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

Option (c), statements 2 and 3 are correct.

Step 1: Understanding Ion Reactions

To determine which statements are correct, we need to understand how ferrous (\(\mathrm{Fe}^{2+}\)) and ferric (\(\mathrm{Fe}^{3+}\)) ions react with potassium ferricyanide and thiocyanate ions. Ferrous ions are known to react with potassium ferricyanide to form a blue precipitate. Ferric ions, on the other hand, react with potassium thiocyanate to give a red coloration.

Step 2: Analyzing Statement 1

Statement 1 claims that \(\mathrm{Fe}^{3+}\) gives a brown color with potassium ferricyanide. This is incorrect because \(\mathrm{Fe}^{3+}\) does not react with potassium ferricyanide to produce a brown color. Instead, potassium ferricyanide reacts with \(\mathrm{Fe}^{2+}\) to produce a blue precipitate.

Step 3: Analyzing Statement 2

Statement 2 states that \(\mathrm{Fe}^{2+}\) gives a blue precipitate with potassium ferricyanide. This is correct, as \(\mathrm{Fe}^{2+}\) reacts with potassium ferricyanide to produce a compound known as Turnbull's Blue.

Step 4: Analyzing Statement 3

Statement 3 mentions that \(\mathrm{Fe}^{3+}\) gives a red color with potassium thiocyanate. This is correct, because \(\mathrm{Fe}^{3+}\) forms a complex with thiocyanate ions, resulting in a blood-red color solution.

Step 5: Analyzing Statement 4

Statement 4 claims that \(\mathrm{Fe}^{2+}\) gives a brown color with ammonium thiocyanate. This is incorrect. \(\mathrm{Fe}^{2+}\) does not typically produce any significant color with thiocyanate; the brown color is more associated with aging or oxidation products of \(\mathrm{Fe}^{3+}\) solutions.

Step 6: Choosing the Correct Option

From the analysis, Statements 2 and 3 are correct. Thus, the correct option is (c), which includes the statements 2 and 3.

Key Concepts

Ferrous Ions

Ferrous ions are represented by the symbol \( \mathrm{Fe}^{2+} \). These are known as divalent iron ions meaning they have lost two electrons.
Ferrous ions are known for their characteristic reactivity with certain compounds. A noteworthy reaction involves potassium ferricyanide. When \( \mathrm{Fe}^{2+} \) ions encounter potassium ferricyanide, they form a striking blue precipitate known as Turnbull's Blue.
This blue precipitate helps in the qualitative analysis of ferrous ions in a solution. This property is often utilized in chemistry labs to detect the presence of \( \mathrm{Fe}^{2+} \) ions.

Ferric Ions

Ferric ions are designated as \( \mathrm{Fe}^{3+} \). They are trivalent iron ions, indicating the loss of three electrons.
Ferric ions exhibitions very distinct chemical behavior compared to their ferrous counterparts. When \( \mathrm{Fe}^{3+} \) ions are introduced to potassium thiocyanate, they form a complex that yields a blood-red coloration.
This reaction is highly specific and serves as a classic test for the presence of \( \mathrm{Fe}^{3+} \) ions in a solution. This distinctive red color is due to the complexation with thiocyanate ions, making it a useful tool in both educational settings and practical applications.

Potassium Ferricyanide

Potassium ferricyanide is a chemical compound with the formula \( \mathrm{K_3[Fe(CN)_6]} \). It is a coordination compound featuring iron in a +3 oxidation state, complexed with six cyanide ions.
When potassium ferricyanide interacts with ferrous ions \( ( \mathrm{Fe}^{2+} ) \), a deep blue precipitate known as Turnbull's Blue is formed. This reaction is specific and characteristic, differentiating \( \mathrm{Fe}^{2+} \) from \( \mathrm{Fe}^{3+} \) ions.

• Used in determining the presence of \( \mathrm{Fe}^{2+} \) ions.
• Not suitable for detecting \( \mathrm{Fe}^{3+} \) ions, as it does not yield significant reactions.

This reagent is highly valuable in chemical analysis and also used in the process of blueprinting.

Potassium Thiocyanate

Potassium thiocyanate, with the chemical formula \( \mathrm{KSCN} \), is a compound known for its ability to form colorful complexes with metal ions.
When it comes into contact with ferric ions \( ( \mathrm{Fe}^{3+} ) \), it forms a complex that exhibits a noticeable blood-red color.

• This is an analytical technique used to confirm the presence of \( \mathrm{Fe}^{3+} \) ions.
• Does not show a significant reaction with \( \mathrm{Fe}^{2+} \) ions, highlighting its specificity for ferric ions.

This reaction is not just a laboratory exercise; it is a practical method for identifying ferric ions in various chemical explorations.