Problem 59 - A Complete Resource Book in Chemistry for JEE Main

Question

When a copper wire is placed in a solution of \(\mathrm{AgNO}_{3}\), the solution acquires blue colour. This is due to the formation of (a) \(\mathrm{Cu}^{+}\)ions (b) \(\mathrm{Cu}^{2+}\) ions (c) soluble complex of copper with \(\mathrm{AgNO}_{3}\) (d) Cu ion by the reduction of \(\mathrm{Cu}\)

Step-by-Step Solution

Answer

The solution acquires a blue color due to the formation of \( \mathrm{Cu}^{2+} \) ions, which corresponds to option (b).

Step 1: Identify the Chemical Reaction

When copper is placed in a solution of silver nitrate (6 ext{AgNO}_3 ight), a displacement reaction occurs where copper displaces silver from its compound, forming copper(II) ions (6 ext{Cu}^{2+} ight) and silver metal. The reaction is: \[ ext{Cu} + 2 ext{AgNO}_3 ightarrow ext{Cu(NO}_3 ext{)}_2 + 2 ext{Ag} \]...

Step 2: Determine the Observation Cause

As a result of the reaction, copper(II) ions (6 ext{Cu}^{2+} ight) are formed, which are blue in color. These ions give the solution a blue color, which indicates the presence of 6 ext{Cu}^{2+} ight) ions in the solution.

Step 3: Select the Correct Option

Based on the chemical reaction, copper forms 6 ext{Cu}^{2+} ight) ions in the solution. Therefore, the blue color in the solution is due to the formation of 6 ext{Cu}^{2+} ight) ions, making option (b) the correct answer.

Key Concepts

Copper and Silver Nitrate Reaction

When a piece of copper wire is introduced into a silver nitrate solution, an interesting chemical exchange takes place. This process is known as a displacement reaction. Here, copper (\(\text{Cu}\)) replaces silver (\(\text{Ag}\)) in the compound \(\text{AgNO}_3\), leading to the formation of copper nitrate (\(\text{Cu(NO}_3)_2\)) and metallic silver. This can be represented by the chemical equation:

• \[ \text{Cu} + 2\text{AgNO}_3 \rightarrow \text{Cu(NO}_3)_2 + 2\text{Ag} \]

The copper atoms lose electrons during this reaction, becoming copper ions while silver ions are reduced to form solid silver metal. The finished reaction results in a visible deposition of silver on the copper surface. The clear chemical transformation marks this as a typical example of a displacement reaction.

Copper II Ions Formation

During the displacement reaction, copper metal is converted into copper(II) ions (\(\text{Cu}^{2+}\)). This happens as the copper atoms lose two electrons each, resulting in positively charged \(\text{Cu}^{2+}\) ions. This can be represented as follows:

• \[ \text{Cu} \rightarrow \text{Cu}^{2+} + 2e^- \]

The formation of copper(II) ions is crucial as it changes the chemical makeup of the solution. Instead of having colorless silver ions, the solution now contains blue copper(II) ions, which imbue the solution with a distinct and vibrant blue color.
This change not only signifies the formation of \(\text{Cu}^{2+}\) ions but also confirms that the reaction has taken place. These ions actively engage in forming copper nitrate (\(\text{Cu(NO}_3)_2\)), solidifying the chemical transformation.

Chemical Reaction Observations

After the copper wire is added to the silver nitrate solution, several key observations can be noted which help in understanding the chemical process:

• Blue Color: The most noticeable change is the blue tint in the solution. This is due to the presence of copper(II) ions (\(\text{Cu}^{2+}\)) that are formed during the reaction.
• Silver Deposit: Silver metal (\(\text{Ag}\)) begins to form and adheres to the copper wire, appearing as a shiny grayish layer.
• Fading of Original Solution: The initial colorlessness of the silver nitrate solution gradually fades into blue, indicating the progression of the reaction.

These observations demonstrate the displacement of silver ions by copper and confirm the chemical transformation within the solution. Both the visual cues and the process underline the principle of conservation of mass, as atoms are neither created nor destroyed, merely rearranged into new compounds.