Problem 102 - A Complete Resource Book in Chemistry for JEE Main

Question

A blue colouration is obtained in which case? (1) on dissolving \(\mathrm{NH}_{4} \mathrm{OH}\) in copper sulphate. (2) on reacting \(\mathrm{CuSO}_{4}\) solution with potassium ferrocyanide. (3) on reacting \(\mathrm{FeCl}_{3}\) with sodium ferrocyanide. (4) on dissolving anhydrous \(\mathrm{CuSO}_{4}\) in water. (a) 2,4 only (b) \(1,3,4\) (c) \(1,2,4\) (d) 3,4 only

Step-by-Step Solution

Answer

Correct answer: (b) 1, 3, 4.

Step 1: Analyze Option 1

Dissolving ammonium hydroxide \(\mathrm{NH}_{4} \mathrm{OH}\) in copper sulphate \(\mathrm{CuSO}_4\) produces a deep blue color. This is because ammonium hydroxide reacts with the copper ions to form a complex ion \([\mathrm{Cu(NH}_3)_4]^{2+}\), which is blue.

Step 2: Analyze Option 2

Reacting copper sulphate \(\mathrm{CuSO}_4\) with potassium ferrocyanide \(\mathrm{K}_4[\mathrm{Fe(CN)}_6]\) results in a dark brown precipitate of copper ferrocyanide \(\mathrm{Cu}_2[\mathrm{Fe(CN)}_6]\). This does not result in a blue solution, so this option does not produce blue color.

Step 3: Analyze Option 3

Reacting ferric chloride \(\mathrm{FeCl}_{3}\) with sodium ferrocyanide \(\mathrm{Na}_4[\mathrm{Fe(CN)}_6]\) typically results in a blue precipitate known as Prussian blue \(\mathrm{Fe}_4[\mathrm{Fe(CN)}_6]_3\), indicating this reaction can produce a blue color.

Step 4: Analyze Option 4

Dissolving anhydrous copper sulphate \(\mathrm{CuSO}_4\) in water results in a blue solution because hydrated copper(II) ions \(\mathrm{Cu}^{2+}\) are blue. Anhydrous copper sulphate itself is white, but when it is dissolved in water, it forms \(\mathrm{CuSO}_4\cdot5\mathrm{H}_2\mathrm{O}\), which is blue.

Step 5: Evaluate Answer Options

From our analysis, options 1, 3, and 4 contribute to blue color formation. Therefore, the correct combination matching one of the provided answer choices is (b) 1, 3, 4.

Key Concepts

Complex Ion Formation

Complex ion formation occurs when central metal atoms bind with surrounding molecules or ions, forming a complex assembly. These assemblies are typically charged, leading to complex ions.

Let's dive into how this works using copper as an example. When copper, commonly encountered in its +2 oxidation state ( Cu^{2+} ), interacts with certain substances, it can form complex ions. A typical case is in the reaction of ammonia with copper sulfate. Ammonia molecules ( NH_3 ) act as ligands, which are species that donate a pair of electrons to the central metal ion.

In this particular reaction, ammonia ligands surround the copper ion, forming a deep blue complex ion known as tetraamminecopper(II) ion, ( [Cu(NH_3)_4]^{2+} ). This complex ion formation is central to the resulting color change and is a hallmark of transition metal chemistry.

Transition Metal Complexes

Transition metal complexes are compounds formed when transition metals coordinate with atoms or groups of atoms called ligands. Transition metals are particularly adept at forming these due to their partially filled d-orbitals.

For instance, copper is a transition metal that often features prominently in complex ion formations. These complexes exhibit fascinating properties, most notably their vibrant colors, which result from electronic transitions in the d-orbitals.

The characteristics of the ligands and the metal at the core of the complex substantially affect the properties of the resulting complex. For example, when copper sulfate reacts with ammonia, the copper ion is surrounded by ammonia ligands, transforming its color to a vivid blue. Other ligands may produce different colors when coordinating with the same metal, showcasing the versatility and beauty of transition metal chemistry.

Color Change in Reactions

Many chemical reactions involving transition metals exhibit noticeable color changes due to the formation of new complexes that absorb and emit various wavelengths of light. This change is primarily due to the alterations in the energy levels of d-electrons within the metal.

During complex ion formation, ligands affect the energy split in the metal's d-orbitals. This split difference determines the color by influencing which wavelengths of light are absorbed and which are reflected. In the case of copper(II) sulfate reacting with ammonia, the resulting deep blue is a result of these intricate electron transitions.

• Blue color formation with ammonia in copper sulfate solutions is due to this exact science.
• Prussian blue, formed in reactions with iron, demonstrates how intricate the coloring of transition metal chemistry can be.

The visible shift in color serves as a visual verification of the chemical processes occurring within the reaction.

Copper Sulfate Reactions

Copper sulfate ( CuSO_4 ) is a versatile compound frequently used in reactions to explore fundamental concepts in inorganic chemistry.

In its anhydrous form, it is a white powder. However, when it absorbs water from its surroundings, it transforms into hydrated copper sulfate ( CuSO_4·5H_2O ), which is blue. This transition from white to blue is a classical demonstration of hydration and the structural impact water can have on a metal salt.

Copper sulfate's reactivity doesn't stop there. When dissolved in water or reacted with other elements like ammonia, it forms complex ions that represent broader principles in transition metal chemistry. These reactions not only produce interesting color changes but also illustrate the deep connections between structure, chemistry, and aesthetic in metal complexes.