Problem 37 - A Complete Resource Book in Chemistry for JEE Main

Question

When a Zn strip is placed in \(\mathrm{CuSO}_{4}\) solution, Cu gets precipitated, because standard oxidation potential of \(\mathrm{Zn}\) is (a) \(<\mathrm{Cu}\) (b) \(>\mathrm{Cu}\) (c) \(<\mathrm{SO}_{4}\) (d) \(>\mathrm{SO}_{4}\)

Step-by-Step Solution

Answer

(b) \(>\mathrm{Cu}\) because Zn oxidizes more readily than Cu.

Step 1: Understanding the Exercise

In this exercise, we need to understand what happens when a zinc (Zn) strip is placed in a copper(II) sulfate (CuSO\(_4\)) solution. The problem involves electrochemistry, focusing on the concept of standard electrode potentials and oxidation/reduction reactions.

Step 2: Concept of Oxidation Potential

The standard oxidation potential indicates the tendency of a species to lose electrons (oxidize). Zn has a standard electrode potential of -0.76 V, and Cu has a lower positive potential of +0.34 V, meaning that Zn has a greater tendency to oxidize compared to Cu.

Step 3: Reaction Analysis

When Zn is introduced to CuSO\(_4\), Zn is oxidized to Zn\(^{2+}\) and releases electrons, while Cu\(^{2+}\) gains the electrons and is reduced to Cu. This electron transfer results in the precipitation of copper as a solid.

Step 4: Determining the Correct Statement

The oxidation potential of Zn being greater than that of Cu means that Zn can displace Cu\(^{2+}\) ions from the solution. This aligns with option (b), which states the standard oxidation potential of Zn is greater than that of Cu.

Key Concepts

Oxidation Potential

The concept of oxidation potential is central to understanding how reactions occur in electrochemistry. Oxidation potential refers to the tendency of a substance to lose electrons and undergo oxidation. It is often measured in volts. Metals like zinc ( Zn ) have higher oxidation potentials, meaning they are more willing to release electrons. When Zn is compared to copper ( Cu ), Zn has a standard oxidation potential of -0.76 V, which is more negative compared to the positive 0.34 V of Cu. This means Zn has a greater tendency to lose electrons than Cu, allowing it to easily oxidize and drive a reaction when placed in a solution containing Cu compounds such as CuSO_4 . Oxidation potential is a driving force in redox reactions where electron exchange is involved.

Standard Electrode Potential

The standard electrode potential is a vital concept in electrochemistry as it offers a quantitative measure of a substance's ability to gain or lose electrons compared to the standard hydrogen electrode. It is denoted by the symbol E° . Measured in volts, it provides a numerical value indicating whether a metal is likely to be oxidized or reduced. Several key points about standard electrode potentials include:

• They are measured under standard conditions: 25°C, 1M concentration, and 1 atm pressure.
• They allow comparison of different substances' tendencies to gain or lose electrons.
• A more positive standard electrode potential implies a greater likelihood of being reduced (gaining electrons), while a more negative potential implies a stronger tendency to be oxidized (losing electrons).

In the case of the Zn and Cu reaction, Zn has a more negative value compared to Cu , thus Zn tends to oxidize more easily than Cu , facilitating the displacement reaction.

Redox Reactions

Redox reactions are chemical reactions involving the transfer of electrons between two species. The term "redox" itself combines the words "reduction" and "oxidation," signifying that one substance is reduced, while another is oxidized. Key features of redox reactions include:

• Oxidation: This process involves the loss of electrons from a substance, leading to an increase in its oxidation state.
• Reduction: This process involves the gain of electrons by a substance, resulting in a decrease in its oxidation state.
• In a balanced redox reaction, the total number of electrons lost equals the total number of electrons gained.

When Zn is placed in CuSO_4 solution, Zn atoms lose electrons (oxidized), becoming Zn^{2+} ions, while the Cu^{2+} ions in the solution gain electrons (reduced) to form solid copper. This exchange of electrons represents the fundamental principle of redox reactions.

Displacement Reaction

A displacement reaction, sometimes referred to as a single replacement reaction, occurs when one element replaces another in a compound. In electrochemistry, this often involves a more reactive metal displacing a less reactive metal from its compound. The displacement reaction involving Zn and CuSO_4 is a classic example:

• Zn displaces Cu^{2+} ions in the solution, forming Zn^{2+} ions and solid Cu .
• This reaction is driven by the relative reactivities of the metals involved, as described by their standard electrode potentials.

Since Zn is more reactive with a higher oxidation potential than Cu , it effectively displaces the copper ions from the solution, precipitating solid Cu . This kind of reaction highlights the practical application of electrochemical principles in predicting and understanding chemical behavior.