Problem 87 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statement is correct with reference to the ferrous and ferric ions? 1\. \(\mathrm{Fe}^{3+}\) gives brown colour with potassium ferricyanide 2\. \(\mathrm{Fe}^{2+}\) gives blue precipitate with potassium ferricyanide 3\. \(\mathrm{Fe}^{3+}\) gives red colour with potassium thiocyanate 4\. \(\mathrm{Fe}^{2+}\) gives brown colour with ammonium thiocyanate (a) 1,4 (b) 1,2 (c) 2,3 (d) all of these

Step-by-Step Solution

Answer

The correct option is (c) 2,3.

Step 1: Understanding Ferric Ion Reactions

Ferric ion, denoted as \( \mathrm{Fe}^{3+} \), reacts with potassium ferricyanide to form a brown coloration. This confirms that statement 1 is correct. \( \mathrm{Fe}^{3+} \) also reacts with potassium thiocyanate to form a blood-red complex, confirming statement 3 is correct.

Step 2: Analyzing Ferrous Ion Reactions

Ferrous ion, denoted as \( \mathrm{Fe}^{2+} \), reacts with potassium ferricyanide to give a blue precipitate, which verifies statement 2 as correct. On the other hand, \( \mathrm{Fe}^{2+} \) does not typically give a brown color with ammonium thiocyanate, invalidating statement 4.

Step 3: Conclusion on Correct Statements

Based on the reactions analyzed for ferric and ferrous ions, the correct statements are 2 and 3. This matches option (c). Therefore, the only correct combination of statements is option (c) 2,3.

Key Concepts

Ferrous Ion Reactions

The ferrous ion, represented by the symbol \( \mathrm{Fe}^{2+} \), is an iron ion in the +2 oxidation state. Understanding the behavior of \( \mathrm{Fe}^{2+} \) in reactions is key to analyzing its chemical properties and applications.

One pivotal reaction involving \( \mathrm{Fe}^{2+} \) is with potassium ferricyanide \( (\mathrm{K}_{3}[\mathrm{Fe} \mathrm{(CN)}_{6}]) \). When \( \mathrm{Fe}^{2+} \) reacts with potassium ferricyanide, it results in the formation of a blue precipitate. This blue compound is known as Turnbull's Blue, an indicator of the transition from ferrous to ferric state through electron transfer.

This reaction is particularly significant in qualitative analysis as it provides a visual representation of the presence of ferrous ions in a solution. Additionally, the formation of this blue precipitate can be used in various industries and laboratories as a means to identify or quantify \( \mathrm{Fe}^{2+} \) ions. While analyzing reactions of ferrous ions, it's crucial to note that not all potentially reactive ions like ammonium thiocyanate lead to a notable product, explaining why no brown color is observed with this reagent.

Ferric Ion Reactions

The ferric ion, denoted by \( \mathrm{Fe}^{3+} \), is an iron ion in the +3 oxidation state. This ion is known for its ability to form distinct compounds with various reagents. Two notable reactions include interactions with potassium ferricyanide and potassium thiocyanate.

When \( \mathrm{Fe}^{3+} \) is introduced to potassium ferricyanide, a brown coloration is produced. This color change indicates the formation of complex species as a result of electron transfer or coordination bonding. This simple visual cue serves as a reliable indicator for detecting the presence of \( \mathrm{Fe}^{3+} \) ions.

Another critical reaction of ferric ions is with potassium thiocyanate \( (\mathrm{KSCN}) \). This reaction results in a blood-red complex, a distinctive change signaling the formation of \( \mathrm{Fe(SCN)}^{2+} \) ion complexes. This blood-red color is not only striking but also absolutely specific to \( \mathrm{Fe}^{3+} \) ions, making this test a specific indicator in complex formation studies. The vivid red coloration, therefore, is a diagnostic tool frequently used in both educational and practical laboratory settings for ferric ion detection.

Complex Formation with Potassium Ferricyanide

Potassium ferricyanide \( (\mathrm{K}_{3}[\mathrm{Fe} \mathrm{(CN)}_{6}]) \) is a pivotal compound used in the study of ionic reactions, especially when combined with metal ions like iron. The interaction of potassium ferricyanide with ferrous and ferric ions produces distinct results due to the complex formation mechanism they partake in.

• **With Ferrous Ions \( (\mathrm{Fe}^{2+}) \):** The reaction produces Turnbull's Blue, a classic precipitate, indicating the oxidation of \( \mathrm{Fe}^{2+} \) ions. This demonstrates electron transfer where \( \mathrm{Fe}^{2+} \) transitions to \( \mathrm{Fe}^{3+} \).
• **With Ferric Ions \( (\mathrm{Fe}^{3+}) \):** Instead of a precipitate, potassium ferricyanide interacts with \( \mathrm{Fe}^{3+} \) to produce a brown solution. This formation is due to complex interactions, rather than physical sedimentation, distinguishing the coloring from \( \mathrm{Fe}^{2+} \) reactions.

Understanding these reactions is vital for chemical studies focusing on transition metal chemistry and serves as an educational tool in illustrating fundamental concepts of coordination chemistry and electron transition processes.