Problem 34 - 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) \(\mathrm{Cu}^{2+}\) (c) \(\mathrm{Fe}^{3+}\) (d) \(\mathrm{Zn}^{2+}\)

Step-by-Step Solution

Answer

(c) Fe^{3+} forms a blood-red color with KCNS.

Step 1: Understanding the Reaction Involved

Potassium thiocyanate ( KCNS ) is used in a qualitative analysis to test for certain metal ions. It forms a distinct blood-red color with Fe^{3+} ions due to the formation of the Fe(SCN)^{2+} complex.

Step 2: Identifying the Ion That Reacts with KCNS

Among the given options, Fe^{3+} (ferric ions) reacts with KCNS to produce the blood-red colored Fe(SCN)^{2+} complex. Other ions do not produce this color or reaction.

Key Concepts

Potassium Thiocyanate Reaction

Potassium thiocyanate, denoted as KCNS, is a chemical reagent commonly used in qualitative chemical analysis, especially to detect the presence of certain metal ions. With its diverse reactivity, it helps in identifying ions by producing visually distinct results. One of the well-known reactions involving KCNS is with ferric ions (\(\mathrm{Fe}^{3+}\)). When potassium thiocyanate is added to a solution containing \(\mathrm{Fe}^{3+}\) ions, a blood-red complex forms. This color is a key indication of the presence of \(\mathrm{Fe}^{3+}\) ions.

The reaction between KCNS and ferric ions can be highlighted as follows:

• Potassium thiocyanate acts as a ligand, attaching itself to \(\mathrm{Fe}^{3+}\) ions.
• This forms the \(\text{Fe(SCN)}^{2+}\) complex, which is responsible for the blood-red coloration.
• The intensity of the red color can also give a rough indication of the concentration of \(\mathrm{Fe}^{3+}\) in the solution.

Understanding the KCNS reaction is essential in chemically analyzing and identifying specific metal ions based on their visual outputs.

Complex Formation

Complex formation is a fundamental process in chemistry where specific species, known as ligands, bind to a central metal ion to form a structured entity known as a complex. These reactions are crucial for understanding the behavior of transition metals and their compounds. In the case of the reaction between potassium thiocyanate and \(\mathrm{Fe}^{3+}\) ions, the ligand, thiocyanate, mixes with the \(\mathrm{Fe}^{3+}\) ion, resulting in the production of the colored complex \(\text{Fe(SCN)}^{2+}\).

Some important aspects of complex formation include:

• Ligands such as thiocyanate possess lone pairs of electrons, which they donate to the metal ion, thus forming a coordination bond.
• The number of ligands and the nature of the metal ion influence the properties of the complex, including its color and stability.
• Complexes, like \(\text{Fe(SCN)}^{2+}\), often have distinct colors due to electronic transitions between different energy levels of the metal ion, a phenomenon particularly observed in transition metals.

Studying these complexes helps in different fields, such as metallurgy, catalysis, and even biochemistry.

Transition Metal Ions

Transition metal ions are a group of elements found in the central block of the periodic table, known for their ability to form various stable and colorful compounds. These ions, which include elements such as iron, copper, and zinc, exhibit unique chemical properties attributed to their d-shell electrons. Some characteristics of transition metal ions include:

• They can exhibit multiple oxidation states, allowing for varied chemical reactivity.
• Many transition metal ions, like \(\mathrm{Fe}^{3+}\), readily form complexes with ligands, leading to diverse colorations and structural formations.
• These metal ions are integral in biological systems; for example, iron forms a part of hemoglobin, crucial for oxygen transport in the body.

Understanding transition metal ions and their interactions is vital for applications ranging from industrial processes to biological macromolecules. The fascinating colors that arise from these metals, such as the blood-red color of \(\text{Fe(SCN)}^{2+}\), are not just visually striking but also provide insights into their electronic structures and bonding nature.