Problem 104 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{Fe}^{3+}\) can be detected by the reaction (1) \(\mathrm{Fe}^{3}+\left[\mathrm{Fe}^{11}(\mathrm{CN})_{6}\right]^{4^{-}} \longrightarrow \mathrm{Fe}^{\mathrm{m}}\left[\mathrm{Fe}^{\mathrm{H}}(\mathrm{CN})_{6}\right]\) Deep blue (2) \(\mathrm{Fe}^{3+}+\left[\mathrm{Fe}^{\mathrm{ml}}(\mathrm{CN})_{6}\right]^{3-} \longrightarrow \mathrm{Fe}^{\mathrm{m}}\left[\mathrm{Fe}^{\mathrm{IH}}(\mathrm{CN})_{6}\right]\) (3) \(\mathrm{Fe}^{3+}+\mathrm{SCN}^{-} \stackrel{\mathrm{H}_{2} \mathrm{O}}{\longrightarrow}\left[\mathrm{Fe}(\mathrm{SCN})\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}\right]^{2}\) Blood red (a) 1 and 2 only (b) 2 and 3 only (c) 1,2 and 3 (d) 3 only

Step-by-Step Solution

Answer

The correct answer is (c) 1, 2, and 3.

Step 1: Identify Reactions Including Fe3+

Examine each given reaction to determine whether \( \mathrm{Fe}^{3+} \) is a reactant and whether it results in the formation of a colored complex.

Step 2: Analyze Reaction (1)

Reaction (1) involves \( \mathrm{Fe}^{3+} \), as it reacts with \( \left[\mathrm{Fe}^{\mathrm{II}}(\mathrm{CN})_{6}\right]^{4-} \) to form a deep blue product. Thus, \( \mathrm{Fe}^{3+} \) is detectable in Reaction (1) with the formation of a colored complex.

Step 3: Analyze Reaction (2)

In Reaction (2), \( \mathrm{Fe}^{3+} \) reacts with another cyanide complex \( \left[\mathrm{Fe}^{\mathrm{III}}(\mathrm{CN})_{6}\right]^{3-} \) to form a new product. The reaction suggests the involvement of \( \mathrm{Fe}^{3+} \), although the specific color of the product isn't provided. Therefore, \( \mathrm{Fe}^{3+} \) is involved here too.

Step 4: Analyze Reaction (3)

In Reaction (3), \( \mathrm{Fe}^{3+} \) reacts with \( \mathrm{SCN}^{-} \) in water to form a complex \( \left[\mathrm{Fe}(\mathrm{SCN})(\mathrm{H}_{2}\mathrm{O})_{5}\right]^{2} \) which is known to produce a blood-red color. This indicates the presence of \( \mathrm{Fe}^{3+} \).

Step 5: Determine Which Reactions Indicate Presence of Fe3+

Review the analysis results: Reaction (1) produces a deep blue complex, Reaction (2) involves a complex but doesn't specify a color, and Reaction (3) produces a blood-red complex, all indicating the detection of \( \mathrm{Fe}^{3+} \). Thus, both Reactions (1), (2), and (3) can be used to detect \( \mathrm{Fe}^{3+} \).

Step 6: Select the Correct Answer Option

Since all reactions (1), (2), and (3) indicate the presence of \( \mathrm{Fe}^{3+} \), the correct answer is (c) 1, 2, and 3.

Key Concepts

Complexation reactions

Complexation reactions are fascinating because they involve the formation of a complex compound. This process occurs when a metal ion, like \(Fe^{3+}\), bonds with molecules or ions termed ligands. In the context of our exercise, the iron ion \(Fe^{3+}\) interacts with various cyanide ions and thiocyanate ions.

These ligands surround the metal ion, creating a coordination complex. The resulting structure often leads to a change in color, which can be indicative of specific reactions. This is especially intriguing because the color shift is due to the specific way the ligands are arranged around the metal. It's much like trying on different outfits, each giving you a different vibe or look!

To detect iron \(Fe^{3+}\), we engage it in complexation reactions, some leading to deep blue outcomes and others to blood-red outcomes. These reactions are pivotal in chemistry due to their practical applications in identifying metal ions.

Colorimetric analysis

Colorimetric analysis is a technique that uses the color of a solution to determine the concentration of a substance. This method relies on the fact that specific metal ions, such as \(Fe^{3+}\), can produce a distinct color when they form complexes with certain ligands. It's like a colorful clue that unveils the presence or absence of a compound.

In our exercise, reactions involving \(Fe^{3+}\) and different ligands like cyanide and thiocyanate lead to noticeable color changes. For instance, when \(Fe^{3+}\) reacts with thiocyanate, a blood-red color indicates its presence.

Using this method, we can determine not only whether a substance is present but also estimate its concentration based on how intense the color appears. It's a visual and straightforward method for detecting substances in a lab setting, making it an excellent choice for quick and efficient analysis.

Coordination chemistry

Coordination chemistry is the branch of chemistry focused on the study of complex compounds formed between metal ions and ligands. Understanding these reactions helps us explore how molecules arrange themselves in three-dimensional space.

In coordination chemistry, central metal ions, like \(Fe^{3+}\), are surrounded by molecules or ions called ligands. These ligands can donate electron pairs to the metal, forming stable structures known as complexes.

The fascination lies in how these complexes exhibit distinct physical and chemical properties, including unique colors. As we see in our exercise, \(Fe^{3+}\) forms complexes with cyanide that leads to a deep blue hue, a hallmark of coordination bonds at work.

The formation of these complexes is not only influenced by simple bond formation but also by the coordination number, geometry, and type of linking atoms involved. It's a rich field of study that opens doors to numerous applications, ranging from materials science to bioinorganic chemistry.

Chemical detection methods

Chemical detection methods involve strategies to identify and quantify chemical substances, often with impressive precision. For iron (III) detection, the choice of method is informed by the characteristics of \(Fe^{3+}\) when it takes part in reactions.

One common method involves forming complexes with reagents that exhibit a color change, as we see with cyanide and thiocyanate in the original exercise.

The reactions which lead to the development of a color, such as deep blue or blood red, provide a visual cue that \(Fe^{3+}\) is present. Techniques like these are advantageous due to their simplicity and effectiveness.

Besides being easy to perform, these methods have the added benefit of requiring minimal equipment, making them accessible for various settings, from educational labs to industrial applications. Such methods underscore the exciting interplay between theoretical chemistry and practical application, demonstrating how fundamental principles can be applied to solve real-world problems.