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 formed is silver sulphide ( Ag_2S ).

Step 1: Understand the Reaction

The reaction takes place between sodium thiosulphate ( Na_2S_2O_3) and silver nitrate ( AgNO_3). When these compounds are mixed, an exchange of ions occurs, leading to the formation of new compounds.

Step 2: Identify Possible Compounds

The main types of products that can form include various silver compounds due to the silver ion ( Ag^+ ) present in silver nitrate reacting with components of sodium thiosulphate. The options provided include silver sulphite, silver sulphate, silver sulphide, and silver thiosulphate.

Step 3: Determine the Black Compound

The question asks for the black compound. Silver sulphide ( Ag_2S ) is well-known for being a black precipitate that often forms in reactions involving silver ions with sulfur-containing ions.

Step 4: Confirm the Correct Option

Among the given options, silver sulphide ( Ag_2S ) is the compound that matches the description of being black. The other possible silver compounds listed, like silver thiosulphate, are not typically black.

Key Concepts

Silver Compounds

Silver compounds are chemical combinations that involve silver, a metallic element known for its excellent conductivity and unique interactions with other elements. Silver can form various compounds that exhibit different properties based on the atoms it bonds with. For example:

• Silver Nitrate (AgNO_3): A colorless compound, silver nitrate is highly soluble in water and releases silver ions, which are very reactive.
• Silver Sulphide (Ag_2S): Known for its distinctive black color, this compound forms when silver ions react with sulfur or sulfur-containing compounds.

Understanding the nature of silver compounds is crucial, as their properties—such as color and solubility—help identify their roles in reactions such as the one with sodium thiosulphate.

Precipitate Formation

Precipitation is a process where soluble ions in a solution react to form an insoluble solid. This solid, known as a precipitate, settles out of the solution. In the reaction between sodium thiosulphate and silver nitrate, a black solid forms, signaling precipitation.

• The silver ion ( Ag^+ ) from silver nitrate reacts with sulfur-containing ions from sodium thiosulphate.
• This reaction forms silver sulphide ( Ag_2S ), a black precipitate.

The color and properties of a precipitate help identify the compounds formed during a chemical reaction. Silver sulphide's distinct black coloration helps verify its presence.

Ion Exchange Reactions

Ion exchange reactions occur when compounds in a solution swap their ions to form new compounds. This kind of reaction is typical in solutions with two or more ionic compounds. During the sodium thiosulphate and silver nitrate reaction:

• The silver ( Ag^+ ) ions from silver nitrate exchange with anions in the sodium thiosulphate.
• As a result, new compounds like silver sulphide ( Ag_2S ) form.

These new compounds often have different solubility properties, which can lead to the formation of a precipitate. Understanding ion exchanges is key to predicting the outcomes of chemical reactions like this one.

Black Silver Sulphide

Black silver sulphide ( Ag_2S ) is a chemical compound renowned for its distinctive black color. This compound forms through the reaction of silver ions with sulfur or sulfur-containing ions. It is a common byproduct in reactions involving silver and compounds that contain sulfur.

• This color change is a reliable indicator of silver sulphide formation.
• Compared to other silver compounds, silver sulphide stands out due to its insolubility and appearance.

In practical situations, identifying the black precipitate as silver sulphide confirms the ion exchange and reaction completion between silver nitrate and sodium thiosulphate.