Problem 142 - 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 statements are 2 and 3 (option c).

Step 1: Identify First Ion and Reaction

Let's start by examining the reaction of the \( \mathrm{Fe}^{3+} \) ion. The given option 1 states: \( \mathrm{Fe}^{3+} \) gives a brown color with potassium ferricyanide. In practice, \( \mathrm{Fe}^{3+} \) does not give a brown color with potassium ferricyanide; instead, \( \mathrm{Fe}^{3+} \) usually does not react significantly with this reagent. Hence, option 1 is incorrect.

Step 2: Analyze Second Ion Reaction

Now, examine option 2: \( \mathrm{Fe}^{2+} \) gives a blue precipitate with potassium ferricyanide. \( \mathrm{Fe}^{2+} \) does react with potassium ferricyanide to form Turnbull's blue, which is a blue precipitate. Thus, option 2 is correct.

Step 3: Assess Third Ion Reaction

Next, consider option 3: \( \mathrm{Fe}^{3+} \) gives a red color with potassium thiocyanate. \( \mathrm{Fe}^{3+} \) reacts with potassium thiocyanate to form a deep red complex called ferric thiocyanate. Therefore, option 3 is correct.

Step 4: Evaluate Fourth Ion Reaction

Finally, examine option 4: \( \mathrm{Fe}^{2+} \) gives a brown color with ammonium thiocyanate. Actually, \( \mathrm{Fe}^{2+} \) does not give a brown color with ammonium thiocyanate, and this reaction does not produce a visible color change. So, option 4 is incorrect.

Key Concepts

Ferrous and Ferric Ions

Ferrous (\(\mathrm{Fe}^{2+}\)) and ferric (\(\mathrm{Fe}^{3+}\)) ions are both forms of iron, differing in oxidation state. The oxidation state refers to the charge of the ion, showing how many electrons the atom has gained or lost compared to its neutral form. Ferrous is in a lower oxidation state compared to ferric.

• Ferrous: \(\mathrm{Fe}^{2+}\) – iron loses two electrons.
• Ferric: \(\mathrm{Fe}^{3+}\) – iron loses three electrons.

This change in electron count affects the way these ions react with other chemicals, resulting in different colors or precipitates in reactions. Understanding the difference is crucial for interpreting reactions, such as those involving potassium ferricyanide or thiocyanate.

Potassium Ferricyanide Reactions

Potassium ferricyanide (\(\mathrm{K_3[Fe(CN)_6]}\)) is a chemical used in reactions with iron ions to identify their presence through color changes or precipitate formation.

• Reaction with \(\mathrm{Fe}^{2+}\): It forms Turnbull's blue, a deep blue-colored precipitate. This occurs because potassium ferricyanide acts as an oxidizing agent, converting \(\mathrm{Fe}^{2+}\) to \(\mathrm{Fe}^{3+}\), which then combines with the ferricyanide ion to form the blue precipitate.
• Reaction with \(\mathrm{Fe}^{3+}\): There is generally no distinct color formation or reaction noted with ferric ions.

This test is often used to positively identify the presence of ferrous ions in a solution.

Potassium Thiocyanate Reactions

Potassium thiocyanate (\(\mathrm{KSCN}\)) is another reagent that reacts distinctly with iron ions. It is especially notable for its colorimetric reaction with ferric ions.

• Reaction with \(\mathrm{Fe}^{3+}\): It forms a deep red complex known as ferric thiocyanate. The red coloration is distinctive and serves as a positive test for \(\mathrm{Fe}^{3+}\) ions.
• Reaction with \(\mathrm{Fe}^{2+}\): There is typically no significant reaction or color change visible with ferrous ions, making it unsuitable for detecting \(\mathrm{Fe}^{2+}\) using this particular reagent.

The vivid color change to red is a straightforward indicator in chemical test identification processes.

Precipitate Formation

In chemistry, a precipitate is a solid formed from a solution during a chemical reaction. Precipitates are often products of reactions between dissolved ionic compounds.

• With potassium ferricyanide and \(\mathrm{Fe}^{2+}\): The reaction results in a blue precipitate known as Turnbull's blue.
• Such reactions are visually noticeable and indicate a change, confirming the presence of specific ions.

Observing precipitate formation is crucial in qualitative analysis as it provides evidence of specific ions or elemental presence in a solution. Each unique precipitate color can lead to insights about the chemical composition of the sample.

Chemical Test Identification

Chemical tests involving color changes or precipitate formations are fundamental techniques in analytical chemistry for identifying ion presence in solutions. Through these tests, specific ions, such as ferrous and ferric ions, can be distinguished based on their reaction products.

• \(\mathrm{Fe}^{2+}\) with potassium ferricyanide forms a blue precipitate.
• \(\mathrm{Fe}^{3+}\) with potassium thiocyanate results in a deep red complex.

The reliability of these tests makes them essential tools for chemists in various fields, from academic research to industrial applications.