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 -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

(a) and (d) are correct.

Step 1: Understanding Acid Basics

To determine the correct statements, we need to understand the properties of the given acids. Discussing their basicity (number of replaceable H ions) and their expected chemical behavior will help analyze each statement.

Step 2: Analyzing Hypo Phosphorous Acid

Hypo phosphorous acid ( ext{H}_3 ext{PO}_2) is known to have reducing properties because it can readily donate hydrogen. This makes statement (a) true.

Step 3: Examining Pyrophosphoric Acid

Pyrophosphoric acid ( ext{H}_4 ext{P}_2 ext{O}_7) has four OH groups, but it is dibasic as only two of these hydrogens are ionizable (contribute to the acid's basicity). Thus, statement (b) is false.

Step 4: Disproportionation of Hypo Phosphorous Acid

Hypo phosphorous acid ( ext{H}_3 ext{PO}_2) can disproportionate to form phosphine ( ext{PH}_3) and orthophosphoric acid ( ext{H}_3 ext{PO}_4), not orthophosphorous acid ( ext{H}_3 ext{PO}_3), making statement (c) false.

Step 5: Heating Meta Phosphoric Acid

On heating, meta phosphoric acid ( ext{HPO}_3) polymerizes to form ext{P}_4 ext{O}_{10}, reinforcing that statement (d) is correct.

Key Concepts

Basicity of Acids

Acids are substances that can donate protons (H⁺ ions) to solutions. The basicity of an acid is determined by the number of ionizable hydrogen atoms a molecule can donate. For example, the basicity of Pyrophosphoric acid (H₄P₂O₇) is represented by the number of hydrogen ions it can donate. Though it contains four hydroxyl (-OH) groups, only two hydrogens are ionizable, meaning it is a dibasic acid.
Understanding this concept is essential in predicting the behavior of acids in various chemical reactions.

• Monobasic acids: Donate one H⁺ ion, such as HCl.
• Dibasic acids: Donate two H⁺ ions, like H₂SO₄.
• Tribasic acids: Can donate three H⁺ ions, an example being H₃PO₄.

Basicity plays a crucial role in determining an acid's strength and its ability to interact or react with bases.

Reducing Properties of Acids

Certain acids have the ability to act as reducing agents by donating electrons during chemical reactions. Hypo phosphorous acid (H₃PO₂) is a prime example. It can readily provide hydrogen atoms which participate in reduction reactions. This property is significant in processes that involve converting positive ions into their elemental form. For instance, in redox reactions, the acid donates electrons to another species, thereby reducing it.
This electron donation capability makes certain acids valuable in chemical synthesis and industrial applications that require the reduction of other compounds.
However, not all acids exhibit reduction properties. It's essential to identify the structure of the acid and its predisposition to give up hydrogen ions when determining its ability to reduce.

Disproportionation Reactions

Disproportionation reactions are special types of redox reactions where a single element is both oxidized and reduced. During these reactions, one substance transforms into two different products. An acid like Hypo phosphorous acid (H₃PO₂) undergoes disproportionation, but contrary to a misconception, it forms phosphine (PH₃) and orthophosphoric acid (H₃PO₄), not orthophosphorous acid (H₃PO₃).
This unique reaction type often involves substances that can provide multiple oxidation states for an element in the molecule.
For students learning about chemical reactions, recognizing this pattern helps understand the variety of outcomes possible from seemingly straightforward reactions.

Polymerization of Acids

Polymerization is a process where small molecules, called monomers, combine to form larger network structures called polymers. In the context of acids, Meta phosphoric acid (HPO₃) demonstrates fascinating behavior. When heated, it undergoes polymerization, leading to the formation of a substance like P₄O₁₀.
The process involves the release of water molecules and the reformation of bonds, turning monomeric forms into a larger complex structure.
Such changes are important in understanding how substances behave under varying conditions and are highly relevant in industrial applications where acid polymerization might be beneficial or necessary.