Problem 114 - A Complete Resource Book in Chemistry for JEE Main

Question

When \(\mathrm{Zn}\) metal is added to \(\mathrm{CuSO}_{4}\) solution, \(\mathrm{Cu}\) is precipitated it is due to (a) ionization of \(\mathrm{CuSO}_{4}\) (b) hydrolysis of \(\mathrm{CuSO}_{4}\) (c) reduction of \(\mathrm{Cu}^{2+}\) (d) oxidation of \(\mathrm{Cu}^{2+}\)

Step-by-Step Solution

Answer

Option (c) reduction of \( \mathrm{Cu}^{2+} \).

Step 1: Identify the Reaction Components

Recognize the components involved in the reaction. Zinc (\( \mathrm{Zn} \)) is a more reactive metal, and it is being added to a solution of copper sulfate (\( \mathrm{CuSO}_4 \)). Copper sulfate dissociates into copper ions (\( \mathrm{Cu}^{2+} \)) and sulfate ions (\( \mathrm{SO}_4^{2-} \)) in solution.

Step 2: Determine Type of Reaction

Understand that \( \mathrm{Zn} \) can displace \( \mathrm{Cu}^{2+} \) ions in the solution since it is more reactive. This is a redox reaction where \( \mathrm{Zn} \) loses electrons (is oxidized) and \( \mathrm{Cu}^{2+} \) gains electrons (is reduced).

Step 3: Identify the Reduction Process

Focus on the reduction part of the redox reaction. \( \mathrm{Cu}^{2+} \) ions in solution gain electrons from \( \mathrm{Zn} \) to form solid copper (\( \mathrm{Cu} \)). This process of gaining electrons corresponds to the reduction of \( \mathrm{Cu}^{2+} \) ions.

Step 4: Choose the Correct Option

Based on the analysis, the correct process responsible for the precipitation of copper metal is the reduction of \( \mathrm{Cu}^{2+} \). Therefore, the correct option is (c) reduction of \( \mathrm{Cu}^{2+} \).

Key Concepts

Zinc-Copper Reaction

The Zinc-Copper reaction is a classic example of a redox reaction, which involves the transfer of electrons between substances. In this type of reaction, zinc metal is added to a solution containing copper sulfate. Copper sulfate dissociates into copper ions (\( \mathrm{Cu}^{2+} \)) and sulfate ions when in solution. What makes this reaction special is the fact that zinc, a more reactive metal, effectively "pushes out" the copper from the solution. This occurs because zinc is capable of donating its electrons to copper ions more efficiently. This exchange of electrons is key in understanding how zinc ends up as zinc ions in solution, while copper, initially part of the solution as copper ions, is deposited as solid copper metal.

Metal Displacement

Metal displacement reactions occur when a more reactive metal displaces a less reactive metal from a compound. In our zinc-copper reaction,\( \mathrm{Zn} \) is more reactive than\( \mathrm{Cu} \), making it capable of displacing copper ions from the solution. When zinc atoms come in contact with copper ions in the solution, they help convert these ions to solid copper. This transformation happens because zinc atoms lose electrons that are then gained by the copper ions. The general idea is that a metal higher in the reactivity series can replace a metal lower in the series, resulting in a displacement, often followed by the metal initially in solution precipitating out as a solid. This displacement showcases zinc's higher reactivity, which plays a crucial role in determining the outcomes of such chemical reactions.

Electron Transfer

At the heart of the zinc-copper reaction is the electron transfer mechanism. This is where the real magic of redox reactions happens! When zinc is added to the \( \mathrm{CuSO}_4 \) solution, zinc atoms donate two electrons each, which are then accepted by copper ions ( \( \mathrm{Cu}^{2+} \)). This donation of electrons by zinc converts it into\( \mathrm{Zn}^{2+} \) ions, signifying oxidation. On the other side, copper ions receiving the electrons are undergoing reduction as they transform into solid copper.

• The electron transfer can be visualized with the half-reactions:
• Oxidation: \( \mathrm{Zn} \rightarrow \mathrm{Zn}^{2+} + 2e^- \)
• Reduction: \( \mathrm{Cu}^{2+} + 2e^- \rightarrow \mathrm{Cu} \)

This electron exchange is crucial in defining the redox nature of the reaction. Without this smooth flow of electrons from zinc to copper ions, the reaction—producing visible solid copper—would not occur. Redox reactions, therefore, not only signify a change in the elemental state but also serve to balance out the charges through this fascinating process of electron transfer.