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

Statement (b) is correct: 8Fe^{3+}9 gives brown color with potassium ferricyanide.

Step 1: Understanding the Reaction with Potassium Ferrocyanide

When potassium ferrocyanide ( ext{K}_4[ ext{Fe(CN)}_6]) is mixed with ext{Fe}^{3+}, a blue color is observed due to the formation of Prussian blue, ext{Fe}_4[ ext{Fe(CN)}_6]_3.

Step 2: Reaction of Fe3+ with Potassium Ferricyanide

When ext{Fe}^{3+} is mixed with potassium ferricyanide ( ext{K}_3[ ext{Fe(CN)}_6]), it results in a brown color precipitate or solution, depending on the conditions, due to the formation of complex iron(III) compounds.

Step 3: Reaction of Fe2+ with Ammonium Thiocyanate

ext{Fe}^{2+} shows minimal color change with ammonium thiocyanate, ext{NH}_4 ext{SCN}. The typical color, if any reaction occurs, is not brown, and any result is weak in color compared to the iron(III) thiocyanate reaction.

Step 4: Reaction of Fe2+ with Potassium Ferricyanide

When ext{Fe}^{2+} reacts with potassium ferricyanide ( ext{K}_3[ ext{Fe(CN)}_6]), it forms Turnball's blue or a similar complex, which has a blue coloration.

Key Concepts

Iron Complexes

Iron complexes are fascinating molecules formed when iron ions, such as \(\mathrm{Fe}^{2+}\) or \(\mathrm{Fe}^{3+}\), coordinate with various ligands. These ligands can be neutral molecules or ions that donate electrons to the metal, stabilizing it and giving it unique properties. In coordination chemistry, iron is known for its ability to exist in multiple oxidation states, commonly as \(\mathrm{Fe}^{2+}\) (ferrous) and \(\mathrm{Fe}^{3+}\) (ferric).

• The coordination number, which is the number of ligand donor atoms attached to the central metal, often influences the structure and reactivity of the complex.
• Typical ligands for iron include cyanides or thiocyanates, which can significantly affect the color and stability of the resulting complex.
• The charge of the complex, its geometry, and the electronic configuration of the metal influence the physical and chemical properties of the iron complex.

Iron complexes are pivotal in biological systems, industrial applications, and chemical reactions, showing richly varied colors and responses.

Colorimetric Reactions

Colorimetric reactions involve chemical processes that result in a color change. These reactions are particularly instrumental in identifying and analyzing specific ions in a solution, such as those involving iron.
Colorimetric analysis is a remarkably simple yet effective tool for chemists because:

• It allows for quick identification based on visible changes without sophisticated equipment.
• The intensity and nature of the color change can provide quantitative information about the concentration of the ions involved.
• It is widely used in educational settings, industrial labs, and even medical fields.

For instance, when \(\mathrm{Fe}^{3+}\) ions react with potassium ferricyanide, a brown color emerges. The nature of these color changes is highly dependent on the ligands involved and the oxidation state of the iron.

Transition Metal Chemistry

Transition metals like iron belong to a unique group in the periodic table, where they exhibit varied oxidation states and form attractive complexes due to their ability to use different electron configurations.
Several distinct features define transition metal chemistry:

• These metals can form complexes with diverse geometries, often due to their unfilled d-orbitals.
• Transition metals exhibit unique electronic transitions that can result in intense colors, such as the blue color of Turnbull's blue in iron complexes.
• They often have catalytic properties and can participate in redox reactions, being suitable for a variety of chemical transformations.

Iron, as a transition metal, is particularly notable for being a part of hemoglobin in blood, providing its red color through complex formations with oxygen.