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

Step 1: Understand the Reaction

When potassium thiocyanate (KCNS) is added to certain metal ions, it can form a complex that has a distinct color. The complex formed with \(\mathrm{Fe}^{3+}\\) ions is known to produce a blood red color.

Step 2: Identify the Ion Producing Blood Red Colour

Out of the options provided, \(\mathrm{Fe}^{3+}\\) ions react with KCNS to form \(\text{Fe(SCN)}^{2+}\\), a complex that exhibits a blood red color. This is a known qualitative test for iron(III) ions.

Step 3: Verify Other Options

Ensure none of the other ions produce a blood red color with KCNS. \(\mathrm{Al}^{3+}, \mathrm{Cu}^{2+},\text{ and } \mathrm{Zn}^{2+}\\) do not form a blood red complex with KCNS, confirming they are not correct options here.

Key Concepts

Colorimetric Analysis

Colorimetric analysis is a method used to determine the concentration of substances by analyzing the color of a solution. When certain chemicals react, they produce characteristic colors that can be measured and compared to a standard. This technique is highly useful in chemical laboratories for quick and semi-quantitative analysis.
It involves observing changes in color intensity which correlates to the concentration of the analyte. By comparing the color intensity of the sample to a set of standards, the concentration of the unknown sample can be deduced.
In the context of detecting iron(III) ions, colorimetric analysis offers a practical approach. Here, the distinct blood red color observed is a tell-tale sign of the presence of these ions when using potassium thiocyanate (KCNS). This makes qualitative tests simple and effective for identifying specific ions based on color change.

Complex Formation

Complex formation is a process where ions combine with other molecules or ions to form a new compound with unique properties. These are known as coordination complexes and are important in both industrial and laboratory chemistry.
When iron(III) ions (\(\mathrm{Fe}^{3+}\)) react with thiocyanate ions (\(\text{SCN}^{-}\)), they form iron thiocyanate (\(\text{Fe(SCN)}^{2+}\)). This is a classic example of a coordination complex.
These complexes often exhibit beautiful colors due to the arrangement of electrons within the metal ion. In this case, the formation of the \(\text{Fe(SCN)}^{2+}\) complex is responsible for the intense blood red color, which is a direct indicator of the presence of iron(III) ions in the solution.

Iron(III) Ions Detection

Detecting iron(III) ions is a common requirement in both educational settings and industrial applications. One of the most straightforward methods for detecting these ions is by their reaction with potassium thiocyanate, resulting in a blood red complex.
This qualitative test takes advantage of the tendency of iron(III) ions to form a colored complex with SCN- ions. Upon addition of KCNS to a solution containing iron(III) ions, the formation of \(\text{Fe(SCN)}^{2+}\) provides visual confirmation.
The reaction is not only useful for detecting iron but also demonstrates a key aspect of qualitative analysis: the identification of substances through their chemical interactions and resulting optical properties. It is important to note that other ions listed, such as \(\mathrm{Al}^{3+}, \mathrm{Cu}^{2+}, ext{ and } \mathrm{Zn}^{2+}\), do not form such a colored complex, making \(\mathrm{Fe}^{3+}\) a unique candidate in this test.