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

Hydrogen sulfide \((\text{H}_2\text{S})\) is evolved.

Step 1: Reactants and Possible Products

When cupric sulphide \(\text{CuS}\) is reacted with dilute hydrochloric acid \(\text{HCl}\), the potential reaction pathway involves the acid reacting with the sulfide ion \(\text{S}^{2-}\) in \(\text{CuS}\), potentially producing hydrogen sulfide gas (\(\text{H}_2\text{S} \)).

Step 2: Write the Chemical Equation

The chemical reaction can be represented as follows: \[\text{CuS} + 2\text{HCl} \rightarrow \text{CuCl}_2 + \text{H}_2\text{S}\uparrow \] This reaction shows that when \(\text{CuS}\) is dissolved in \(\text{HCl}\), it forms cupric chloride (\(\text{CuCl}_2\)) and evolves hydrogen sulfide gas (\(\text{H}_2\text{S}\)).

Step 3: Identify the Evolved Gas

The balanced equation from the previous step reveals that the gas evolved during this reaction is \(\text{H}_2\text{S}\), which is also indicated by the gaseous arrow symbol (\(\uparrow\)).

Key Concepts

Cupric Sulfide Reaction

Cupric sulfide, with the chemical formula CuS, is a compound made up of copper and sulfur. When engaging in chemical reactions, cupric sulfide can react with various substances to form different products. In the context of our exercise, it reacts with dilute hydrochloric acid (HCl). This is an example of a type of reaction known as a "double displacement reaction.”
This type of reaction typically involves the exchange of ions between two compounds. In the case of CuS and HCl, the reaction results in the formation of cupric chloride (CuCl2) and hydrogen sulfide (H2S) gas. Understanding the nature of this reaction helps in predicting the products and identifying the characteristics of the involved reactants.

Chemical Reaction Equation

Chemical equations are vital in summarizing the details of a chemical reaction. They provide a clear representation of the reactants involved and the products formed, plus the stoichiometry of the reaction. In our case, the chemical equation is:

• The reactants: CuS and HCl
• The products: CuCl2 and H2S

This can be written as:\[\text{CuS} + 2\text{HCl} \rightarrow \text{CuCl}_2 + \text{H}_2\text{S}\uparrow\]The arrow \(\rightarrow\) indicates the direction of the reaction, with reactants on the left converting into products on the right. The balancing of the chemical equation also assures the conservation of mass, meaning the number of each type of atom on both sides of the equation is the same. Furthermore, the upwards arrow (\(\uparrow\)) specifically denotes that hydrogen sulfide gas (\(\text{H}_2\text{S}\)) is produced and released as a gas during the reaction.

Dilute Hydrochloric Acid Reaction

Dilute hydrochloric acid reacts with many metal sulfides to form gaseous products. In the reaction with cupric sulfide, it serves as an acid that donates protons (H\(^+\)) to sulfide ions (S\(^{2-}\)), triggering the evolution of hydrogen sulfide gas. This type of reaction is part of a broader category termed "acid-base reactions," where the acid (HCl) reacts with the base (\(\text{S}^{2-}\) in CuS) to form new compounds.

In this process, it’s important to note:

• Hydrochloric acid is in a diluted form, meaning it is mixed with water, thereby reducing its concentration.
• The reaction is vigorous but controlled since the dilute acid limits the rate and extent of the reaction.

These reactions are commonly used in industry and laboratories to produce hydrogen sulfide gas because of its various applications and properties.