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

The answer is option (b): 1,3,4.

Step 1: Understanding Copper Sulfate and Ammonium Hydroxide

When \ \(\mathrm{NH}_4\mathrm{OH}\) is added to a copper sulfate solution (\(\mathrm{CuSO}_4\)), a complex reaction occurs forming the compound Cu(NH₃)₄(H₂O)₂SO₄, which is also known as "tetraamminecopper(II) sulfate," resulting in a characteristic deep blue color.

Step 2: Reaction of Copper Sulfate with Potassium Ferrocyanide

When \ \(\mathrm{CuSO}_4\) reacts with potassium ferrocyanide (\(\mathrm{K_4[Fe(CN)_6]}\)), it forms copper ferrocyanide, a reddish-brown precipitate, not a blue color.

Step 3: Reaction of Ferric Chloride with Sodium Ferrocyanide

Ferric chloride (\(\mathrm{FeCl}_3\)) reacts with sodium ferrocyanide (\(\mathrm{Na_4[Fe(CN)_6]}\)) to form a blue complex known as "Prussian blue," specifically in conditions where the anion is able to form this particular complex.

Step 4: Dissolution of Anhydrous Copper Sulfate in Water

Anhydrous copper sulfate (\(\mathrm{CuSO}_4\)) initially appears white, but when it's dissolved in water, it forms copper sulfate pentahydrate, which is bright blue due to the hydration involving water molecules.

Step 5: Identifying the Correct Answers

Based on the analysis above: 1. Dissolving \ \(\mathrm{NH}_4\mathrm{OH}\) in \ \(\mathrm{CuSO}_4\) gives a blue color; hence option \(1\) contributes to a blue solution.2. Reacting \ \(\mathrm{CuSO}_4\) with potassium ferrocyanide does not give a blue solution; hence, option \(2\) is not applicable.3. Reacting \ \(\mathrm{FeCl}_3\) with sodium ferrocyanide gives a blue color (Prussian blue); thus, option \(3\) applies.4. Dissolving anhydrous \ \(\mathrm{CuSO}_4\) in water gives a blue solution; thus, option \(4\) is correct.Hence, options 1, 3, and 4 are blue. Answer is option (b): \(1,3,4\).

Key Concepts

Complex Formation in Chemistry

Complex formation in chemistry involves the interaction between molecules or ions to form a complex compound. These complexes are often associated with color changes because of the rearrangement of electrons, particularly in metal ion complexes. A well-known example is the interaction between copper ions and ammonia molecules. When ammonium hydroxide (\( \text{NH}_4\text{OH} \)), which releases ammonia \( (\text{NH}_3)\), is added to a copper sulfate solution (\(\text{CuSO}_4\)), a complex is formed: \[\text{Cu}(\text{NH}_3)_4(\text{H}_2\text{O})_2^{2+}\].
This complex, known as tetraamminecopper(II), is responsible for the deep blue color observed in the solution.

• Complexes occur when central metal ions bond with surrounding molecules or anions.
• Color changes stem from electron transitions influenced by the complex environment.

Copper Sulfate Reactions

Copper sulfate is a versatile compound that can undergo various reactions leading to distinct color changes. When dissolved in water, anhydrous copper sulfate (\(\text{CuSO}_4\)) transforms from a white solid to a bright blue solution, forming copper sulfate pentahydrate, \[\text{CuSO}_4\cdot5\text{H}_2\text{O}\].
This transformation is due to the hydration process involving water molecules, which also reaffirms copper sulfate's role as an effective drying agent when it is anhydrous.

• Hydration adds water molecules to copper sulfate, affecting its color.
• This reaction showcases copper sulfate's utility in both chemical and practical applications.

Prussian Blue Formation

Prussian blue is a deep blue pigment resulting from a chemical reaction between iron ions and ferrocyanide ions. This compound is often used in paints and dyes due to its stable and intense color.
When ferric chloride (\(\text{FeCl}_3\)) reacts with sodium ferrocyanide (\(\text{Na}_4[\text{Fe(CN)}_6]\)), Prussian blue is formed through a process where iron(III) ions (\(\text{Fe}^{3+}\)) interact with ferrocyanide ions:
\[\text{Fe}^{3+} + \text{Na}_4[\text{Fe(CN)}_6] \rightarrow \text{Fe}_4[\text{Fe(CN)}_6]_3\text{(Prussian Blue)}\]

• Prussian blue is a classic reaction outcome in chemical tests for iron.
• It shows the vivid colors that can result from chemical reactions, useful for visual identification.

Chemical Solution Reactions

Chemical solution reactions involve substances dissolving and reacting with each other at the molecular level to form new compounds. This often results in color changes that help in determining the nature of the reaction and the products formed.
For instance, when copper sulfate reacts with ammonia, the solution turns deep blue due to complex formation.
The color change signifies the development of the copper-ammonia complex.

• Color changes in solutions indicate specific chemical interactions and products.
• Understanding chemical reactions is a fundamental aspect of chemistry, helping to predict and explain outcomes.