Problem 30 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

30\. For \(\mathrm{H}_{3} \mathrm{PO}_{3}\) and \(\mathrm{H}_{3} \mathrm{PO}_{4}\), the correct choice is: [2003S] (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) \(\mathrm{H}_{3} \mathrm{PO}_{3}\) 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 the Acids

The basicity of an acid is defined by the number of ionizable hydrogen ions it can donate. - **Phosphorous acid (\(\mathrm{H}_3\mathrm{PO}_3\))** can donate two hydrogen ions. Its structure is \(\mathrm{P(OH)_2H}\), featuring two \(\mathrm{OH}\) groups, making it dibasic (able to release two protons, \(\mathrm{H}^+\)).- **Phosphoric acid (\(\mathrm{H}_3\mathrm{PO}_4\))** can donate three hydrogen ions. Its structure is \(\mathrm{PO(OH)_3}\), featuring three \(\mathrm{OH}\) groups, making it tribasic.

Step 2: Determine the Nature of Each Acid's Reducing Property

A reducing agent can donate electrons in a reaction. - \(\mathrm{H}_3\mathrm{PO}_3\) (phosphorous acid) is a known reducing agent because it can easily oxidize to \(\mathrm{H}_3\mathrm{PO}_4\) by donating electrons and forming a stronger P=O bond.- \(\mathrm{H}_3\mathrm{PO}_4\) (phosphoric acid) does not act as a reducing agent because it is already in a fully oxidized form and lacks an available site for oxidization.

Step 3: Match the Acid Properties with Given Choices

- Based on the assessment: - \(\mathrm{H}_3\mathrm{PO}_3\) is dibasic and acts as a reducing agent. - \(\mathrm{H}_3\mathrm{PO}_4\) is tribasic but not a reducing agent.- Comparing these findings with the choices given: - Option (a) states \(\mathrm{H}_3\mathrm{PO}_3\) is dibasic and reducing, which matches our assessment.

Key Concepts

Basicity

Basicity refers to the number of hydrogen ions (protons, \(\mathrm{H}^+\)) an acid can donate. This concept is essential in acid-base chemistry as it helps determine how an acid will react in a solution. Two important acids to understand when discussing basicity in phosphorus compounds are phosphorous acid \((\mathrm{H}_3\mathrm{PO}_3)\) and phosphoric acid \((\mathrm{H}_3\mathrm{PO}_4)\).Phosphorous acid is dibasic, which means it can donate two hydrogen ions. This is due to its molecular structure, \(\mathrm{P(OH)_2H}\), where two \(\mathrm{OH}\) groups are present. These hydroxyl groups can each release a proton, making the compound capable of contributing two ions to a chemical reaction.In contrast, phosphoric acid is tribasic. It can donate three hydrogen ions, which is attributed to its structure \(\mathrm{PO(OH)_3}\). Here, all three hydroxyl groups are able to release a proton each, which reflects its higher basicity compared to phosphorous acid.Understanding basicity is crucial as it influences how these acids behave in different reactions, including their strength as acids and how they react with bases.

Reducing Agents

Reducing agents are substances that can donate electrons in a chemical reaction, often causing reduction by decreasing the oxidation state of another chemical species. In the context of our phosphorus-based acids, the reducing properties of \(\mathrm{H}_3\mathrm{PO}_3\) and \(\mathrm{H}_3\mathrm{PO}_4\) highlight interesting contrasts.Phosphorous acid \((\mathrm{H}_3\mathrm{PO}_3)\) acts as a reducing agent because it undergoes oxidation to form phosphoric acid \((\mathrm{H}_3\mathrm{PO}_4)\). During this process, \(\mathrm{H}_3\mathrm{PO}_3\) donates electrons, facilitating its own oxidation while reducing another substance in the reaction. This capability is largely due to the presence of \(\mathrm{P-H}\) bonds in its molecular structure, enabling electron donation.However, phosphoric acid \((\mathrm{H}_3\mathrm{PO}_4)\) is not a reducing agent. It is already in its most oxidized form. Without any available bonds to donate electrons, \(\mathrm{H}_3\mathrm{PO}_4\) does not have the ability to act as a reducing agent, contrasting its less oxidized counterpart, \(\mathrm{H}_3\mathrm{PO}_3\). Understanding this concept is key in grasping the differing chemical properties and reactivity of these acids.

Phosphorus Compounds

Phosphorus compounds play a significant role in chemistry due to their varied chemical properties and reactivity patterns. Two common phosphorus compounds are phosphorous acid \((\mathrm{H}_3\mathrm{PO}_3)\) and phosphoric acid \((\mathrm{H}_3\mathrm{PO}_4)\). They offer a fascinating study of both basicity and redox behavior.These acids illustrate different bonding and electron configurations in phosphorus chemistry:- **Phosphorous Acid (\(\mathrm{H}_3\mathrm{PO}_3\))**: - Contains \(\mathrm{P-H}\) bonds, which makes it capable of acting as a reducing agent. This feature allows it to donate electrons readily. - Its structure \(\mathrm{P(OH)_2H}\) entails two hydroxyl groups, contributing to its dibasic nature.- **Phosphoric Acid (\(\mathrm{H}_3\mathrm{PO}_4\))**: - Features three hydroxyl groups \(\mathrm{PO(OH)_3}\), enabling it to be tribasic, but lacks \(\mathrm{P-H}\) bonds to act as a reducer. - As the maximum oxidized state of phosphorus in this acid, it doesn’t undergo further oxidation.By studying these compounds, one gains insight into essential principles of acid-base chemistry, the nature of reducing agents, and the structural dynamics that influence these properties. Bridging these ideas provides clarity on their real-world applications and significance within the chemical sciences.