Problem 43 - A Complete Resource Book in Chemistry for JEE Main

Question

What is formed when \(\mathrm{H}_{2} \mathrm{~S}\) is passed through acidified solution of \(\mathrm{CuSO}_{4}\) ? (a) \(\mathrm{CuSO}_{4}\) (b) \(\mathrm{CuS}\) (c) \(\mathrm{CuHSO}_{4}\) (d) \(\mathrm{Cu}_{2} \mathrm{~S}\)

Step-by-Step Solution

Answer

The product formed is \(\mathrm{CuS}\), the correct answer is (b).

Step 1: Understanding the Reaction

The problem involves passing hydrogen sulfide (\(\mathrm{H}_2 \mathrm{~S}\)) through an acidified copper(II) sulfate (\(\mathrm{CuSO}_4\)) solution. We need to determine the product of this chemical reaction.

Step 2: Chemical Equation Setup

When \(\mathrm{H}_2 \mathrm{~S}\) is added to \(\mathrm{CuSO}_4\), a double displacement reaction occurs. Copper(II) ions (\(\mathrm{Cu^{2+}}\)) react with \(\mathrm{H}_2 \mathrm{~S}\) to form copper(II) sulfide (\(\mathrm{CuS}\)) and hydrogen sulfate ions (\(\mathrm{H}^{+}\)). The reaction is: \[ \mathrm{CuSO}_4 + \mathrm{H}_2\mathrm{S} \rightarrow \mathrm{CuS} + \mathrm{H}_2\mathrm{SO}_4 \]

Step 3: Identifying the Product

The copper(II) sulfide (\(\mathrm{CuS}\)) formed is an insoluble black precipitate, indicating a typical reaction in this context, where copper ions react with sulfide ions to form a precipitate.

Key Concepts

Double Displacement Reaction

In chemistry, a double displacement reaction, also known as a metathesis reaction, occurs when parts of two ionic compounds are exchanged to form two new compounds. These reactions are characterized by the exchange of ions between the reacting substances. In essence, it involves the anions and cations of two different molecules swapping places, leading to the formation of two entirely new compounds.
In the case of copper(II) sulfate (\(\mathrm{CuSO}_4\)) and hydrogen sulfide (\(\mathrm{H}_2 \mathrm{~S}\)), the copper ions (\(\mathrm{Cu}^{2+}\)) and the sulfate ions (\(\mathrm{SO}_4^{2-}\)) from the copper sulfate react with the hydrogen and sulfide ions from the hydrogen sulfide. The copper ions form a new compound with the sulfide ions. Similarly, the hydrogen ions combine with the sulfate ions to form sulfuric acid (\(\mathrm{H}_2 \mathrm{SO}_4\)). This type of reaction is predictable and often results in the formation of at least one insoluble product, called a precipitate. Understanding this concept can help in predicting products of such reactions easily.

Precipitation Reaction

A precipitation reaction occurs when two soluble salts are mixed together, resulting in an insoluble solid known as a precipitate. This happens when the product of the reaction does not dissolve in water, causing it to separate from the liquid solution into a solid form.
In the scenario described, when hydrogen sulfide (\(\mathrm{H}_2 \mathrm{~S}\)) reacts with copper(II) sulfate (\(\mathrm{CuSO}_4\)), copper(II) sulfide (\(\mathrm{CuS}\)) forms as a precipitate. This black solid is insoluble in water, and its formation is evidence of a successful precipitation reaction.
Precipitation reactions are useful in a variety of contexts, including chemical analysis and synthesis. They allow chemists to isolate certain compounds and study their properties. Recognizing these reactions is crucial, as the formation of a precipitate often indicates that a specific ionic interaction has taken place.

Copper Sulfide Formation

Copper sulfide (\(\mathrm{CuS}\)) is a chemical compound that results from the reaction between copper ions (\(\mathrm{Cu}^{2+}\)) and sulfide ions (\(\mathrm{S}^{2-}\)). This formation is a crucial part of many chemical reactions involving sulfide ions. When hydrogen sulfide gas is bubbled through an acidified copper sulfate solution, copper(II) ions from the solution react with sulfide ions from the gas.
This process essentially swaps the ions, forming copper sulfide and liberating sulfuric acid in the process. The resulting copper sulfide appears as a black precipitate, indicating the reaction's completion. The solid nature of copper sulfide highlights that it is not soluble in the aqueous medium in which the reaction occurs.
This reaction and the production of copper sulfide are important in various industrial applications and laboratories, where the formation of this compound can be utilized for separations and purifications based on insolubility.