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 reactions that produce a blue color are 1, 3, and 4. Option (b) is correct.

Step 1: Analyze Compound Reactions

First, examine the reactions to determine the products they form. 1. \[\text{NH}_4\text{OH} + \text{CuSO}_4 \rightarrow \text{Cu(OH)}_2 \text{(blue precipitate)} + \text{(NH)}_4\text{}_2\text{SO}_4\] This reaction forms a blue precipitate known as copper hydroxide. 2. \[\text{CuSO}_4 + \text{K}_4\text{Fe(CN)}_6 \rightarrow \text{Cu}_2[\text{Fe(CN)}_6] + \text{K}_2\text{SO}_4\] Copper ferrocyanide, \(Cu_2[Fe(CN)_6]\), forms, which is usually brown, not blue.3. \[\text{FeCl}_3 + \text{Na}_4\text{[Fe(CN)}_6] \rightarrow \text{Fe}_4[\text{Fe(CN)}_6]_3 + \text{NaCl}\] Prussian blue, \(Fe_4[Fe(CN)_6]_3\), a deep blue compound, is formed.4. \[\text{CuSO}_4 ext{(anhydrous)} + \text{H}_2\text{O} \rightarrow \text{CuSO}_4\cdot5\text{H}_2\text{O}\] Hydrated copper sulphate forms, which is blue.

Step 2: Determine Blue Coloration Cases

Based on the analysis from Step 1, identify which of the reactions produce blue compounds:- Reaction 1 (\(\text{NH}_4\text{OH} + \text{CuSO}_4\)): forms \(\text{Cu(OH)}_2\), blue precipitate.- Reaction 3 (\(\text{FeCl}_3 + \text{Na}_4\text{Fe(CN)}_6\)): forms Prussian blue.- Reaction 4 (anhydrous \(\text{CuSO}_4\) becoming hydrated): forms blue \(\text{CuSO}_4\cdot5\text{H}_2\text{O}\).

Key Concepts

Copper Reactions

Copper reactions are fundamental in chemistry. These reactions often involve changes in color, making them visually striking and significant in identifying compounds. For instance, copper sulfate (\( \text{CuSO}_4 \)), a common copper compound, is known for its vibrant blue color when hydrated. When copper sulfate is initially in its anhydrous form or dehydrated state, it appears white. However, upon exposure to water, it transforms into a blue crystal known as hydrated copper sulfate (\( \text{CuSO}_4 \cdot 5\text{H}_2\text{O} \)).
This transformation is a classic example of a reaction involving copper, emphasizing its color change as a key indicator. Similarly, reactions involving copper often lead to the formation of complex ions, which can drastically change in appearance and properties.
Understanding these reactions helps in various applications, from chemical analysis to art, where the distinct blue of copper compounds is often utilized for pigmentation.

Precipitate Formation

Precipitate formation is a crucial concept in chemistry that refers to the creation of a solid from a solution during a chemical reaction. This process occurs when the concentration of a compound exceeds its solubility in the solution, leading to its separation as a solid.
In the case of copper reactions, for instance, when ammonia (\( \text{NH}_4\text{OH} \)) reacts with copper sulfate (\( \text{CuSO}_4 \)), a blue precipitate known as copper hydroxide (\( \text{Cu(OH)}_2 \)) is formed. This reaction highlights how precipitates can signify a successful interaction between different chemical species.
Precipitation reactions are not just limited to color changes; they are essential in various industries. In water treatment, precipitation is used to remove contaminants, while in manufacturing, it's used to create materials with desired properties.

Complex Ion Reactions

Complex ion reactions involve the formation of stable structures known as complex ions, where a central metal ion is surrounded by molecules or ions called ligands. These reactions can result in striking color changes.
For example, when copper sulfate reacts with potassium ferrocyanide (\( \text{K}_4\text{[Fe(CN)}_6] \)), a compound called copper ferrocyanide is formed. Although this compound typically appears brown, it serves as an example of how complex ion reactions can alter the original characteristics of the reactants.

• Complex ions often alter the solubility and color of the solutions, allowing them to be easily identified in reactions.
• These reactions are instrumental in analytical chemistry, where they are used for qualitative and quantitative analysis of metal ions.

Understanding the principles behind complex ion reactions enables chemists to predict and manipulate outcomes in various chemical processes.

Coordination Chemistry

Coordination chemistry is the study of compounds with a central metal atom bonded to surrounding ligands through coordinate covalent bonds. This area of chemistry explains how compounds like Prussian blue are formed.
Prussian blue (\( \text{Fe}_4[\text{Fe(CN)}_6]_3 \)) is a well-known coordination compound that displays a deep blue color. It is produced when \( \text{FeCl}_3 \) reacts with sodium ferrocyanide (\( \text{Na}_4[\text{Fe(CN)}_6] \)). The resulting structure is stable and is utilized not just for its color but also in industrial applications, such as pigments for paints and inks.

• Coordination compounds can show unique magnetic and electronic characteristics.
• They are pivotal in biological systems, where metals like iron play essential roles in biochemistry.

The study of coordination chemistry is vital for understanding reactions and compounds in both synthetic and natural environments.