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) C ext{Fe}^{3+}e gives a blood red color with KCNS.

Step 1: Understanding the Reaction

The question asks for an ion that gives a blood red color in solution when reacted with potassium thiocyanate (KCNS). This is a qualitative analysis test to identify certain metal ions based on color changes.

Step 2: Identifying the Responsible Ion

The blood red color in presence of KCNS is a well-known chemical test for the presence of iron(III) ions, C ext{Fe}^{3+}e. When C ext{Fe}^{3+}e reacts with thiocyanate ions C ext{SCN}^{-}e, a complex ion C ext{Fe(SCN)}^{2+}e is formed, which exhibits a deep red color.

Step 3: Confirming the Correct Option

Options (a) C ext{Al}^{3+}e, (b) C ext{Zn}^{2+}e, and (d) C ext{Cu}^{2+}e do not form a blood red colored complex with KCNS. Therefore, option (c) C ext{Fe}^{3+}e is the correct answer as it reacts with KCNS to give the blood red color.

Key Concepts

Fe(SCN)²⁺ complex formation

The formation of the Fe(SCN)²⁺ complex is an intriguing reaction that arises during the interaction of iron(III) ions with thiocyanate ions. This process is significant in qualitative analysis because it yields a distinct deep red color, allowing for straightforward identification of iron ions in solution.

As a brief introduction, when the iron(III) ions, denoted as \( \text{Fe}^{3+} \), encounter thiocyanate ions (\( \text{SCN}^- \)), they form a complex ion known as \( \text{Fe(SCN)}^{2+} \). This complex is responsible for the deep red coloration. The reaction can be represented as follows:

• \( \text{Fe}^{3+} + \text{SCN}^- \rightarrow \text{Fe(SCN)}^{2+} \)

The bold color change serves as both a visual cue and evidence of this specific complex's formation. The red hue is particularly useful in laboratory techniques for detecting the presence of iron(III) in a mixture, making it easier to analyze various samples.

Potassium thiocyanate reaction

Potassium thiocyanate (KCNS) plays a vital role in qualitative chemical analysis due to its reactions with certain metal ions. When KCNS is added to a solution containing iron(III) ions, it provides thiocyanate ions (\( \text{SCN}^- \)) that are central to forming a colored complex. This interaction is important as it turns colorless or slightly yellow solutions into a visually striking blood red color.

This color transformation happens because thiocyanate ions react with \( \text{Fe}^{3+} \) ions in the solution to form the \( \text{Fe(SCN)}^{2+} \) complex, which has a strong red hue. The reaction's specificity is instrumental in reliably testing for the presence of iron(III), distinguishing it from other metal ions that do not produce such distinct coloration. In fact, other metal ions like \( \text{Al}^{3+}, \text{Zn}^{2+}, \) or \( \text{Cu}^{2+} \) do not produce this red color with KCNS, highlighting the reaction's specificity.

Iron(III) ion identification

Identifying iron(III) ions in a solution is an essential part of various chemical analyses and educational experiments. The reaction involving iron(III) ions and potassium thiocyanate is one of the classical methods for achieving this. It is both simple and highly effective.

When you suspect the presence of \( \text{Fe}^{3+} \) ions in your solution, adding KCNS can quickly confirm it. The appearance of a blood red color indicates the formation of the \( \text{Fe(SCN)}^{2+} \) complex. This is a clear, immediate signal, making it advantageous for students and chemists who need a rapid and straightforward diagnostic technique.

Because the reaction reliably showcases its results through the vivid coloring, it is commonly included in instructional lab exercises to help students understand the principles of complex formation and ion identification in liquid solutions. This reaction serves as a powerful tool in both educational and professional settings for determining the presence of specific ions.