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

Statements 2 and 3 are correct, so the answer is (c) 2,3.

Step 1: Understanding the Chemistry

Before solving the exercise, we should understand that ferrous ions (\(\mathrm{Fe}^{2+}\)) and ferric ions (\(\mathrm{Fe}^{3+}\)) react differently with reagents. Knowing the colours of these reactions can help us determine the correct statements.

Step 2: Analyzing Statement 1

Statement 1 says that \(\mathrm{Fe}^{3+}\) gives a brown colour with potassium ferricyanide, \(\mathrm{K}_3[\mathrm{Fe(CN)}_6]\). This is incorrect as potassium ferricyanide does not give a brown colour with \(\mathrm{Fe}^{3+}\).

Step 3: Analyzing Statement 2

Statement 2 claims \(\mathrm{Fe}^{2+}\) gives a blue precipitate with potassium ferricyanide. This is correct because \(\mathrm{Fe}^{2+}\) reacts with potassium ferricyanide to form Turnbull's blue.

Step 4: Analyzing Statement 3

Statement 3 suggests \(\mathrm{Fe}^{3+}\) gives a red colour with potassium thiocyanate, \(\mathrm{KSCN}\). This is correct because \(\mathrm{Fe}^{3+}\) forms a deep red complex with thiocyanate ions.

Step 5: Analyzing Statement 4

Statement 4 suggests \(\mathrm{Fe}^{2+}\) gives a brown colour with ammonium thiocyanate. This is incorrect; \(\mathrm{Fe}^{2+}\) does not form a brown colour with thiocyanates.

Key Concepts

Ferrous ions reactions

Ferrous ions, also known as (Fe^{2+} ions, have specific chemical behaviors that are distinct from ferric ions. A notable reaction involving ferrous ions is with potassium ferricyanide (K_3[Fe(CN)_6]). When ferrous ions encounter potassium ferricyanide, they form a compound known as Turnbull's blue. This distinctive blue precipitate is a result of a chemical change where the ferrocyanide complex absorbs specific wavelengths of light, reflecting blue.

• Turnbull's blue is an important indicator in qualitative analysis.
• The reaction is useful in detecting the presence of ferrous ions in a solution.

Understanding the behavior of Fe^{2+} ions in various reactions helps in identifying and differentiating them in a mixture of ions, which is pivotal in laboratory chemistry practices.

Ferric ions reactions

Ferric ions, denoted as Fe^{3+} ions, exhibit unique reaction characteristics compared to their ferrous counterparts. An important reaction to note is with potassium thiocyanate (KSCN). When Fe^{3+} ions react with thiocyanate, they form a blood-red complex. This reaction is characterized by a distinct deep red color, making it visually identifiable.

• The reaction with KSCN serves as a diagnostic tool for detecting ferric ions.
• It is commonly employed in laboratories for differentiating ferric ions from other metal ions.

Familiarity with these reactions aids in the identification and analysis of ferric ions, especially in qualitative chemical analysis settings.

Color reactions in coordination compounds

Color reactions in coordination chemistry are integral for understanding how metal ions interact with different ligands and reactants. These reactions not only provide visual insights but also clue into the subtle chemical processes happening at a molecular level. Specifically, the color changes are due to electronic transitions within the d-orbitals of the metal-ligand complexes.

• For instance, ferrous ions reacting with potassium ferricyanide result in Turnbull's blue, illustrating a bond between the iron and cyanide ligand.
• Similarly, ferric ions with potassium thiocyanate yield a red color, indicating a strong interaction and complex formation between the ion and the thiocyanate.

These color changes form a foundational part of coordination chemistry, allowing researchers and chemists to infer structural and bonding information simply from a shift in color, making it a valuable tool in spectroscopy and qualitative analysis.