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) \(\mathrm{Cu}\) ion by the reduction of \(\mathrm{Cu}\)

Step-by-Step Solution

Answer

The solution turns blue due to the formation of \( \mathrm{Cu}^{2+} \) ions, making option (b) correct.

Step 1: Understanding the Reaction Context

When a copper wire is introduced into a silver nitrate \( \mathrm{AgNO}_3 \) solution, a chemical reaction takes place. The silver ions \( \mathrm{Ag}^+ \) in the solution react with the copper metal \( \mathrm{Cu} \), leading to the deposition of silver on the copper wire and formation of copper ions in solution.

Step 2: Identifying the Ions Formed

During the reaction between \( \mathrm{Cu} \) and \( \mathrm{Ag}^+ \) ions, copper donates two electrons to two silver ions, resulting in the production of \( \mathrm{Cu}^{2+} \) ions. This is evident from the half-reaction: \( \mathrm{Cu} \rightarrow \mathrm{Cu}^{2+} + 2 \mathrm{e}^- \). The presence of \( \mathrm{Cu}^{2+} \) ions gives the solution its characteristic blue color.

Step 3: Eliminating Other Options

Option (a) \( \mathrm{Cu}^+ \) ions, do not typically form as a stable product in this reaction under normal circumstances. Option (c) refers to a soluble complex of copper with \( \mathrm{AgNO}_3 \), which is unlikely because the blue color typically indicates free \( \mathrm{Cu}^{2+} \) ions. Option (d) suggesting reduction of \( \mathrm{Cu} \) to form \( \mathrm{Cu} \) ion is incorrect since \( \mathrm{Cu} \) is oxidized, not reduced.

Step 4: Choosing the Correct Answer

The correct option is (b), as the blue color in solution is due to the formation of \( \mathrm{Cu}^{2+} \) ions which are formed when copper is oxidized in the presence of silver nitrate.

Key Concepts

Oxidation-Reduction Reactions

When two substances interact in a chemical reaction, they can often exchange electrons, leading to changes in their oxidation states. This is what we refer to as an oxidation-reduction (redox) reaction. In the reaction between copper \( \mathrm{Cu} \) and silver nitrate \( \mathrm{AgNO}_3 \), copper is oxidized, which means it loses electrons. On the other hand, silver ions \( \mathrm{Ag}^+ \) are reduced, gaining the electrons that copper loses. This exchange is described by half-reactions: - **Oxidation Half-Reaction:** \( \mathrm{Cu} \rightarrow \mathrm{Cu}^{2+} + 2 \mathrm{e}^- \)- **Reduction Half-Reaction:** \( 2 \mathrm{Ag}^+ + 2 \mathrm{e}^- \rightarrow 2 \mathrm{Ag} \)Through these half-reactions, we see a clear path of electron flow. Copper gives up electrons, resulting in the formation of \( \mathrm{Cu}^{2+} \) ions, while silver accepts these electrons to convert into metallic silver. What's fascinating about this interaction is its natural tendency to happen when copper and silver nitrate are in contact, highlighting the principle of redox chemistry where a more reactive metal (copper in this case) displaces another (silver) from its compound.

Ion Formation

In chemistry, ions are atoms or molecules that carry a net electric charge, due to the loss or gain of electrons. In the reaction between copper \( \mathrm{Cu} \) and silver nitrate \( \mathrm{AgNO}_3 \), the formation of ions is central to the process. As copper loses electrons, it transforms from a neutral atom to a positively charged ion, \( \mathrm{Cu}^{2+} \). This process of losing electrons is known as oxidation. Copper, upon interaction with the silver ions \( \mathrm{Ag}^+ \) present in \( \mathrm{AgNO}_3 \), undergoes a change:- **Electron Loss:** From copper to \( \mathrm{Cu}^{2+} \), gaining a positive charge - **Resulting Ion:** \( \mathrm{Cu}^{2+} \) contributes to the blue color observed in the solutionSimultaneously, silver ions in the solution undergo a reduction, gaining electrons to form solid silver metal. This exchange explains why we see a deposition of metallic silver on the wire. Understanding ion formation helps us comprehend various chemical phenomena that rely on the transfer of electrons.

Chemical Reaction Analysis

Examining chemical reactions closely allows us to understand the changes in substance and energy. The reaction between copper \( \mathrm{Cu} \) and silver nitrate solution \( \mathrm{AgNO}_3 \) gives us a live example worthy of analysis. First, it's essential to identify the key players:

• **Copper \( \mathrm{Cu} \):** Initially in metallic form, oxidized to form \( \mathrm{Cu}^{2+} \)
• **Silver \( \mathrm{Ag}^+ \):** Initially ionic, reduced to form metallic silver

The solution's change to blue signals the formation of \( \mathrm{Cu}^{2+} \) ions, which occurs as a result of the oxidation of copper. By examining this reaction, we see foundational principles of chemistry at work:

• The balance between electron loss and gain
• The transformation from an ionic form to a metallic form and vice versa
• The role of oxidation-reduction as a driving force for chemical change

This analysis helps us understand what we observe on a macroscopic level—the color change, precipitate formation, and even the energy changes accompanying this reaction. Beyond explanation, it offers a gateway into predicting other reactions using these foundational concepts.