Problem 41 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following gas is evolved when cupric sulphide (CuS) is dissolved in dilute hydrochloric acid \((\mathrm{HCl}) ?\) (a) \(\mathrm{S}\) (b) \(\mathrm{Cl}_{2}\) (c) \(\mathrm{H}_{2} \mathrm{~S}\) (d) \(\mathrm{H}_{2}\)

Step-by-Step Solution

Answer

The gas evolved is hydrogen sulfide, \(\mathrm{H}_2\mathrm{S}\).

Step 1: Identify the Reaction Components

Cupric sulphide is known as \(\text{CuS}\), and we are considering its reaction with dilute hydrochloric acid \(\text{HCl}\). Our task is to determine which gas is produced when these two react.

Step 2: Write the Reaction Equation

The reaction between cupric sulphide \(\text{CuS}\) and hydrochloric acid \(\text{HCl}\) is: \[ \text{CuS} + 2\text{HCl} \rightarrow \text{CuCl}_2 + \text{H}_2\text{S} \] Here, \(\text{CuCl}_2\) (cupric chloride) is formed, and \(\text{H}_2\text{S}\) (hydrogen sulfide gas) is released.

Step 3: Analyze the Reaction Products

Upon analyzing the chemical reaction, it is evident that when cupric sulphide reacts with hydrochloric acid, hydrogen sulfide \(\text{H}_2\text{S}\) gas is evolved as one of the products.

Step 4: Verify the Answer

Given options are: (a) \(\mathrm{S}\), (b) \(\mathrm{Cl}_{2}\), (c) \(\mathrm{H}_{2} \mathrm{~S}\), (d) \(\mathrm{H}_{2}\). From the reaction, we established that hydrogen sulfide \(\mathrm{H}_{2}\mathrm{S}\) is the gas released. Thus, option (c) is correct.

Key Concepts

Chemical Reactions

Chemical reactions involve the transformation of reactants into products, incorporating changes at the molecular level. When substances interact, the atoms rearrange, forming new compounds as a result of this reorganization. Such reactions can be identified by

• The formation of gas
• Precipitate formation
• Color change
• Temperature change

In the specific case of cupric sulphide reacting with dilute hydrochloric acid, we see the formation of a new compound, cupric chloride, and the evolution of hydrogen sulfide gas. Understanding these changes allows chemists to predict the outcomes of chemical reactions and manipulate them for practical applications.

Cupric Sulphide

Cupric sulphide is a chemical compound represented by the formula \[\text{CuS}\]. It consists of copper and sulfur elements bonded together. This compound is often seen as a black solid, indicative of its metallic nature.
Cupric sulphide is commonly found in its natural state as the mineral covellite. Its applications extend to:

• Catalysis in organic synthesis
• Electrodes in certain types of batteries
• Photovoltaic materials

The distinct reactions and interactions of cupric sulphide, especially in solution, make it an important substance in studying inorganic chemistry.

Dilute Hydrochloric Acid

Dilute hydrochloric acid is simply hydrochloric acid with a reduced concentration. This dilution often makes it safer to handle and react with various substances without causing violent reactions. It retains the fundamental properties of \(\text{HCl}\) and is noted for being:

• A strong acid
• Highly corrosive
• Reactive with metals and bases

In laboratory settings, it's frequently used due to its ability to provide chloride ions and protons, influencing various chemical reactions. In the reaction with cupric sulphide, it effectively breaks down the compound to release hydrogen sulfide gas.

Hydrogen Sulfide Gas

Hydrogen sulfide \(\text{H}_2\text{S}\) is a colorless gas known for its characteristic "rotten egg" smell. It naturally occurs in crude petroleum and natural gas, as well as from bacterial breakdown of organic materials. While toxic and flammable, it has distinct uses:

• Precursors in synthetic chemistry
• Production of heavy water
• Lab scale production of many organosulfur compounds

In the chemical reaction between cupric sulphide and dilute hydrochloric acid, \(\text{H}_2\text{S}\) gas is notable for being the evolution product, marking the reaction's completion. Recognizing this gas' presence is essential in both detecting successful reactions and ensuring safety in handling.