Problem 136 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

The total number of diprotic acids is 6.

Step 1: Understand Diprotic Acids

A diprotic acid is an acid that can donate two protons (hydrogen ions) per molecule in an aqueous solution. This means we need to identify acids from the list that can release two H⁺ ions.

Step 2: List Each Acid's Formula

We will go through each provided acid's formula to determine the number of hydrogen ions that can potentially be released. The formulas given are: 1. \( \mathrm{H}_{3} \mathrm{PO}_{4} \)2. \( \mathrm{H}_{2} \mathrm{SO}_{4} \)3. \( \mathrm{H}_{3} \mathrm{PO}_{3} \)4. \( \mathrm{H}_{2} \mathrm{CO}_{3} \)5. \( \mathrm{H}_{2} \mathrm{S}_{2} \mathrm{O}_{7} \)6. \( \mathrm{H}_{3} \mathrm{BO}_{3} \)7. \( \mathrm{H}_{3} \mathrm{PO}_{2} \)8. \( \mathrm{H}_{2} \mathrm{CrO}_{4} \)9. \( \mathrm{H}_{2} \mathrm{SO}_{3} \)

Step 3: Evaluate Each Acid

Identify which acids among the listed ones have two hydrogen atoms that can be donated as protons:- \( \mathrm{H}_{3} \mathrm{PO}_{4} \): Triprotic, releases 3 H⁺- \( \mathrm{H}_{2} \mathrm{SO}_{4} \): Diprotic, releases 2 H⁺- \( \mathrm{H}_{3} \mathrm{PO}_{3} \): Diprotic, releases 2 H⁺- \( \mathrm{H}_{2} \mathrm{CO}_{3} \): Diprotic, releases 2 H⁺- \( \mathrm{H}_{2} \mathrm{S}_{2} \mathrm{O}_{7} \): Diprotic, releases 2 H⁺- \( \mathrm{H}_{3} \mathrm{BO}_{3} \): Typically acts like a monoprotic acid, releases 1 H⁺- \( \mathrm{H}_{3} \mathrm{PO}_{2} \): Monoprotic, releases 1 H⁺- \( \mathrm{H}_{2} \mathrm{CrO}_{4} \): Diprotic, releases 2 H⁺- \( \mathrm{H}_{2} \mathrm{SO}_{3} \): Diprotic, releases 2 H⁺

Step 4: Count the Diprotic Acids

Count the acids from the list that can release exactly 2 hydrogen ions: 1. \( \mathrm{H}_{2} \mathrm{SO}_{4} \)2. \( \mathrm{H}_{3} \mathrm{PO}_{3} \)3. \( \mathrm{H}_{2} \mathrm{CO}_{3} \)4. \( \mathrm{H}_{2} \mathrm{S}_{2} \mathrm{O}_{7} \)5. \( \mathrm{H}_{2} \mathrm{CrO}_{4} \)6. \( \mathrm{H}_{2} \mathrm{SO}_{3} \) There are 6 diprotic acids in total.

Key Concepts

Acid Strength

Understanding acid strength is crucial for anyone studying chemistry, especially when dealing with diprotic acids. Acid strength refers to an acid's ability to donate protons (hydrogen ions). The strength can vary significantly among different acids. Strong acids, like sulfuric acid (\(\mathrm{H}_2\mathrm{SO}_4\)), dissociate completely in water, releasing hydrogen ions freely.

On the other hand, weak acids only partially dissociate, releasing fewer hydrogen ions into the solution. The degree of dissociation, or ionization, largely determines how strongly an acid will behave in an aqueous solution. For diprotic acids, which can donate two protons, understanding their acid strength helps predict how easily each hydrogen ion can be donated.

• Strong acids: Dissociate completely, stronger electrolytes.
• Weak acids: Dissociate partially, weaker electrolytes.

Proton Donation

Proton donation is a fundamental concept to grasp when studying acid behavior. In chemistry, a proton refers to a hydrogen ion (\(\mathrm{H}^+\)), the donation of which is what characterizes an acid. Diprotic acids have the unique ability to donate two protons per molecule. This means that upon dissolution in water, diprotic acids can go through two sequential ionization steps.

For example:

• First ionization: \(\mathrm{H}_2\mathrm{SO}_4 \rightarrow \mathrm{H}^+ + \mathrm{HSO}_4^-\)
• Second ionization: \(\mathrm{HSO}_4^- \rightarrow \mathrm{H}^+ + \mathrm{SO}_4^{2-}\)

Each step involves the donation of one hydrogen ion. Recognizing this process is vital to understanding how acids react in various chemical environments, particularly in acid-base reactions.

Chemical Formulas

Chemical formulas are shorthand notations used to represent molecules, including acids. These formulas provide essential information about the composition of a substance, including the number of each type of atom within a molecule. For acids, the chemical formula reveals the number of hydrogen atoms available for donation as protons.

Analyzing the provided exercise, we see a list of chemical formulas representing different acids. Identifying a diprotic acid involves checking the subscripts on the hydrogen atom in the formula. For instance:

• \(\mathrm{H}_2\mathrm{SO}_3\) indicates two hydrogen atoms, suggesting it could be diprotic.
• \(\mathrm{H}_3\mathrm{PO}_4\) has three hydrogen atoms, suggesting a triprotic nature.

Thus, knowing how to read and interpret chemical formulas is crucial in determining an acid's proton donation capabilities.

Acid-Base Reactions

Acid-base reactions are central to the study of chemistry and involve the transfer of protons between reactants. In Brønsted-Lowry theory, an acid is defined as a proton donor, while a base is a proton acceptor. Diprotic acids add complexity to this dynamic because they can participate in two proton transfer reactions due to their ability to donate two hydrogen ions.

In a typical acid-base reaction, the proton from the acid is transferred to the base. When diprotic acids are involved, they can result in:

• First proton transfer: \(\mathrm{H}_2\mathrm{CO}_3 + \mathrm{OH}^- \rightarrow \mathrm{HCO}_3^- + \mathrm{H}_2\mathrm{O}\)
• Second proton transfer: \(\mathrm{HCO}_3^- + \mathrm{OH}^- \rightarrow \mathrm{CO}_3^{2-} + \mathrm{H}_2\mathrm{O}\)

Such reactions are crucial in countless scientific processes, from physiological pathways in living organisms to industrial chemical applications.