Problem 163 - A Complete Resource Book in Chemistry for JEE Main

Question

The electrochemical series is the arrangement of various electrode systems in the increasing order of their standard reduction potentials. It has several important features. On moving from the top to the bottom in the series, tendency to gain electrons, i.e., to get reduced increases. The electrode systems having negative values of standard reduction potentials act as anode when connected to a standard hydrogen electrode, while those having positive values act as cathode. If \(\mathrm{E}_{\mathrm{Fe}^{2+} / \mathrm{re}}^{0}=-0.44 \mathrm{~V}\) and \(\left.\mathrm{E}_{\mathrm{Mg}^{+2} \mathrm{Mg}}^{0}\right)=-2.37 \mathrm{~V}, \mathrm{E}_{\mathrm{Cu}^{2+}}^{0}\) \(=+0.34 \mathrm{~V}\) and \(\mathrm{E}_{\mathrm{Ag}^{+} / \mathrm{A}_{\mathrm{E}}}^{\mathrm{n}}=+0.80 \mathrm{~V}\), the correct order in which the metals displace each other is (a) \(\mathrm{Fe}>\mathrm{Cu}>\mathrm{Ag}>\mathrm{Mg}\) (b) \(\mathrm{Ag}>\mathrm{Cu}>\mathrm{Fe}>\mathrm{Mg}\) (c) \(\mathrm{Mg}>\mathrm{Fe}>\mathrm{Cu}>\mathrm{Ag}\) (d) \(\mathrm{Fe}>\mathrm{Ag}>\mathrm{Cu}>\mathrm{Mg}\)

Step-by-Step Solution

Answer

The correct displacement order is \(\mathrm{Ag} > \mathrm{Cu} > \mathrm{Fe} > \mathrm{Mg}\), so option (b) is correct.

Step 1: Understand the Series

The electrochemical series arranges electrode systems by their standard reduction potentials. A higher potential indicates a greater tendency to gain electrons and be reduced. The series progresses from negative to positive values as you move from anode-like behavior to cathode-like behavior.

Step 2: Analyze Given Potentials

We are given the standard reduction potentials: \(\mathrm{E}_{\mathrm{Fe}^{2+} / \mathrm{Fe}}^{0} = -0.44 \, \mathrm{V}\), \(\mathrm{E}_{\mathrm{Mg}^{2+} / \mathrm{Mg}}^{0} = -2.37 \, \mathrm{V}\), \(\mathrm{E}_{\mathrm{Cu}^{2+} / \mathrm{Cu}}^{0} = +0.34 \, \mathrm{V}\), and \(\mathrm{E}_{\mathrm{Ag}^{+} / \mathrm{Ag}}^{0} = +0.80 \, \mathrm{V}\).

Step 3: Order the Potentials

Arrange the metals by their standard reduction potentials from highest to lowest: \(+0.80 \, \mathrm{V} \) for Ag, \(+0.34 \, \mathrm{V} \) for Cu, \(-0.44 \, \mathrm{V} \) for Fe, and \(-2.37 \, \mathrm{V} \) for Mg.

Step 4: Determine the Displacement Order

A metal with a higher reduction potential can displace one with a lower potential from a solution of its ions. Therefore, Ag can displace Cu, Cu can displace Fe, and Fe can displace Mg. The order is \(\mathrm{Ag} > \mathrm{Cu} > \mathrm{Fe} > \mathrm{Mg}\).

Step 5: Select the Correct Option

Based on the order derived, the metals displace each other in the following sequence: \(\mathrm{Ag} > \mathrm{Cu} > \mathrm{Fe} > \mathrm{Mg}\). Thus, the correct option is (b).

Key Concepts

Electrode Potential

Electrode potential is the measure of the tendency of an electrode to either gain or lose electrons when it is in contact with its own ions in a solution. In essence, it indicates the reactivity of an electrode in electrochemical reactions.
Understanding electrode potential is crucial because it helps predict the direction in which electrons flow between different electrodes, which in turn determines which reactions will occur.

• Higher electrode potential means a greater ability to gain electrons (reduction).
• Lower electrode potential suggests a greater likelihood of losing electrons (oxidation).
• The difference between electrode potentials of two electrodes in a cell determines the cell potential, influencing the cell's ability to do work.

Electrode potential values are standardized and usually reported against the standard hydrogen electrode (SHE) as a reference. This setup helps chemists and engineers understand and compare the relative reactivity of different electrode systems.

Reduction Potential

Reduction potential, also known as standard reduction potential when measured under standard conditions, is a key concept in electrochemistry. It is a measure of the propensity of a chemical species to gain electrons and be chemically reduced.
The more positive the reduction potential value, the higher the affinity for electrons, indicating a stronger oxidizing ability:

• Measured in volts (V) and often symbolized as E°.
• Substances with higher reduction potentials can oxidize those with lower reduction potentials.
• Used to predict the spontaneity of a redox reaction.

Understanding reduction potentials is essential when assessing how metals interact in solutions or during galvanic chemical processes. For example, metals with higher reduction potentials will be reduced before those with lower potentials, explaining the series' arrangement from negative to positive values.

Metal Displacement

Metal displacement occurs when a metal ion in solution is replaced by another, more reactive metal. This reaction is explained through the concept of displacement reactions, where metals exhibit different tendencies to give off electrons based on their positions in the electrochemical series.
This behavior can be predicted using standard reduction potentials:

• Metals with higher reduction potential values can displace metals with lower reduction potentials from a solution.
• Metal displacement is a practical application of the electrochemical series, often observed in metallurgy and other industrial processes.
• Provides insight into the reactivity and stability of metals in chemical processes.

The order in which metals displace each other, as shown in the original exercise, reflects their relative reactivity. For example, silver with the highest reduction potential will displace copper and others lower in the series.