Problem 20 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following gives blood red colour with KCNS? (a) \(\mathrm{Al}^{3+}\) (b) \(\mathrm{Zn}^{2+}\) (c) \(\mathrm{Fe}^{3+}\) (d) \(\mathrm{Cu}^{2+}\)

Step-by-Step Solution

Answer

(c) \(\mathrm{Fe}^{3+}\)

Step 1: Introduction to KCNS Reaction

KCNS (potassium thiocyanate) is a compound used in chemistry to produce a blood red complex in the presence of certain metal ions. When certain metal ions are added to a solution of KCNS, they form a colored complex indicating the presence of that ion.

Step 2: Analyzing the Options

Let's examine each option to see which one forms a blood red color when reacted with KCNS: - Option (a) \(\mathrm{Al}^{3+}\) does not react with KCNS to form a colored complex. - Option (b) \(\mathrm{Zn}^{2+}\) also does not produce a colored complex with KCNS. - Option (c) \(\mathrm{Fe}^{3+}\) reacts with KCNS to form a blood red complex. - Option (d) \(\mathrm{Cu}^{2+}\) does not produce a red complex with KCNS.

Step 3: Identifying the Correct Answer

From the analysis, \(\mathrm{Fe}^{3+}\) is the ion that reacts with KCNS to give the characteristic blood red color. This is due to the formation of the blood red complex \([\mathrm{Fe(SCN)]^{2+}}\).

Key Concepts

Potassium Thiocyanate Reaction

Potassium thiocyanate, often abbreviated as KCNS, is widely utilized in inorganic chemistry for its ability to form colored complexes with transition metal ions. These complexes arise because the thiocyanate ion ( ext{SCN}^-), part of KCNS, acts as a ligand. A ligand is a molecule that can donate a pair of electrons to a metal ion, forming a coordination bond.
In practice, when you mix KCNS with an appropriate metal ion in a solution, it's like adding a detective tool to uncover which ions are present. This is because certain reactions lead to distinct color changes. However, not all metal ions respond, which means only specific combinations will form complexes that exhibit characteristic colors.
It's essential to understand that the nature of the metal ion and the environment, like the solvent or the pH, can impact whether a colored complex will form. With KCNS, we primarily look for complexes with transition metals, which often show vibrant colors due to the electronic transitions that occur within the metal's electron orbitals.

Metal Ion Complexes

Metal ion complexes, also known as coordination compounds, play a significant role in many chemical processes. These compounds consist of a central metal ion bonded to surrounding molecules or ions known as ligands, forming a coordination sphere.
The number of ligands attached to the metal ion is known as the coordination number. The geometry of these complexes, such as octahedral, tetrahedral, or linear, can vary greatly depending on the metal ion and the type of ligands.

• Formation: Ligands donate a pair of electrons to the metal ion, establishing a coordinate covalent bond.
• Properties: Complexes often have unique colors, magnetic properties, and reactivity based on their metal ions.
• Applications: They are crucial in areas ranging from catalysis and biological processes to materials science and medicine.

In the context of KCNS reactions, understanding metal ion complexes helps explain the formation of colored complexes, such as the blood red complex formed between \( ext{Fe}^{3+}\) ions and thiocyanate ions.

Iron(III) Thiocyanate Complex

The reaction of \( ext{Fe}^{3+}\) ions with potassium thiocyanate is frequently highlighted in chemical studies because it provides a vivid example of how metal ion complexes can be identified through color changes.
When \( ext{Fe}^{3+}\) ions encounter thiocyanate ions, they form a complex represented by \[ ext{Fe(SCN)}^{2+} \] that exhibits a deep blood red color. The nature of this color is due to an electronic transition known as charge transfer, which occurs when an electron moves between the metal and the ligand.
This specific iron thiocyanate reaction is not just a classroom demonstration but also has practical applications. It's used in qualitative chemical analysis to confirm the presence of iron ions, and in industrial settings, it's involved in various processes that require the detection and measurement of iron content.
Moreover, this complex's formation can be influenced by several factors, such as concentration, pH, and the presence of other ions, which can shift the equilibrium of the reaction and alter the intensity of the color. Understanding these nuances is vital for interpreting results accurately in both educational and practical chemical analyses scenarios.