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 1, 2, and 3, so the answer is (c) 2,3.

Step 1: Examine Fe3+ with potassium ferricyanide

The reaction between \( \mathrm{Fe}^{3+} \) and potassium ferricyanide typically forms a brown coloration. This indicates that statement 1 is correct.

Step 2: Examine Fe2+ with potassium ferricyanide

The reaction between \( \mathrm{Fe}^{2+} \) and potassium ferricyanide forms a blue precipitate, which confirms that statement 2 is correct.

Step 3: Examine Fe3+ with potassium thiocyanate

\( \mathrm{Fe}^{3+} \) reacts with potassium thiocyanate to form a deep red color solution due to the formation of \( \mathrm{Fe(SCN)}^{2+} \). Thus, statement 3 is correct.

Step 4: Examine Fe2+ with ammonium thiocyanate

\( \mathrm{Fe}^{2+} \) does not produce a brown color with ammonium thiocyanate, making statement 4 incorrect.

Key Concepts

Fe2+ and Fe3+ Reactions

Inorganic chemistry often focuses on the behavior of metal ions like ferrous (\( \mathrm{Fe}^{2+} \)) and ferric (\( \mathrm{Fe}^{3+} \)) ions due to their widespread occurrence in nature and industrial processes. These ions are oxidation states of iron, with the ferrous ion being less oxidized than the ferric ion. These states differ in electron configuration, which dictates their distinct chemical properties and reactions. When it comes to reactions with different chemical agents, these ions exhibit diverse behaviors that are significant for both qualitative analysis and practical applications. Understanding how each ion reacts with various substances helps in identifying them and predicting their behavior in chemical processes.

Color Indications in Chemistry

Color changes in chemical reactions often serve as a visual cue for chemists to detect the presence of specific ions or compounds. These color indications can result from electronic transitions within the ion or complex formation. For example, between ferrous ions \( \mathrm{Fe}^{2+} \) and potassium ferricyanide (\( \mathrm{K}_3[\mathrm{Fe(CN)}_6] \)), a blue precipitate known as Turnbull's blue forms. Similarly, ferric ions \( \mathrm{Fe}^{3+} \) react to form brown or red hues depending on the reacting agent. Recognizing these visual indications simplifies the process of conducting chemical tests and experiments, allowing for quick, qualitative analyses without complex instruments. Thus, knowledge of these color changes is vital for students and practitioners in chemistry.

Chemical Ion Reactions

Chemical reactions involving ions such as \( \mathrm{Fe}^{2+} \) and \( \mathrm{Fe}^{3+} \) are primarily driven by electron interactions. When these ions react, they often form new compounds through processes such as precipitation, redox reactions or complexation. Precipitation occurs when a substance forms a solid from a solution, as in the case of \( \mathrm{Fe}^{2+} \) with potassium ferricyanide, resulting in a blue precipitate.

• Ferrous ions tend to undergo oxidation, converting to ferric ions while donating electrons.
• Ferric ions can also form complexes with other molecules, such as thiocyanate, providing a visual indication of their presence.

Understanding these reactions aids in harnessing their applications, such as in water treatment, metallurgy, and sensors.

Potassium Ferricyanide Reactions

Potassium ferricyanide (\( \mathrm{K}_3[\mathrm{Fe(CN)}_6] \)) is a complex compound commonly used in analytical chemistry. It serves as a powerful reagent due to its ability to form colorful complexes with iron ions. Its interaction with ferrous ions leads to the famous blue color or precipitate, providing a straightforward method for detecting \( \mathrm{Fe}^{2+} \). On the other hand, its reaction with ferric ions results in a brown coloring, a feature useful in various laboratory analyses.

• In the laboratory, potassium ferricyanide functions not only in analyses but also in pharmaceuticals and photography.
• It is non-toxic contrary to earlier beliefs, thus providing a safer alternative for chemical operations.

The knowledge of how potassium ferricyanide reacts with both \( \mathrm{Fe}^{2+} \) and \( \mathrm{Fe}^{3+} \) is essential for effective laboratory techniques and industrial processes.