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 is silver sulphide \\(\text{Ag}_2\text{S}\\).

Step 1: Identify the Reactants

The question involves a reaction between sodium thiosulphate \(\text{Na}_2\text{S}_2\text{O}_3\) and silver nitrate \(\text{AgNO}_3\). Our goal is to find out which black compound is formed from these reactants.

Step 2: Reaction Products Formation

When sodium thiosulphate \(\text{Na}_2\text{S}_2\text{O}_3\) reacts with silver nitrate \(\text{AgNO}_3\), one of the key products is silver sulphide \(\text{Ag}_2\text{S}\). The black coloration is typically associated with silver sulphide due to its formation as a precipitate.

Step 3: Confirm Bypass of Other Compounds

Other compounds such as silver sulphite, silver sulphate, and silver thiosulphate do not form in this context because they do not account for the distinct black color relevant to reaction dynamics. Silver sulphide is known for being black.

Step 4: Identify the Chemical Equation

The simplified chemical equation for this reaction is: \[2 \text{AgNO}_3 + \text{Na}_2\text{S}_2\text{O}_3 \rightarrow \text{Ag}_2\text{S} + 2 \text{NaNO}_3 + \text{SO}_2 + \text{S}\]This confirms that silver sulphide (black compound) is produced.

Key Concepts

Sodium Thiosulphate

Sodium thiosulphate, represented as \( \text{Na}_2\text{S}_2\text{O}_3 \), is a versatile and interesting compound often used in chemical reactions. It is a white crystalline substance that readily dissolves in water. Sodium thiosulphate is commonly used in the titration of iodine solutions because it reacts with iodine to form sodium iodide and tetrathionate, a reaction that is easily and accurately analyzed.

In this particular reaction with silver nitrate, sodium thiosulphate serves not just as a reactant but as an essential component in facilitating the formation of new compounds. It is involved in reactions where it can influence the color and nature of the final product, such as in photographic processing, where it helps "fix" images. Understanding sodium thiosulphate’s role in reactions is key to grasping how chemical changes and color transitions occur, as is the case with silver sulphide formation.

Silver Nitrate

Silver nitrate, \( \text{AgNO}_3 \), is a well-known inorganic compound, and its crystalline form appears colorless. However, it is a highly reactive compound that can be a powerful oxidizing agent and reacts readily with various other substances to form new compounds.

One of its critical reactions involves mixing with sodium thiosulphate leading to the formation of silver sulphide. Silver nitrate’s reactivity is attributable to the silver ion \( \text{Ag}^+ \), which interacts with anions to form different types of bonds, often precipitating into distinct, sometimes colorful, compounds. Silver nitrate is used extensively in chemical experiments, medical applications, and even in jewelry making for its reactivity and versatility.

Silver Sulphide Formation

The formation of silver sulphide, \( \text{Ag}_2\text{S} \), from the reaction between sodium thiosulphate and silver nitrate is a fascinating example of chemical transformation. In this reaction, the silver ions are reduced and sulfur ions precipitate out forming silver sulphide, which is recognized by its characteristic black color.

The balanced chemical equation for this reaction is:\[2 \text{AgNO}_3 + \text{Na}_2\text{S}_2\text{O}_3 \rightarrow \text{Ag}_2\text{S} + 2 \text{NaNO}_3 + \text{SO}_2 + \text{S}\]

• The silver nitrate provides \( \text{Ag}^+ \) ions which combine with sulfur ions \( \text{S}^{-2} \) from sodium thiosulphate.
• The result is silver sulphide \( \text{Ag}_2\text{S} \), a black precipitate, indicating its formation.

Understanding this transformation helps us see the interplay between different ions and how they result in distinct compounds. Such reactions are important in various applications, from industrial processes to artistic endeavors, due to the unique properties of the new compounds formed.