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

Step 1: Identify the reaction with KCNS

Potassium thiocyanate (KCNS) is known to form a deep blood red colored complex when it reacts with certain metal ions. One well-known example is its reaction with iron(III) ions,  ext{Fe}^{3+}, to form the compound [ ext{Fe}(SCN)]^{2+}.

Step 2: Determine which ion forms the red complex

Among the given options, we need to check which metal ion is capable of forming this blood red complex with  ext{KCNS}. This is typically due to the formation of [ ext{Fe}(SCN)]^{2+} when  ext{Fe}^{3+} ions react with  ext{KCNS}.

Step 3: Conclusion on red colored complex formation

Considering the chemical behavior of the ions listed, of  ext{Al}^{3+},  ext{Cu}^{2+},  ext{Fe}^{3+}, and  ext{Zn}^{2+}, only the iron(III) ion,  ext{Fe}^{3+}, is known to react with thiocyanate ions,  ext{SCN}^{-}, to give a deep blood red solution.

Key Concepts

Coordination Compounds

Coordination compounds form when central metal atoms or ions bind to surrounding molecules or ions, known as ligands, through coordinate covalent bonds. These compounds are also known as complex ions. The structure of coordination compounds can be quite intricate due to the coordination number, which denotes the number of ligand bonds to the central atom. In the context of the discussed exercise, [Fe\(\mathrm{(SCN)}\)]^{2+} is a coordination compound where Fe^{3+} is the central metal ion and SCN^{-} is the ligand.

Coordination compounds have diverse applications in various fields, one of which is in colorimetric analysis. They exhibit vibrant colors, such as the blood-red hue observed through the interaction of Fe^{3+} with SCN^{-}. This specific bonding arrangement is responsible for the characteristic color of the complex, which arises from electronic transitions within the compound. As electrons absorb light, they move to higher energy levels, and the corresponding release of energy produces visible color.

These features make coordination compounds not only interesting to study but also incredibly useful for the identification of substances and in catalytic processes.

Transition Metals

Transition metals are elements found in the central block of the periodic table and are known for their ability to form various oxidation states and complexes. They include familiar metals such as iron, copper, and zinc. The transition metals are characterized by the presence of d-electrons, which allow the formation of colorful compounds and multiple oxidation states.

The reaction of Fe^{3+} with KCNS, which yields a blood-red complex, is an excellent example of this characteristic behavior. Transition metals like iron ( Fe) can easily lose or gain electrons, which enables complex formation. As seen with Fe^{3+}, when combined with SCN^{-} ligands, the d-orbital's vacant spaces are filled, resulting in a color change. This property is extensively exploited in analytical chemistry to indicate the presence of specific metals through colorimetric tests.

Similarly, other transition metals, such as copper and zinc, also form complexes, but not all combinations produce visible color changes. The unique electronic configuration in transition metals makes these reactions quite versatile and variable.

Colorimetric Analysis

Colorimetric analysis is a method used in chemistry to determine the concentration of colored compounds within a solution. This technique relies on the principle that the amount of light absorbed is proportional to the concentration of the compound present. It is commonly applied in laboratories for quantitative analysis and quality control.

The basis of colorimetric analysis often involves coordination compounds, as these tend to exhibit strong and distinct colors. In the exercise, the formation of the blood-red color from the reaction of Fe^{3+} and SCN^{-} is a practical example. Through such reactions, one can infer the presence and often the amount of iron(III) in a solution. By measuring the intensity of the red color using spectrophotometry, chemists can determine the concentration of the complex ions.

This kind of analysis is crucial in areas such as water quality testing, clinical tests, and industrial processes, making it an invaluable tool in both qualitative and quantitative procedures. Its ability to provide clear visual results allows easy interpretation and is a testament to the utility of coordination chemistry combined with technology.