Question

\(\mathrm{Fe}^{3+}\) can be detected by the reaction (1) \(\mathrm{Fe}^{3+}+\left[\mathrm{Fe}^{\mathrm{II}}(\mathrm{CN})_{6}\right]^{4} \longrightarrow \mathrm{Fe}^{\mathrm{III}}\left[\mathrm{Fe}^{\mathrm{ll}}(\mathrm{CN})_{6}\right]\) Deep blue (2) \(\mathrm{Fe}^{3+}+\left[\mathrm{Fe}^{\mathrm{ll}}(\mathrm{CN})_{6}\right]^{3} \longrightarrow \mathrm{Fe}^{\mathrm{Il}}\left[\mathrm{Fe}^{\mathrm{III}}(\mathrm{CN})_{6}\right]\) (3) \(\mathrm{Fe}^{3+}+\mathrm{SCN}^{-} \stackrel{\mathrm{H}_{2} \mathrm{O}}{\mathrm{C}} \begin{gathered}\text { Brown } \\\ {\left[\mathrm{Fe}(\mathrm{SCN})\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}\right]^{2^{+}}}\end{gathered}\) Blood red (a) 1 and 2 only (b) 2 and 3 only (c) 1,2 and 3 (d) 3 only

Short answer

(d) 3 only.

Step-by-step solution

Understand the Reaction Conditions

In the first reaction, an iron(III) ion (\(\mathrm{Fe}^{3+}\)) reacts with \([\mathrm{Fe}^{\mathrm{II}}(\mathrm{CN})_{6}]^{4-}\) to form a deep blue compound. This indicates a distinct color change, signifying the presence of \(\mathrm{Fe}^{3+}\). It shows a clear detection method for \(\mathrm{Fe}^{3+}\).

Analyze the Second Reaction

The second reaction involves \(\mathrm{Fe}^{3+}\) reacting with \([\mathrm{Fe}^{\mathrm{II}}(\mathrm{CN})_{6}]^{3-}\) to form another iron compound. However, only the transition to a deep blue color is explicitly mentioned and is potentially less distinctive for detection.

Review the Third Reaction

In the third reaction, \(\mathrm{Fe}^{3+}\) reacts with \(\mathrm{SCN}^{-}\) in water to form a complex with a blood-red color. This color change is a clear indication of \(\mathrm{Fe}^{3+}\) presence, serving as a good detection method.

Determine which Reactions are Effective for Detection

Reactions 1 and 3 both result in significant and noticeable color changes (deep blue and blood-red, respectively) upon the presence of \(\mathrm{Fe}^{3+}\). Reaction 2 involves \(\mathrm{Fe}^{3+}\) but lacks a distinct color description.

Identify the Correct Answer

Based on the analysis, only reactions 1 and 3 provide distinctive color changes for \(\mathrm{Fe}^{3+}\) detection. Reactions 2 and 3 have less distinctive descriptions, and reaction 2 alone may not provide a clear color change. Therefore, the correct answer is (d) 3 only.

Key concepts

Colorimetric Detection

Colorimetric detection is a popular analytical method used in chemistry to identify the presence of specific ions or compounds in a solution by observing a color change. This technique leverages the unique color properties that result when certain chemicals are introduced to analytes. By watching these shifts in color, researchers can gain insights into which substances are present.
Colorimetric detection is beneficial because it is often quick, requires minimal equipment, and can usually be observed with the naked eye. Its simplicity makes it ideal for situations where advanced lab instruments may not be available.
Key points of colorimetric detection include:

• Reactions involve a color change due to the formation of a new compound or complex.
• These color shifts can be used as visual indicators for the presence or concentration of certain ions like \( ext{Fe}^{3+}\).
• Color changes can range from subtle to dramatic, allowing for easy detection in qualitative analysis.

Additionally, colorimetric detection not only shows if a reaction has occurred but can also suggest the intensity or concentration of the analyte based on the depth or shade of the color.

Iron(III) Ion Detection

Detecting iron(III) ions (\( ext{Fe}^{3+}\)) is crucial in many chemical processes and environmental analyses. Iron, in its tri-positive form, presents unique characteristics that allow for its presence to be confirmed through specific reactions. In the exercise example, \(\text{Fe}^{3+}\) detection prominently features two reactions where color change clearly signifies its presence.
One effective method involves reacting \(\text{Fe}^{3+}\) with \(\text{SCN}^{-}\) ions. This results in a blood-red complex—specifically, \([\text{Fe(SCN)(H}_{2}\text{O})_{5}]^{2+}\)—that is easily recognizable.

• The transition to a deep blue color, although less widely mentioned, can also signify \(\text{Fe}^{3+}\) when it forms complexes with CN ligands.

This specificity in reaction highlights the critical sensitivity of these colorimetric techniques for iron detection. Furthermore, the unique chromatic transitions that accompany these reactions are directly linked to the distinct electron configurations in \(\text{Fe}^{3+}\) and its resultant complexes, enriching its detection methods.

Complex Formation Reactions

Complex formation reactions involve the interaction between central metal ions and surrounding ligands to form complex ions. In the context of this exercise, such reactions are vital for the detection of \(\text{Fe}^{3+}\). Specifically, \(\text{Fe}^{3+}\) reacts with ligands like cyanide (CN) and thiocyanate (SCN) to form colored complexes. These reactions are pivotal in qualitative analysis as they provide the physical basis for distinguishing different ions based on appearance.
For example, \(\text{Fe}^{3+}\) can form:

• a deep blue complex with cyanide ligands, \(\text{Fe}^{\text{II}}\left[\text{Fe}^{\text{III}}(\text{CN})_{6}\right]\)
• a blood-red complex with thiocyanate, \(\left[\text{Fe(SCN)(H}_{2}\text{O})_{5}\right]^{2+}\).

The formation of these complexes not only underscores the versatility of iron ions in chemistry but also accentuates the careful balance between metal ions and ligands needed to achieve a noticeable color change. This balance is crucial in successfully utilizing these reactions for qualitative evaluations and educational exercises.
By understanding complex formation, students get a deeper grasp of ligand coordination chemistry and its critical role in qualitative analysis.