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

Question

\(\mathrm{Fe}^{3+}\) can be detected by the reaction (1) \(\mathrm{Fe}^{3_{+}}+\left[\mathrm{Fe}^{11}(\mathrm{CN})_{6}\right]^{4^{-}} \longrightarrow \mathrm{Fe}^{\mathrm{IH}}\left[\mathrm{Fe}^{\mathrm{I}}(\mathrm{CN})_{6}\right]\) Deep blue (2) \(\mathrm{Fe}^{3+}+\left[\mathrm{Fe}^{\mathrm{ll}}(\mathrm{CN})_{6}\right]^{3} \longrightarrow \mathrm{Fe}^{\mathrm{Uu}}\left[\mathrm{Fe}^{\mathrm{III}}(\mathrm{CN})_{6}\right]\) (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

Option (c) 1, 2, and 3.

Step 1: Understanding the Context

The question involves reactions of ferric ion (\(\mathrm{Fe}^{3+}\)) that create visible color changes. These reactions are used to detect the presence of \(\mathrm{Fe}^{3+}\) by the formation of colorful complexes.

Step 2: Analyzing Reaction (1)

In Reaction (1), \(\mathrm{Fe}^{3+}\) reacts with \(\left[\mathrm{Fe}^{\mathrm{II}}(\mathrm{CN})_6\right]^{4-}\) to form a compound that is described as deep blue. This is known as the formation of Prussian blue, a common indicator of \(\mathrm{Fe}^{3+}\).

Step 3: Analyzing Reaction (2)

Reaction (2) involves \(\mathrm{Fe}^{3+}\) reacting with \(\left[\mathrm{Fe}^{\mathrm{II}}(\mathrm{CN})_6\right]^{3-}\). This also forms a similar deep blue compound, confirming that this reaction is used to detect \(\mathrm{Fe}^{3+}\) based on its color change.

Step 4: Analyzing Reaction (3)

For Reaction (3), \(\mathrm{Fe}^{3+}\) forms a blood-red complex with \(\mathrm{SCN}^-\). This reaction is another standard test for \(\mathrm{Fe}^{3+}\), producing a distinct color change that confirms the presence of the ferric ion.

Step 5: Selecting the Correct Options

Reactions (1), (2), and (3) all result in color changes that confirm the detection of \(\mathrm{Fe}^{3+}\). Therefore, all three reactions can be used for detection, indicating the correct choice is the option containing all three reactions.

Key Concepts

Fe3+ detection

Detecting \(\mathrm{Fe}^{3+}\) ions involves observing distinct color changes when they react with specific compounds. This type of chemical detection is commonly applied in chemistry to identify the presence of particular ions in a solution. \ \(\mathrm{Fe}^{3+}\) is iron in a 3+ oxidation state, also known as ferric iron. This transition metal ion is well-known for its ability to create colorful complexes through chemical reactions. \ As these reactions occur, \(\mathrm{Fe}^{3+}\) interacts with other ions or molecules, leading to noticeable color transformations. These transformations arise because the \(\mathrm{Fe}^{3+}\) ion forms new compounds that have different electronic arrangements, which absorb light differently and thus display distinctive colors. \ To detect \(\mathrm{Fe}^{3+}\), scientists often leverage this property, using color change as an effective signal during analysis. \ Some standardized tests involve reagents like thiocyanate (SCN) and various cyanide complexes, each producing a specific color change characteristic of their chemical interactions with \(\mathrm{Fe}^{3+}\).

color change reactions

Color change reactions are special kinds of chemical reactions where the color of the solution or material involved visibly changes. These changes frequently occur during complex formations, as the electronic structure and light absorption characteristics of the compound vary. \ \ Iron ions, particularly \(\mathrm{Fe}^{3+}\), are notorious for their dramatic color changes when forming chemical complexes. In iron complexes, the ligands involved can significantly alter the color observed. For instance, \(\mathrm{Fe}^{3+}\) with \(\mathrm{SCN}^{-}\) results in a blood-red complex, while interaction with ferrocyanide leads to a deep blue color indication of Prussian blue. \ \ Color changes in chemical reactions are not just visually stunning but also provide essential qualitative data about the presence or concentration of species such as ions in a solution. Due to their relatively straightforward observation without the need for elaborate equipment, color change reactions serve as an efficient and cost-effective method for ion detection and chemical analysis.

Prussian blue formation

Prussian blue is a famous compound in chemistry known for its intense blue color. The compound is generated when \(\mathrm{Fe}^{3+}\) ions react with ferrocyanide ions \[\left[\mathrm{Fe}^{II}(\mathrm{CN})_6\right]^{4-}\]. This reaction results in the formation of a solid complex with a vivid blue hue. \ \ Prussian blue formation is a classical example of selective ion detection based on color change mechanisms. Often used historically as a pigment, this compound has found its applications in modern chemistry for scientifically important tasks such as detoxifying heavy metal poisoning. \ \ The reaction involves oxidation, where the \(\mathrm{Fe}^{II}\) in \[\left[\mathrm{Fe}^{II}(\mathrm{CN})_6\right]^{4-}\] gets oxidized to \(\mathrm{Fe}^{III}\) while \(\mathrm{Fe}^{3+}\) might be partially reduced in special structures forming Prussian Blue or its analogs. This ability to form such structured compounds highlights the unique role of \(\mathrm{Fe}^{3+}\) in coordination chemistry.

complex ion chemistry

Complex ion chemistry is a field that explores how transition metal ions, like \(\mathrm{Fe}^{3+}\), engage with surrounding molecules called ligands. These interactions lead to the formation of complex ions. A complex ion consists of a central metal ion bonded to one or more ligands – typically neutral molecules or anions – through coordinate covalent bonds. \ \ \(\mathrm{Fe}^{3+}\) is an archetypal example in complex ion chemistry due to its ability to form magnificent complexes with a variety of ligands, resulting in diverse colors. This diversity stems from variations in ligand field effects which change the energy levels of electrons within the metal ion. \ \ In essence, when a ligand approaches the metal ion it impacts its electronic arrangement, often leading to changes in color due to light absorption properties. Such properties are utilized in detection methods for ions because the color formed upon complexing offers clues about the specific interactions taking place. \ \ Understanding these interactions requires an appreciation of principles such as ligand field theory, coordination numbers, and the geometry of complexes. These contribute to explaining why ions like \(\mathrm{Fe}^{3+}\) display such distinctive and vibrant colors upon forming complexes.