Problem 39 - A Complete Resource Book in Chemistry for JEE Main

Question

The black compound formed during the reaction between sodium thiosulphate and silver nitrate is (a) silver sulphite \(\left(\mathrm{Ag}_{2} \mathrm{SO}_{3}\right)\) (b) silver sulphate \(\left(\mathrm{Ag}_{2} \mathrm{SO}_{4}\right)\) (c) silver sulphide \(\left(\mathrm{Ag}_{2} \mathrm{~S}\right)\) (d) silver thiosulphate \(\left(\mathrm{Ag}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}\right)\)

Step-by-Step Solution

Answer

The black compound is silver sulphide (Ag₂S).

Step 1: Understanding the Chemical Reaction

Silver nitrate (AgNO₃) reacts with sodium thiosulphate (Na₂S₂O₃) to form a black precipitate. It's essential to determine which compound forms this black color.

Step 2: Black Compound Identification

The black color in chemical reactions involving silver typically indicates the formation of silver sulfide (Ag₂S). This is especially true in solutions where sulfide ions are involved in the reaction.

Step 3: Identify the Sulfide Ion

In this reaction, thiosulphate (S₂O₃²⁻) eventually decomposes to give sulfide ions which combines with silver ions to form the black silver sulfide (Ag₂S) precipitate.

Step 4: Conclusion on Chemical Reaction

Considering the possible compounds — silver sulphite, silver sulphate, silver sulphide, and silver thiosulphate — the black compound is identified as silver sulphide (Ag₂S).

Key Concepts

Sodium Thiosulphate Reaction

Sodium thiosulphate, provides a fascinating component in the world of chemical reactions. This compound, represented as Na₂S₂O₃, is essential in various chemical processes due to its versatility and reactivity. One of its critical features is its ability to act as a reducing agent. This means it can donate electrons to other substances, making it valuable in reactions where reduction is necessary.

When it comes to its interaction with silver nitrate (AgNO₃), sodium thiosulphate doesn’t directly produce sulfide ions. Instead, it undergoes decomposition to eventually yield sulfide ions (S²⁻). This ability to decompose into sulfide ions under certain conditions is particularly important, especially in the context of forming compounds like silver sulfide, which is characterized by its distinctive black color.

• Decomposes to provide sulfide ions.
• Acts as a reducing agent.
• Essential in photographic and analytical applications.

Understanding the behavior of sodium thiosulphate in chemical reactions is crucial for any chemistry enthusiast, especially when exploring reactions involving silver ions.

Silver Nitrate Reaction

Silver nitrate, AgNO₃, is a quintessential reagent in chemistry laboratories primarily due to its high solubility and reactivity. It's widely used for qualitative analysis and its characteristic to form precipitates with various ions. When mixed with sodium thiosulphate, it engages in a fascinating reaction leading to the production of a solid compound.

The interaction between silver nitrate and sodium thiosulphate is significant due to the formation of a black precipitate, which is indicative of a change in chemical composition.

• Forms white precipitates with chloride, bromide, and iodide ions.
• Reacts with thiosulphate to yield black silver sulfide.
• Used as a disinfectant and in photography.

In this reaction, silver ions (Ag⁺) in the silver nitrate combine with sulfide ions (S²⁻) released by thiosulphate decomposition to form silver sulfide (Ag₂S). This specific process highlights the critical role silver nitrate plays in yielding visual identifiers for specific reactions.

Silver Sulfide Formation

The formation of silver sulfide (Ag₂S) is a key reaction result in the chemistry of silver nitrate. This compound is well-known for its striking black appearance and occurs naturally as the tarnish on silver items. The chemistry behind its formation involves a transfer of ions that results in an insoluble and stable product.

In the context of the reaction between sodium thiosulphate and silver nitrate, silver sulfide formation is the final chemical outcome of the interaction. The thiosulphate ions break down to release sulfide ions, which then combine with silver ions present in the solution.

• Insoluble in water, hence precipitates out.
• Forms a black compound, marking the presence of sulfide ions.
• Occurs naturally in mineral deposits like acanthite.

The production of silver sulfide serves as a perfect example for visually identifying reactions through precipitation. It is widely used for educational demonstrations, showcasing both the principles of chemical reactions and the characteristics of ionic compounds.