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

Question

When 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

The precipitation of Cu is due to the reduction of \(\mathrm{Cu^{2+}}\).

Step 1: Understand the Context

We begin by recognizing this is a single replacement reaction where zinc (Zn) displaces copper (Cu) from the copper sulfate (\(\mathrm{CuSO}_4\)) solution. This type of reaction is common in redox chemistry, which involves the exchange of electrons between species.

Step 2: Identify the Reactants and Products

The chemical reaction is \(\mathrm{Zn} + \mathrm{CuSO}_4 \rightarrow \mathrm{ZnSO}_4 + \mathrm{Cu}\). When zinc is added to the copper sulfate solution, zinc sulfate (ZnSO₄) and copper (Cu) metal are produced. The products indicate that copper is changing its oxidation state.

Step 3: Determine Changes in Oxidation States

In the reactants, zinc (Zn^0) is in its elemental form and copper in \(\mathrm{CuSO}_4\) is in the form of \(\mathrm{Cu^{2+}}\). In the products, zinc changes to \(\mathrm{Zn^{2+}}\) in \(\mathrm{ZnSO}_4\) and \(\mathrm{Cu^{2+}}\) is deposited as elemental copper (\(\mathrm{Cu^0}\)).

Key Concepts

Oxidation States

The concept of oxidation states is vital in understanding redox reactions. An oxidation state, or oxidation number, is a value assigned to an element in a chemical compound that represents its ability to lose, gain, or share electrons when forming bonds.
Oxidation states can help us track electron transfer in a chemical reaction. For instance, in the reaction between zinc and copper sulfate, the zinc metal starts with an oxidation state of zero (Zn\(^0\)) because it's in its elemental form.
When zinc reacts, it loses two electrons, increasing its oxidation state to +2 as it forms zinc sulfate (ZnSO\(_4\)). This process is known as oxidation. Simultaneously, copper ions in copper sulfate (Cu\(^{2+}\)) gain these two electrons, decreasing their oxidation state to zero, and are deposited as solid copper metal (Cu\(^0\)). This process is called reduction. Thus, tracking changes in oxidation states helps us identify which substances are oxidized and which are reduced in redox reactions.

Single Replacement Reactions

Single replacement reactions, also known as single displacement reactions, occur when one element displaces another in a compound. These reactions are a subset of redox reactions and often involve metals.
In the example of zinc reacting with copper sulfate, zinc (the more reactive element) replaces copper in the compound. The general form of these reactions can be summarized as:

• A + BC \(\rightarrow\) AC + B

Where element A displaces element B from the compound BC. In this scenario, zinc (Zn) replaces copper (Cu) in the copper sulfate solution (CuSO\(_4\)). The reaction produces zinc sulfate (ZnSO\(_4\)) and solid copper (Cu).
The activity series of metals can predict whether a single replacement reaction will occur. Metals higher in the activity series will displace those below them from compounds. Because zinc is above copper in the activity series, it readily displaces copper from copper sulfate.

Electrochemistry

Electrochemistry is the branch of chemistry that studies the relationship between electrical energy and chemical reactions. In the context of redox reactions, electrochemistry helps explain the movement of electrons during the reaction, which is fundamental to understanding how batteries and electrochemical cells work.
In our zinc and copper sulfate reaction, the transfer of electrons is critical. When zinc is oxidized (Zn\(^0\) to Zn\(^{2+}\)), it releases electrons. These electrons are captured by copper ions (Cu\(^{2+}\)) in the solution, leading to the reduction of copper ions to copper metal (Cu\(^0\)).
This electron transfer can create an electric current, making such redox reactions useful in designing galvanic cells or batteries. By separating the oxidation and reduction processes into different compartments, and allowing electron flow through an external circuit, we can harness chemical energy and convert it into electrical energy used for powering devices.