Problem 160 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statement(s) is correct here: (a) Hypo phosphorous acid shows reducing properties (b) Pyrophosphoric acid has although \(4-\mathrm{OH}\) groups but it is a dibasic acid (c) Orthophosphorous acid, is obtained during disproportionation of hypo phosphorous acid (d) Meta phosphoric acid on heating gives \(\mathrm{P}_{4} \mathrm{O}_{10}\)

Step-by-Step Solution

Answer

Statements (a) and (b) are correct.

Step 1: Understanding Chemical Properties of Hypo Phosphorous Acid

Hypo phosphorous acid (H₃PO₂) acts as a powerful reducing agent. This characteristic arises because it has one hydrogen directly bonded to phosphorus, which can be oxidized, making it a reducing agent. Therefore, statement (a) that it shows reducing properties is correct.

Step 2: Analyzing Pyrophosphoric Acid's Basicity

Pyrophosphoric acid ( P_2O_7H_4 ) contains four hydroxyl (OH) groups, but only two of these can dissociate to release protons. This means it is classified as a dibasic acid despite having four OH groups, making statement (b) correct.

Step 3: Examining Disproportionation of Hypo Phosphorous Acid

The compound orthophosphorous acid gets involved in reactions of disproportionation but is not specifically formed from the disproportionation of hypo phosphorous acid. Therefore, statement (c) is incorrect.

Step 4: Determining Meta Phosphoric Acid's Behavior on Heating

Meta phosphoric acid ( HPO_3 ) can polymerize upon heating but does not produce P_4O_{10} directly as a product. Therefore, statement (d) is incorrect.

Key Concepts

Reducing Properties of Hypo Phosphorous Acid

Hypo phosphorous acid, with the chemical formula \( \text{H}_3\text{PO}_2 \), is known for its strong reducing properties. This is due to the presence of one hydrogen atom that is directly bonded to the phosphorus atom. In chemical terms, reducing agents donate electrons to other substances, decreasing their oxidation state. In \( \text{H}_3\text{PO}_2 \), the hydrogen attached to phosphorus can act in this electron-donating fashion, allowing the acid to reduce other compounds.

This single hydrogen bond makes hypo phosphorous acid unique compared to other acids where hydrogen is typically bonded to oxygen, as in hydroxyl groups. Thus, when \( \text{H}_3\text{PO}_2 \) participates in chemical reactions, it can effectively reduce substances by transferring the hydrogen in the form of electrons, hence its classification as a reducing agent.

Basicity of Pyrophosphoric Acid

Pyrophosphoric acid, chemically represented as \( \text{H}_4\text{P}_2\text{O}_7 \), contains four hydroxyl (\( \text{OH}\)) groups. One might assume that the presence of four \( \text{OH} \) groups indicates tetrabasicity; however, this is not the case. Only two of these hydroxyl groups are capable of dissociating to release protons \((\text{H}^+)\).

A substance is termed dibasic when it can release two protons during its dissociation process. Although pyrophosphoric acid contains multiple hydroxyl groups, due to the structure and stability of the acid, only two out of four hydroxyl groups actually participate in releasing protons, thus classifying it as dibasic. Understanding acid dissociation is key to grasping the behavior and classification of acids such as pyrophosphoric acid.

Disproportionation of Phosphorous Acids

Disproportionation is a chemical reaction in which a single substance is simultaneously oxidized and reduced, forming two different products. With phosphorous acids, these reactions are fascinating in that they often involve unique structural transformations.

For example, orthophosphoric acid \( (\text{H}_3\text{PO}_4) \) is sometimes mistaken to be a by-product of disproprtionation of hypo phosphorous acid but the topic actually gets more nuanced. Hypo phosphorous acid \((\text{H}_3\text{PO}_2)\) does not directly produce orthophosphorous acid \((\text{H}_3\text{PO}_3)\) by disproportionation. Instead, varying conditions lead to different products and can include both oxidation and reduction reactions in separate pathways.

Disproportionation embodies the dual-electron transfer processes, often resulting in a mix of higher and lower oxidation states, which distinguishes phosphorous chemistry. By grasping these reactions, students can gain deeper insights into the dynamics and complexity of phosphorus compounds in chemistry.