Problem 128 - A Complete Resource Book in Chemistry for JEE Main

Question

Phosphorus on reaction with conc. HNO \(_{3}\) gives an acid A which can also be formed by the action of dil. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) on powdered phosphorite rock, the acid \(\mathrm{A}\) is (a) \(\mathrm{H}_{3} \mathrm{PO}_{3}\) (b) \(\mathrm{H}_{3} \mathrm{PO}_{4}\) (c) \(\mathrm{H}_{3} \mathrm{PO}_{2}\) (d) \(\mathrm{HPO}_{3}\)

Step-by-Step Solution

Answer

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

Step 1: Understand the Reaction

When phosphorus reacts with concentrated nitric acid \( \text{HNO}_3 \), it oxidizes to form phosphoric acid, which is our acid \( A \).

Step 2: Recall the Formation Process from Phosphorite

Dilute sulfuric acid \( \text{H}_2\text{SO}_4 \) acting on phosphorite rock also produces phosphoric acid. This indicates that the acid formed is phosphoric acid.

Step 3: Identify the Chemical Formula of Acid A

Phosphoric acid is chemically represented as \( \mathrm{H}_{3} \mathrm{PO}_{4} \).

Step 4: Match with Given Options

The options provided list \( \mathrm{H}_{3} \mathrm{PO}_{4} \) as one of the choices, which confirms our earlier deduction.

Key Concepts

Reaction of Phosphorus with Nitric Acid

Phosphorus is a reactive non-metal and when exposed to concentrated nitric acid (\( \text{HNO}_3 \)), it undergoes a chemical reaction. In this process, phosphorus is oxidized by the nitric acid to form phosphoric acid (\( \mathrm{H}_3\mathrm{PO}_4 \)). This reaction is significant because it demonstrates the oxidizing power of nitric acid. The conversion of phosphorus to phosphoric acid involves a redox process where phosphorus atoms lose electrons (oxidation) and are transformed into a higher oxidation state within the phosphoric acid structure.

• Reaction: Phosphorus + Concentrated \( \text{HNO}_3 \) → Phosphoric Acid (\( \mathrm{H}_3\mathrm{PO}_4 \))
• This reaction exemplifies how strong oxidizing agents like nitric acid can create more complex acids from simple elements.
• Phosphoric acid produced can be used in fertilizers and industrial applications.

Formation of Phosphoric Acid from Phosphorite

Phosphorite, a type of phosphate rock, is a crucial source for producing phosphoric acid commercially. When dilute sulfuric acid (\( \mathrm{H}_2 \mathrm{SO}_4 \)) interacts with phosphorite rock, a reaction occurs that yields phosphoric acid among the products. This process is a key step in the fertilizer industry because phosphoric acid serves not only as a major component of agricultural fertilizers but also as a starting point for other phosphorus-based products.

• Process: Phosphorite + Dilute \( \text{H}_2\text{SO}_4 \) → Phosphoric Acid (\( \mathrm{H}_3\mathrm{PO}_4 \))
• Sulfuric acid plays a role in extracting phosphorus from insoluble mineral forms to create a soluble product that can be used in various industries.
• This chemical process is economical and efficient for large-scale production of phosphoric acid.

Chemical Reactions of Phosphorus

Phosphorus is known for its high reactivity and ability to form a variety of compounds through chemical reactions. It reacts with several oxidizing agents, acids, and other elements to form different phosphorus-containing compounds. Understanding these reactions helps in grasping the versatility of phosphorus in chemical processes and industrial applications.

• With Oxygen: Phosphorus burns in air to form phosphorus pentoxide (\( \text{P}_2\text{O}_5 \)), a starting material for phosphoric acid production.
• With Halogens: Reacts with chlorine to form phosphorus trichloride (\( \text{PCl}_3 \)) or phosphorus pentachloride (\( \text{PCl}_5 \)), both used in organic synthesis.
• With Metals: Forms phosphides, which have applications in semiconductor technology.

Phosphorus's ability to react with such a wide range of substances makes it a fundamental element in both nature and industry. It's essential to understand these reactions to appreciate its role in the environment and technological advancements.