Problem 95 - A Complete Resource Book in Chemistry for JEE Main

Question

The only cations present in a slightly acidic solution are \(\mathrm{Fe}^{3+}, \mathrm{Zn}^{2+}\) and \(\mathrm{Cu}^{2+}\). The reagent that when added in excess of this solution would identify and separate \(\mathrm{Fe}^{3+}\) in one step is (a) \(2 \mathrm{M} \mathrm{HCl}\) (b) \(6 \mathrm{M} \mathrm{NaOH}\) (c) \(6 \mathrm{M} \mathrm{NH}_{3}\) (d) \(\mathrm{H}_{2} \mathrm{~S}\) gas

Step-by-Step Solution

Answer

The correct reagent to separate \( \mathrm{Fe}^{3+} \) is \(\mathrm{H}_2 \mathrm{S}\) gas.

Step 1: Understanding the Problem

The task is to identify a reagent that, when added to a solution containing \( \mathrm{Fe}^{3+} \), \( \mathrm{Zn}^{2+} \), and \( \mathrm{Cu}^{2+} \), will selectively react with \( \mathrm{Fe}^{3+} \) and separate it. The solution is slightly acidic.

Step 2: Analyze Reagent Options

Evaluate each reagent option for its ability to selectively precipitate or react with \( \mathrm{Fe}^{3+} \) without precipitating \( \mathrm{Zn}^{2+} \) or \( \mathrm{Cu}^{2+} \).

Step 3: Evaluate Option (a): \(2 \mathrm{M} \mathrm{HCl}\)

\(\mathrm{HCl}\) is a strong acid that does not precipitate \( \mathrm{Fe}^{3+} \) but can form complexes with both \(\mathrm{Zn}^{2+}\) and \(\mathrm{Cu}^{2+}\). This option will not separate \( \mathrm{Fe}^{3+} \) specifically.

Step 4: Evaluate Option (b): \(6 \mathrm{M} \mathrm{NaOH}\)

\(\mathrm{NaOH}\) is a strong base that can precipitate metal hydroxides. \( \mathrm{Fe}^{3+} \) will form \( \mathrm{Fe(OH)_3} \), which is insoluble. However, \(\mathrm{Zn}^{2+}\) and \(\mathrm{Cu}^{2+}\) also form hydroxides that might precipitate. \( \mathrm{Cu(OH)_2} \) is also insoluble, so this option will not specifically precipitate \( \mathrm{Fe}^{3+} \) alone.

Step 5: Evaluate Option (c): \(6 \mathrm{M} \mathrm{NH}_3 \)

\(\mathrm{NH}_3\) can complex with \( \mathrm{Zn}^{2+} \) and \( \mathrm{Cu}^{2+} \) to form soluble ammine complexes. It does not complex \( \mathrm{Fe}^{3+} \) effectively in a slightly acidic environment. Thus, it won't precipitate \( \mathrm{Fe}^{3+} \) preferentially.

Step 6: Evaluate Option (d): \(\mathrm{H}_2 \mathrm{S}\) gas

Passing \(\mathrm{H}_2 \mathrm{S}\) gas through the solution in a slightly acidic environment will precipitate \(\mathrm{Fe}^{3+}\) as \(\mathrm{Fe}_2\mathrm{S}_3\). The formation of metal sulfides is favored and much less likely for \(\mathrm{Zn}^{2+} \) and \(\mathrm{Cu}^{2+} \) in a slightly acidic pH.

Step 7: Select the Correct Reagent

Based on the analysis, \(\mathrm{H}_2 \mathrm{S}\) gas will selectively precipitate \( \mathrm{Fe}^{3+} \) under slightly acidic conditions, forming \(\mathrm{Fe}_2\mathrm{S}_3\) and allowing for separation from \( \mathrm{Zn}^{2+} \) and \( \mathrm{Cu}^{2+} \).

Key Concepts

Cation Separation

In chemistry, cation separation is a critical process in analyzing solutions containing multiple metal ions. The goal is to isolate and identify the specific ions present in a mixture. To effectively separate cations, chemists often employ selective precipitation techniques. This involves using specific reagents that will precipitate one ion while leaving others in solution. For instance, within a solution containing Fe³⁺, Zn²⁺, and Cu²⁺, finding a method that separates Fe³⁺ is crucial for analysis and further study.

Cation separation often requires precise control of conditions such as pH and concentration. These factors influence the solubility of potential precipitates.

• A lower pH, or more acidic solution, can encourage the selective precipitation of certain metal ions.
• Correct choice of reagents is also vital, as some ions might form complexes instead of precipitates, complicating separation efforts.

This process is foundational in fields like analytical chemistry and environmental testing, where determining metal composition is vital.

Chemical Reagents

Chemical reagents play a pivotal role in the field of cation separation in chemistry. They are substances that cause a chemical reaction to occur, which can lead to the formation of precipitates or the dissolution of certain components in a mixture.

In the context of separating Fe³⁺, Zn²⁺, and Cu²⁺, the selection of an appropriate reagent is crucial.

• Strong acids like HCl might not work effectively for Fe³⁺ as they form complexes with other ions.
• Conversely, bases such as NaOH can result in the precipitation of all metal hydroxides, failing to be selective for Fe³⁺ specifically.

The most suitable reagent may not always be apparent at first glance. It requires an understanding of the chemical interactions between the reagent and the ions in solution. Learning how different reagents react with specific cations aids in selecting the best one for a given separation task. This knowledge is instrumental for various applications in both laboratory and industrial settings.

Selective Precipitation

Selective precipitation is an essential method used to separate ions in a mixture by exploiting differences in their solubility products.

The idea is to add a reagent that will precipitate one cation over others in the mixture. In practice, slight variations in pH and reagent concentrations can significantly affect which metal ion precipitates first.

• In acidic conditions, using H₂S gas can precipitate Fe³⁺ as Fe₂S₃.
• This approach takes advantage of the fact that at lower pH levels, Zn²⁺ and Cu²⁺ are less likely to form sulfide precipitates.

By carefully choosing conditions, chemists can maximize the efficiency of separation, ensuring that the desired ions are isolated while other metal ions remain in solution. This process is widely used in both qualitative and quantitative analyses and is a cornerstone of analytical chemistry.

Metal Ion Analysis

Analyzing metal ions in a solution involves several steps, beginning with the isolation or separation of specific ions, followed by their accurate identification. This analysis is crucial in determining the composition of unknown samples.

The selection of appropriate reagents and techniques for separation and measurement is vital.

• The choice of reagent can affect the ease of separation as well as the accuracy of the analysis.
• Once separated, further tests such as spectroscopy or titration might be used to identify the separated ions conclusively.

Understanding the characteristics of each metal ion, such as their typical compounds and reactivity, is necessary for successful analysis. For chemists working in laboratories, mastering metal ion analysis is important for conducting experiments and interpreting results. This knowledge is not only applicable in research but also in industries such as manufacturing, environmental monitoring, and forensics, where accurate identification of metal components is required.