Problem 197 - A Complete Resource Book in Chemistry for JEE Main

Question

How is H_{2} S \text { prepared in laboratory? } [2004] (a) \(\mathrm{FeS}+\operatorname{dil} \mathrm{H}_{2} \mathrm{SO}_{4}\) (b) \(\mathrm{FeSO}_{4}+\mathrm{H}_{2} \mathrm{SO}_{4}\) (c) \(\mathrm{FeS}+\) conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (d) elementary \(\mathrm{H}_{2}+\) elementary \(\mathrm{S}\)

Step-by-Step Solution

Answer

The correct method of preparation in the laboratory is (a) \(\mathrm{FeS} + \operatorname{dil} \mathrm{H}_2 \mathrm{SO}_4\).

Step 1: Understanding the reaction for H2S preparation

Hydrogen sulfide (\(\mathrm{H}_2\mathrm{S}\)) is typically prepared in the laboratory by reacting an iron (II) sulfide with a dilute acid.

Step 2: Evaluating the options

Let's evaluate the given options: - Option (a): Reacting \(\mathrm{FeS}\) with dilute \(\mathrm{H}_2\mathrm{SO}_4\).- Option (b): Reacting \(\mathrm{FeSO}_4\) with \(\mathrm{H}_2\mathrm{SO}_4\).- Option (c): Reacting \(\mathrm{FeS}\) with concentrated \(\mathrm{H}_2\mathrm{SO}_4\).- Option (d): Combining elementary \(\mathrm{H}_2\) and \(\mathrm{S}\).

Step 3: Identifying the correct reaction for H2S

In the laboratory, \(\mathrm{H}_2\mathrm{S}\) is typically prepared by reacting \(\mathrm{FeS}\) (iron sulfide) with a dilute acid, such as dilute sulfuric acid. This matches option (a).

Step 4: Analyzing the results

Option (a) produces \(\mathrm{FeSO}_4\) and \(\mathrm{H}_2\mathrm{S}\) gas as follows:\[\mathrm{FeS} + \mathrm{H}_2\mathrm{SO}_4 \rightarrow \mathrm{FeSO}_4 + \mathrm{H}_2\mathrm{S}\uparrow\]Option (c) is incorrect because concentrated acid could lead to side reactions and doesn't generally produce \(\mathrm{H}_2\mathrm{S}\) effectively in this context. Option (b) and (d) do not apply for \(\mathrm{H}_2\mathrm{S}\) preparation as described.

Key Concepts

Iron Sulfide Reaction

The reaction between iron sulfide (FeS) and a dilute acid is a classic method for producing hydrogen sulfide gas in the laboratory. When iron sulfide is treated with a dilute acid—often sulfuric acid (\( \text{H}_2\text{SO}_4 \))—a chemical reaction takes place. This reaction involves the breaking of bonds in \( \text{FeS} \) and the formation of new bonds, leading to the creation of \( \text{FeSO}_4 \) and hydrogen sulfide gas (\( \text{H}_2\text{S} \)). The chemical equation representing this reaction is:\[\text{FeS} + \text{H}_2\text{SO}_4 \rightarrow \text{FeSO}_4 + \text{H}_2\text{S}\uparrow\]Key points about this reaction:

• It generates \( \text{H}_2\text{S} \) as a gas, which is why you see the gas symbol (\(\uparrow\)).
• It's important to use a dilute acid to avoid excess heat and unwanted side products.

This reaction shows how compounds interact to produce new substances in a controlled manner.

Dilute Sulfuric Acid

Sulfuric acid is one of the most versatile and widely used acids in chemistry. In its dilute form, it acts as a reagent in many reactions, including the preparation of hydrogen sulfide gas. Using dilute sulfuric acid is crucial because:

• It provides the necessary hydrogen ions (H⁺) to react with iron sulfide (\( \text{FeS} \)).
• It helps in controlling the reaction rate to ensure safety in laboratory settings.
• It minimizes the risk of side reactions that might occur with concentrated acids.

In contrast, using concentrated sulfuric acid might cause unwanted reactions due to its strong oxidizing nature. It could lead to the oxidation of \( \text{FeS} \) or even the decomposition of the desired product. Thus, for effective and safe preparation of \( \text{H}_2\text{S} \)n gas, dilute sulfuric acid is preferred.

Laboratory Preparation

Preparing chemicals in the laboratory requires careful handling and understanding of chemical properties and reactions. When preparing hydrogen sulfide (\( \text{H}_2\text{S} \)) gas, laboratory safety, and technique are especially important:

• Ensure proper ventilation because \( \text{H}_2\text{S} \) is a toxic gas.
• Use small quantities to control the reaction more safely.
• Be aware of the characteristic rotten egg smell as a signal of a \( \text{H}_2\text{S} \) leak.
• Always conduct the experiment under the supervision of an experienced individual or an instructor.

Typically, setups involving the production of \( \text{H}_2\text{S} \) are part of introductory chemistry labs, teaching students about gas generation and collection techniques. By performing these preparations safely, students learn valuable skills for future work in chemistry.