Problem 45 - A Complete Resource Book in Chemistry for JEE Main

Question

For \(\mathrm{H}_{3} \mathrm{PO}_{3}\) and \(\mathrm{H}_{3} \mathrm{PO}_{4}\), the correct choice is: (a) \(\mathrm{H}_{3} \mathrm{PO}_{3}\) is dibasic and reducing (b) \(\mathrm{H}_{3} \mathrm{PO}_{3}\) is dibasic and non-reducing (c) \(\mathrm{H}_{3} \mathrm{PO}_{4}\) is tribasic and reducing (d) \(\mathrm{H}_{3} \mathrm{PO}_{4}\) is tribasic and non-reducing

Step-by-Step Solution

Answer

The correct choice is (a) \( \mathrm{H}_{3} \mathrm{PO}_{3} \) is dibasic and reducing.

Step 1: Identify the basicity of H3PO3

The basicity of an acid is determined by the number of ionizable hydrogen atoms per molecule. For \( \mathrm{H}_{3} \mathrm{PO}_{3}\), the structure shows two ionizable hydrogen atoms linked to oxygen atoms. Therefore, it is a dibasic acid.

Step 2: Determine reducing properties of H3PO3

\( \mathrm{H}_{3} \mathrm{PO}_{3} \) is a reducing agent because it contains an additional P-H bond. This bond can be oxidized, making the compound capable of reducing other substances.

Step 3: Identify the basicity of H3PO4

In \( \mathrm{H}_{3} \mathrm{PO}_{4} \), all three hydrogen atoms are ionizable as they are attached to oxygen atoms, meaning \( \mathrm{H}_{3} \mathrm{PO}_{4} \) is tribasic.

Step 4: Determine reducing properties of H3PO4

\( \mathrm{H}_{3} \mathrm{PO}_{4} \) is not a reducing agent, as it does not have a P-H bond that can reduce other substances during reactions.

Step 5: Final Decision

Comparing options with the properties of the acids: H3PO3 is dibasic and reducing, while H3PO4 is tribasic and non-reducing. The correct choice is (a) \( \mathrm{H}_{3} \mathrm{PO}_{3} \) is dibasic and reducing.

Key Concepts

H3PO3 Dibasic Nature

Phosphorous acid, or \( \mathrm{H}_{3}\mathrm{PO}_{3} \), is classified as a dibasic acid. The term "dibasic" refers to the number of hydrogen atoms in the molecule that are capable of being ionized. In the case of \( \mathrm{H}_{3}\mathrm{PO}_{3} \), only two of the three hydrogen atoms are bound to oxygen (* O-H bonds), and these are the ones that can ionize during a reaction.This stems from the molecular structure, where one of the hydrogen atoms is bound directly to phosphorus in a P-H bond, which is not ionizable. As a result, when \( \mathrm{H}_{3}\mathrm{PO}_{3} \) dissolves in water, it releases two protons \( (\mathrm{H}^+) \), confirming its dibasic nature. This is an important distinction from tribasic acids, which can donate three protons in solution.

H3PO4 Tribasic Nature

In stark contrast to \( \mathrm{H}_{3}\mathrm{PO}_{3} \), phosphoric acid, or \( \mathrm{H}_{3}\mathrm{PO}_{4} \), is considered a tribasic acid. This designation is due to its ability to ionize three hydrogen atoms.* Each of these hydrogens is bonded to an oxygen atom, forming three ionizable O-H bonds.When \( \mathrm{H}_{3}\mathrm{PO}_{4} \) is in solution, it can lose all three hydrogen ions (*\( \mathrm{H}^+ \)) sequentially, making it a tribasic acid. This characteristic is significant in many chemical processes, especially those involving neutralization reactions. Tribasic acids like \( \mathrm{H}_{3}\mathrm{PO}_{4} \) are able to engage more reactively in solutions that require the donation of three protons, which can also affect their buffering capacity and the pH levels of solutions.

Reducing Agents in Chemistry

In chemistry, reducing agents are substances that can donate electrons to another substance, thereby reducing that substance. When this process occurs, the reducing agent itself is oxidized. In this context, \( \mathrm{H}_{3}\mathrm{PO}_{3} \) serves as a reducing agent primarily because of its P-H bond.* This unique P-H bond can undergo oxidation, meaning \( \mathrm{H}_{3}\mathrm{PO}_{3} \) is able to give up electrons and reduce other substances.On the other hand, \( \mathrm{H}_{3}\mathrm{PO}_{4} \) lacks such a bond, making it non-reducing. Understanding the roles of reducing agents is crucial in chemical reactions, as they help drive reactions by offering electrons to oxidizing substances, which, in turn, affects the reaction's pathway and end products. Hence, lengthening the comprehension range for various chemical processes and synthesizing complex molecules.