Problem 118 - 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 formed is silver sulphide \((Ag_2S)\).

Step 1: Understand the Reaction

When sodium thiosulphate \((Na_2S_2O_3)\) reacts with silver nitrate \((AgNO_3)\), a double displacement reaction occurs. It's important to identify the products that result from this reaction.

Step 2: Identify the Formed Compound

In a reaction between sodium thiosulphate and silver nitrate, one of the key products is known to be a black precipitate. Based on typical reactions involving silver and sulfide, silver sulfide \((Ag_2S)\) is commonly known to form as a black compound.

Step 3: Compare the Products with Options

Among the given options, identify the compound which is black in color. The compound that matches this description is silver sulphide \((Ag_2S)\), fitting with both the color description and the known reaction product.

Key Concepts

Sodium Thiosulphate

Sodium Thiosulphate, denoted chemically as \(Na_2S_2O_3\), is known for its versatility in various chemical reactions. In the context of a double displacement reaction with silver nitrate, sodium thiosulphate plays a crucial role. Here, it acts as a source of sulfate ions. These ions participate in the exchange process during the chemical reaction.

A few notable properties of sodium thiosulphate include:

• It is a white or colorless crystalline solid.
• Highly soluble in water.
• Often used as a photographic fixative and in iodometry.

When involved in reactions, sodium thiosulphate is preferred for its ability to undergo displacement and form binding compounds with metals, such as silver in the reaction at hand. Understanding its basic structure and behavior can aid in visualizing how it interacts with other compounds.

Silver Nitrate

Silver Nitrate, represented as \(AgNO_3\), is a highly reactive compound widely used in chemical experiments. Its property as a strong oxidizing agent makes it a key player in double displacement reactions. When introduced to sodium thiosulphate, silver nitrate initiates a reaction that results in the exchange of ions, forming new compounds.

Key characteristics of silver nitrate include:

• Colorless crystalline appearance.
• Highly soluble in water, enabling easy mixing with solutions.
• Notable for causing black stains on organic matter, due to the formation of silver particles.

In reactions involving sulfuric components, like sodium thiosulphate, silver nitrate binds with sulfur ions, leading to precipitation. The silver sulfide that forms in this reaction is black, and this transformation makes silver nitrate a powerful tool in identifying reactions with sulfur-containing substances.

Silver Sulphide

Silver Sulphide, known by its chemical formula \(Ag_2S\), is the black compound produced in the reaction between sodium thiosulphate and silver nitrate. Its characteristic black color helps in easily identifying its formation in laboratory settings.

Silver sulfide is renowned for certain properties:

• It is insoluble in water.
• Often observed in tarnished silver objects as a result of the metal reacting with hydrogen sulfide in the air.
• Has semiconductor facets, useful in various technological applications.

This compound is pivotal as it provides a visual indicator of the completion of the double displacement reaction. When silver sulfide precipitates, it signals the exchange of ions has successfully occurred. Moreover, recognizing its formation helps students verify the reaction's accuracy in experiments with sulfur and silver-based compounds.