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 sulfide \((\text{Ag}_2\text{S})\).

Step 1: Identify the Reaction

The reaction in question is between sodium thiosulphate \((\text{Na}_2\text{S}_2\text{O}_3)\) and silver nitrate \((\text{AgNO}_3)\). The known reaction must form a compound containing sulfur and silver.

Step 2: Determine the Product Characteristics

Out of the given options, the description highlights a black compound as the resulting product. Silver sulfide \((\text{Ag}_2\text{S})\) is known for its black color.

Step 3: Recognize Typical Outcomes

Commonly, reactions of thiosulphate with silver ions often lead to the formation of silver sulfide due to the reduction of the thiosulphate ion and precipitation of silver ions as silver sulfide.

Step 4: Conclusion Based on Chemical Knowledge

Given the characteristics of the reaction between sodium thiosulphate and silver nitrate (formation of a black compound and typical outcomes), silver sulfide \((\text{Ag}_2\text{S})\) is the correct answer.

Key Concepts

Silver sulfide formation

When sodium thiosulphate reacts with silver nitrate, the resulting product is silver sulfide, denoted as \( ext{Ag}_2 ext{S}\). This compound is notable for its distinctive black color. The formation of silver sulfide takes place because sulfur from the thiosulphate ion couples with silver ions from silver nitrate. This creates a stable compound where silver forms a bond with sulfur atoms.

• Silver sulfide is a common compound that appears on silver jewelry, often seen as a black tarnish.
• The black coloration of silver sulfide is a significant identifier, helping in quick recognition during chemical experiments.

Understanding the creation of silver sulfide is crucial in identifying and explaining chemical processes involving silver and sulfur.

Precipitation reactions

In chemical reactions, precipitation is the process in which a solid is formed in a solution. When sodium thiosulphate and silver nitrate interact, a precipitation reaction occurs. This reaction results in the formation of solid silver sulfide. The silver ions \( ext{Ag}^+\) from silver nitrate react with sulfide ions \( ext{S}^{2-}\) to form the insoluble compound \( ext{Ag}_2 ext{S}\).

• Precipitation happens because of the low solubility of silver sulfide in water.
• Observing the black precipitate forms a visual cue for the completion of the reaction.

This type of reaction is crucial in many chemical processes, including wastewater treatment and qualitative chemical analysis.

Reduction of thiosulphate

During the reaction of sodium thiosulphate with silver nitrate, the thiosulphate ion \( ext{S}_2 ext{O}_3^{2-}\) undergoes reduction. In a redox reaction, reduction refers to the gain of electrons by a molecule. Here, one sulfur atom in the thiosulphate ion gets reduced, meaning it gains electrons, contributing to the formation of sulfide \( ext{S}^{2-}\).

• This sulfide ion is critical for bonding with silver ions to form silver sulfide.
• Understanding reduction is important for insight into the behavior and transformation of chemical substances.

The reduction of thiosulphate is a key step that enables the precipitation of silver sulfide.

Characteristic compound colors in chemistry

Colors in chemistry are often used as indicators for compounds' identity and properties. Silver sulfide \( ext{Ag}_2 ext{S}\), for example, is known for its black color, making it easily recognizable in reactions.

• Colors arise due to specific wavelengths of light absorbed or reflected by compounds.
• Understanding the color properties can aid in predicting and identifying chemical substances during experiments.

The characteristic color of chemical compounds is an essential element in educational practices, providing a reliable and straightforward method to identify and learn about various substances.