Problem 93 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statement is correct? (a) \(\mathrm{Fe}^{3+}\) gives red colour with potassium ferrocyanide (b) \(\mathrm{Fe}^{3+}\) gives brown colour with potassium ferricyanide (c) \(\mathrm{Fe}^{2+}\) gives brown colour with ammonium thiocyanate (d) \(\mathrm{Fe}^{2+}\) gives blue colour with potassium ferricyanide

Step-by-Step Solution

Answer

Statements (b) and (d) are correct.

Step 1: Understanding the Reactants

We need to look at each reaction carefully to determine the correct color that forms when each ion is reacted with the given reagents. We'll evaluate each statement one by one by considering which ion gives which color with which reagent.

Step 2: Evaluating Option (a)

In option (a), \(\mathrm{Fe}^{3+}\) ion is known to produce a blue color when reacted with potassium ferrocyanide (\(\text{K}_4[ ext{Fe(CN)}_6]\)), not red. This makes option (a) incorrect, as it suggests a red color is formed.

Step 3: Evaluating Option (b)

In option (b), \(\mathrm{Fe}^{3+}\) ion does form a brown precipitate when reacted with potassium ferricyanide (\(\text{K}_3[ ext{Fe(CN)}_6]\)). This statement correctly describes the reaction and the color produced.

Step 4: Evaluating Option (c)

In option (c), \(\mathrm{Fe}^{2+}\) ion actually produces a blood-red color when reacted with ammonium thiocyanate (\(\text{NH}_4 ext{SCN}\)). The statement mentions brown color, which is incorrect.

Step 5: Evaluating Option (d)

In option (d), \(\mathrm{Fe}^{2+}\) ion forms a deep blue color when it reacts with potassium ferricyanide (\(\text{K}_3[ ext{Fe(CN)}_6]\)). The statement correctly describes the color produced in this reaction.

Step 6: Verification of Color Reactions

After evaluating each option, we conclude that both option (b) and option (d) correctly describe the reactions of \(\mathrm{Fe}^{3+}\) with potassium ferricyanide and \(\mathrm{Fe}^{2+}\) with potassium ferricyanide, respectively.

Key Concepts

Potassium Ferrocyanide Reactions

Potassium ferrocyanide, (\[\text{K}_4\text{[Fe(CN)}_6]\]), is a compound that forms distinctive colors when it reacts with iron ions. In the realm of chemical reactions, this substance plays a crucial role as a reagent for identifying iron ions. When \(\mathrm{Fe}^{2+}\) binds with potassium ferrocyanide, a dramatic change occurs. It forms a classic blue precipitate known as Prussian blue. This precipitate is intense in color and indicates the presence of \(\mathrm{Fe}^{2+}\) ions. It's essential to remember that potassium ferrocyanide does not produce a red color with \(\mathrm{Fe}^{3+}\) ions; instead, the reaction with \(\mathrm{Fe}^{3+}\) can subtly impact colors depending on the conditions. To properly analyze reactions, consider the specific ions and the conditions under which they are reacting. This ensures the correct identification of materials based on reaction colors.

• The reaction with \(\mathrm{Fe}^{2+}\) forms Prussian blue.
• It diverges with \(\mathrm{Fe}^{3+}\), and no intense color is typical.

Potassium Ferricyanide Reactions

Potassium ferricyanide, (\[\text{K}_3\text{[Fe(CN)}_6]\]), interacts differently with \(\mathrm{Fe}^{2+}\) and \(\mathrm{Fe}^{3+}\) ions compared to its ferrocyanide counterpart. When \(\mathrm{Fe}^{2+}\) is added to potassium ferricyanide, it creates a deep blue solution known as Turnbull's blue. This reaction is characteristically used to detect the presence of \(\mathrm{Fe}^{2+}\) ions, owing to its vibrant blue hue. Unlike ferrocyanide, with \(\mathrm{Fe}^{3+}\) ions, potassium ferricyanide forms a brown precipitate. This brownish color is typical for the reaction with ferric ions and serves as an effective distinguishing factor in qualitative analysis of these ions.

• \(\mathrm{Fe}^{2+}\) yields Turnbull's blue.
• \(\mathrm{Fe}^{3+}\) leads to a brown precipitate.

Ammonium Thiocyanate Reaction

Ammonium thiocyanate, (\[\text{NH}_4\text{SCN}\]), is known for its reaction with iron, especially \(\mathrm{Fe}^{3+}\) ions. In this reaction, \(\mathrm{Fe}^{3+}\) reacts with thiocyanate ions to produce a blood-red colored complex. This red color is quite distinctive and is a reliable indicator of ferric ions in a solution. On the other hand, \(\mathrm{Fe}^{2+}\) ions do not form this red complex with ammonium thiocyanate, so if a red color appears after adding thiocyanate, \(\mathrm{Fe}^{3+}\) ions are present. This specific reaction is often employed in the laboratory during analytical chemistry procedures to ascertain the presence of ferric ions.

• \(\mathrm{Fe}^{3+}\) results in a blood-red complex.
• No significant reaction color is seen with \(\mathrm{Fe}^{2+}\).