Problem 47 - A Complete Resource Book in Chemistry for JEE Main

Question

Group reagent for the precipitation of group II basic radicals for the qualitative analysis is (a) \(\mathrm{NH}_{4} \mathrm{OH}+\mathrm{H}_{2} \mathrm{~S}\) (b) dil. \(\mathrm{HCl}+\mathrm{H}_{2} \mathrm{~S}\) (c) only \(\mathrm{H}_{2} \mathrm{~S}\) (d) none of these

Step-by-Step Solution

Answer

The answer is (b) dil. HCl + H₂S.

Step 1: Understand Group II Radicals

Group II basic radicals in qualitative inorganic analysis are those which precipitate as sulfides when treated with hydrogen sulfide (H₂S) in the presence of dilute acid. They typically include metals such as Cu²⁺, Pb²⁺, Cd²⁺, Hg²⁺ (in some schemes), and Sn²⁺.

Step 2: Identify the Reagents Used

The task requires determining the reagent or combination of reagents used for precipitating Group II radicals in qualitative analysis. From the options provided, we need to focus on the use of hydrogen sulfide (H₂S) and other possible reagents like NH₄OH or HCl.

Step 3: Effect of Reagents

Hydrogen sulfide ( nH₂S ) is used in the precipitation of sulfides of Group II cations. However, the presence of a weak acid such as dilute HCl is essential to avoid complete ionization of H₂S, allowing it to react with the cations to form insoluble sulfides.

Step 4: Choose the Correct Option

In the qualitative analysis of Group II radicals, the combination of dil. HCl and H₂S is used because it controls the sulfide ion concentration just right for the targeted precipitation of Group II metals as their respective sulfides.

Key Concepts

Understanding Group II Basic Radicals

Group II basic radicals are specific metal ions analyzed in qualitative inorganic analysis. They include copper (Cu²⁺), lead (Pb²⁺), cadmium (Cd²⁺), and tin (Sn²⁺), among others. These ions are unique because they form insoluble sulfides when they interact with hydrogen sulfide (H₂S) in a slightly acidic environment. This reaction forms the basis for their identification and separation from other groups. This concept is crucial because it helps scientists and students distinguish these ions based on their chemical behavior. Here are some characteristics:

• They form insoluble compounds with sulfide ions.
• Precipitation occurs in the presence of a weak acid.
• Key ions include Cu²⁺, Pb²⁺, Cd²⁺, and Sn²⁺.

Understanding these basics makes it easier to identify and separate them during qualitative analysis. In the context of a lab setting, knowing these ions and their behaviors ensures that tests run smoothly and the right conclusions are drawn. This is the first step in the qualitative analysis process that emphasizes observation and systematic identification.

Hydrogen Sulfide Precipitation in Qualitative Analysis

The role of hydrogen sulfide (H₂S) in qualitative analysis centers around its ability to precipitate certain metal ions. When H₂S gas is introduced to a solution containing Group II basic radicals, it reacts with these ions to form metal sulfides. These sulfides, being insoluble, form precipitates that can be easily observed. This process is vital for separating and identifying Group II metal ions. Let's delve into how this works:

• H₂S provides sulfide ions necessary for precipitating metal sulfides.
• The precipitation is specific to certain metal ions, aiding in identification.
• Reactions typically occur under controlled acidic conditions to prevent full ionization of H₂S.

The process of utilizing H₂S is integral for qualitative inorganic analysis because of its specificity and reliability. It allows chemists to leverage the distinct formation of sulfide precipitates for confirming the presence of Group II metal ions. By ensuring that conditions are suitably acidic, H₂S functions effectively, heralding a specific and orderly analysis process.

The Role of Dilute HCl in Qualitative Analysis

Dilute hydrochloric acid (HCl) plays a strategic role in the qualitative analysis of metal ions. Its use is crucial for maintaining the right acidity in solutions during the test for Group II basic radicals. Let's explore some key aspects:

• Dilute HCl provides a weakly acidic environment necessary for H₂S to work efficiently.
• It helps control the concentration of sulfide ions by limiting complete ionization of H₂S.
• A proper acidic environment ensures selective precipitation of metal sulfides.

Using dilute HCl ensures that the faint acidity slows down the complete ionization of H₂S. This is important because fully ionized H₂S cannot precipitate the sulfides properly, potentially leading to false negatives. The acidity maintained by HCl not only stabilizes the system but also enhances the precipitation reactions. This careful balance is crucial for the successful identification and precipitation of Group II basic radicals.