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

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

Step 1: Identify the chemical reaction

When zinc metal (\(\mathrm{Zn}\)) is added to copper (II) sulfate solution (\(\mathrm{CuSO}_4\)), a single displacement reaction occurs. The equation for the reaction is:\[ \mathrm{Zn} + \mathrm{CuSO}_4 \rightarrow \mathrm{ZnSO}_4 + \mathrm{Cu} \]This indicates that \(\mathrm{Cu}^{2+}\) ions in the solution are being replaced by \(\mathrm{Zn}^{2+}\) ions.

Step 2: Analyze the process occurring with copper ions

In the reaction, copper ions (\(\mathrm{Cu}^{2+}\)) in the solution gain electrons to form solid copper (\(\mathrm{Cu}\)). The reaction can be broken down into two half-reactions:- Reduction: \(\mathrm{Cu}^{2+} + 2e^- \rightarrow \mathrm{Cu}\)- Oxidation: \(\mathrm{Zn} \rightarrow \mathrm{Zn}^{2+} + 2e^-\)Thus, \(\mathrm{Cu}^{2+}\) is reduced to copper metal.

Step 3: Select the correct answer based on the analysis

From the half-reactions, it is evident that the key process responsible for the precipitation of copper is the reduction of \(\mathrm{Cu}^{2+}\) ions to copper (\(\mathrm{Cu}\)). This matches option (c): reduction of \(\mathrm{Cu}^{2+}\).

Key Concepts

Single Displacement Reaction

A single displacement reaction is a type of chemical reaction where one element in a compound is replaced by another element. In this reaction, an element that is more reactive replaces a less reactive element in the compound, leading to the displacement.
In the example provided, zinc (\(\mathrm{Zn}\)) displaces copper from copper sulfate (\(\mathrm{CuSO}_{4}\)) solution. The reaction can be represented as:

• \[\mathrm{Zn} + \mathrm{CuSO}_4 \rightarrow \mathrm{ZnSO}_4 + \mathrm{Cu}\]

Zinc (\(\mathrm{Zn}\)) is more reactive than copper, which is why it can replace copper (\(\mathrm{Cu}^{2+}\)) from the copper sulfate. This is a characteristic feature of single displacement reactions, and it is often used to understand the reactivity series of metals.

Oxidation and Reduction

In any reaction, if one substance is oxidized, another is reduced. Oxidation involves the loss of electrons, while reduction is the gain of electrons. It's important to remember that these processes always occur simultaneously.
Using the zinc and copper sulfate reaction as an example:

• Oxidation: Zinc (\(\mathrm{Zn}\)) loses electrons to form zinc ions (\(\mathrm{Zn}^{2+}\)).
• Reduction: Copper ions (\(\mathrm{Cu}^{2+}\)) gain electrons to form solid copper (\(\mathrm{Cu}\)).

Thus, in the reaction, zinc is oxidized, and copper is reduced. This exchange of electrons is fundamental to many processes in chemistry, including reactions in batteries and metabolic processes in living organisms.

Half-Reactions

Half-reactions are a way of breaking down a redox reaction into its two components: oxidation and reduction. By separating these components, it becomes easier to see the transfer of electrons and understand the complete process.
When analyzing the example with zinc and copper sulfate:

• Reduction half-reaction: \[\mathrm{Cu}^{2+} + 2e^- \rightarrow \mathrm{Cu}\]
• Oxidation half-reaction: \[\mathrm{Zn} \rightarrow \mathrm{Zn}^{2+} + 2e^-\]

Here, the copper ions are gaining electrons in the reduction half-reaction, while the zinc atoms are losing electrons in the oxidation half-reaction. Understanding half-reactions is essential in electrochemistry, as it helps predict how electrons will flow in an electrochemical cell.

Electrochemistry

Electrochemistry is the branch of chemistry that deals with the interaction between electrical energy and chemical changes. This involves processes like the movement of electrons within a reaction, as seen in redox reactions.
The zinc and copper sulfate reaction is electrochemical in nature, as it involves the transfer of electrons. When one metal is more reactive than another, it can induce a flow of electrons from one side of the reaction to the other.
In electrochemical cells, these principles are applied to generate electricity or drive chemical reactions. For example, in a galvanic cell, the spontaneous reactions that occur produce electrical energy, whereas in an electrolytic cell, electrical energy is used to prompt chemical reactions. This is widely utilized in batteries, electroplating, and even purification processes.