Problem 11 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is a tribasic acid? (a) \(\mathrm{H}_{3} \mathrm{PO}_{4}\) (b) \(\mathrm{H}_{3} \mathrm{PO}_{3}\) (c) \(\mathrm{H}_{3} \mathrm{PO}_{2}\) (d) \(\mathrm{HPO}_{3}\)

Step-by-Step Solution

Answer

The tribasic acid is \(\mathrm{H}_{3} \mathrm{PO}_{4}\).

Step 1: Understand the Definition of Tribasic Acid

A tribasic acid is an acid that has three ionizable hydrogen atoms. This means it can donate three protons (H⁺ ions) in a chemical reaction. It is important to examine each given option to determine if it meets this criterion.

Step 2: Analyze Each Option

Go through each compound:- (a) \(\mathrm{H}_{3} \mathrm{PO}_{4}\): This contains three hydrogen atoms attached to the phosphorus atom, all of which can be ionized, making it a tribasic acid.- (b) \(\mathrm{H}_{3} \mathrm{PO}_{3}\): Though it appears to have three hydrogens, only two are ionizable because one is bonded to oxygen, making it a dibasic acid.- (c) \(\mathrm{H}_{3} \mathrm{PO}_{2}\): Here, only one hydrogen is ionizable, so it is a monobasic acid.- (d) \(\mathrm{HPO}_{3}\): Lacks sufficient hydrogen to be tribasic.

Step 3: Compare Definitions with Compounds

From Step 2, identify that only \(\mathrm{H}_{3} \mathrm{PO}_{4}\) (option a) has three ionizable hydrogen atoms. The other options either have less or their molecular structure prevents all hydrogens from being ionizable.

Step 4: Confirm the Correct Answer

Ensure that the reasoning is correct: Option (a) \(\mathrm{H}_{3} \mathrm{PO}_{4}\) is the only compound with three ionizable hydrogen ions, therefore confirming that it is a tribasic acid.

Key Concepts

Ionizable Hydrogen Atoms

In the context of acids, ionizable hydrogen atoms refer to the hydrogen atoms in a molecule that can be released as protons, or \( \text{H}^+ \) ions, in a chemical reaction. Not every hydrogen atom in a compound is ionizable. Their ionizability depends on their bonding and molecular structure. For example, in the molecule \( \text{H}_3\text{PO}_4 \), all three hydrogen atoms are bonded in such a way that they can ionize readily in an aqueous solution.
In contrast, \( \text{H}_3\text{PO}_3 \), although seemingly similar, only allows two of its hydrogens to ionize because one hydrogen atom is more tightly bonded, reducing its ability to ionize. Such differences occur because of how hydrogen atoms are attached within the molecular structure.

• When all hydrogen atoms can ionize, like in \( \text{H}_3\text{PO}_4 \), the compound is called a tribasic acid.
• If fewer hydrogens can ionize, such as the two in \( \text{H}_3\text{PO}_3 \), it is considered a dibasic acid.
• Further, if just one hydrogen ionizes, it becomes a monobasic acid like \( \text{H}_3\text{PO}_2 \).

Understanding which hydrogens are ionizable helps in determining the acid's strength and classification.

Chemical Reactions

Chemical reactions involve changes where substances transform into completely new entities with different properties. Acids participate in a variety of chemical reactions, often involving the donation of protons. When we say an acid is 'ionizing,' we're specifically talking about it undergoing a reaction where it releases \( \text{H}^+ \) ions into a solution.
Consider the reaction of a tribasic acid like \( \text{H}_3\text{PO}_4 \). In water, each ionizable hydrogen atom can be dissociated, demonstrating the acid's ability to create a series of reactions for each hydrogen atom removed:- The first removal releases one proton, forming \( \text{H}_2\text{PO}_4^- \)- The second removal produces \( \text{HPO}_4^{2-} \)- Finally, the third ionization results in \( \text{PO}_4^{3-} \)These steps illustrate the acid's tribasic nature, as it can undergo three successive ionization processes, each requiring distinct energy levels and conditions.

Acid-Base Concepts

Acids and bases are fundamental concepts in chemistry, governed by their ability to donate or accept protons, respectively. Acids like \( \text{H}_3\text{PO}_4 \) have multiple ionizable hydrogens, making them able to donate more protons, which classifies them as having higher acidity or being stronger acids in multistep ionization processes.
The concept of tribasic acid fits into broader acid-base theories like the Bronsted-Lowry theory, which describes acids as proton donors and bases as proton acceptors. Key to this theory is understanding how effectively an acid like \( \text{H}_3\text{PO}_4 \) can donate its three protons across distinct reactions, interacting with bases to form salts and water. This is fundamental to:

• Predicting the direction and completion of reactions
• Understanding buffer systems, which control the pH in biological and chemical systems
• Calculating the equilibrium concentrations in solutions which involve strong and weak acids and bases

By grasping these concepts, one can better appreciate the balance of chemical reactions and the vital role acids play in various chemical processes.