2} \mathrm{CrO}_{4}\) and \(\mathrm{H}_{2} \mathrm{SO}_{3}\)

Step-by-Step Solution

Answer

The total number of diprotic acids is 6.

Step 1: Understand the Definition of Diprotic Acid

Diprotic acids are acids that can donate two protons (H⁺ ions) per molecule during the dissociation process. Focus on identifying which acids have only two replaceable hydrogen ions.

Step 2: Analyze Each Acid

Evaluate each given acid to determine if it is diprotic: - **H₃PO₄**: This is a triprotic acid since it can donate three protons. - **H₂SO₄**: This is a diprotic acid. - **H₃PO₃**: This can donate two protons due to its structure; thus, it is diprotic. - **H₂CO₃**: This is diprotic. - **H₂S₂O₇**: This is diprotic. - **H₃BO₃**: This is monoprotic; it does not donate more than one proton readily. - **H₃PO₂**: This is monoprotic, as it generally donates only one hydrogen readily. - **H₂CrO₄**: This is diprotic. - **H₂SO₃**: This is diprotic.

Step 3: Count the Diprotic Acids

From the analysis: - **H₂SO₄**: Diprotic - **H₃PO₃**: Diprotic - **H₂CO₃**: Diprotic - **H₂S₂O₇**: Diprotic - **H₂CrO₄**: Diprotic - **H₂SO₃**: Diprotic Count the number of diprotic acids: 6.

Key Concepts

Acid Dissociation

Acid dissociation is a fundamental concept in chemistry that deals with how acids release protons into a solution. When an acid dissolves in water, it can donate hydrogen ions, also known as protons, to the solution.
This process is known as dissociation. It is essential for understanding how acids behave in different environments. In the case of diprotic acids, these are unique because they have the ability to donate two protons per molecule during dissociation. Upon dissociation, the acid releases its first hydrogen ion to form an intermediate ion. Subsequently, the intermediate can dissociate further to release the second hydrogen ion. When considering dissociation in equations, the dissociation of a diprotic acid typically involves two steps:

• The first step: \( ext{HA}_2 ightarrow ext{HA}^{-} + ext{H}^{+} \)
• The second step: \( ext{HA}^{-} ightarrow ext{A}^{2-} + ext{H}^{+} \)

Knowing how this process works can help in predicting how the acid will affect the pH level of a solution, as each proton donated influences acidity.

Proton Donation

Proton donation is a key feature of acids. It refers to the ability of an acid to donate a proton or hydrogen ion (H⁺) to another compound, typically a base.
This behavior is central to the Brønsted-Lowry acid-base theory, which defines acids as proton donors. Diprotic acids, such as sulfuric acid (\( ext{H}_2 ext{SO}_4 \)), can donate two protons in a stepwise manner. This means that in a solution, a diprotic acid does not release both its protons at once. Instead, it first donates one proton then, after reaching equilibrium, it can donate the second proton. Understanding proton donation in diprotic acids is crucial when predicting the chemical environment of a solution:

• The first proton donation generally has a stronger effect on the acidity of the solution.
• The second proton is often less aggressive due to the remaining anion's stability.
• The ability of an acid to donate protons can determine its strength and the pH of its solutions.

Recognizing how proton donation happens in stages helps one in understanding reactions and calculations involving multiple-step dissociations.

Acid Strength

The strength of an acid is determined by its ability to donate protons. It measures how completely an acid dissociates in water. A strong acid dissociates entirely, releasing all its hydrogen ions. In contrast, a weak acid releases few protons, keeping most of them bound to the anion.Diprotic acids often illustrate varying strengths between their dissociation steps. The first dissociation typically exhibits stronger acid behavior than the second. Each step of ionization is characterized by its own dissociation constant, \( K_a \), which is a numerical measure of the acid's strength at that step.For example:

• In \( ext{H}_2 ext{CO}_3 \), the first dissociation has a larger \( K_a \), indicating a stronger tendency to lose its first proton than the second.
• This means the first proton donation significantly affects the solution’s pH, while the second contributes less.

To determine acidity, compare the \( K_a \) values:

• Larger \( K_a \), stronger acid behavior in that step.
• Smaller \( K_a \), weaker dissociation in subsequent steps.

Understanding how acid strength varies across dissociation steps aids in predicting the behavior of diprotic acids and similar compounds in various reactions and solutions.