Problem 161 - 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. HCl cannot be stored in an aluminium vessel because (a) \(\mathrm{Al}\) is a highly reactive metal. (b) \(\mathrm{HCl}\) is an oxidizing acid (c) \(\mathrm{E}_{\mathrm{A}^{3+} / / \mathrm{I} 1}^{0}\) is much smaller than \(E_{\mathrm{H}}^{\mathrm{o}} / \mathrm{H}_{2}\) (d) All of these

Step-by-Step Solution

Answer

(d) All of these

Step 1: Understanding the Electrochemical Series

The electrochemical series arranges elements according to their standard reduction potentials. Elements at the top have negative potentials, meaning they are more likely to lose electrons, while those at the bottom have positive potentials, indicating a higher tendency to gain electrons.

Step 2: Aluminum's Position in the Series

Aluminum has a negative standard reduction potential (\(E_{Al^{3+}/Al}\)) compared to hydrogen, meaning it can lose electrons and acts as an anode in reaction to acids like HCl.

Step 3: Identifying Reaction of HCl with Aluminum

HCl is a strong acid that can oxidize metals with lower reduction potentials. Since aluminum has a lower standard reduction potential than hydrogen, it would react with HCl, leading to the dissolution of aluminum.

Step 4: Evaluating the Given Options

- (a) "Al is a highly reactive metal" - This is accurate since aluminum can indeed react readily with HCl.- (b) "HCl is an oxidizing acid" - This is true because it can oxidize aluminum.- (c) "\(E_{Al^{3+}/Al}^0\) is much smaller than \(E_{H^+/H_2}^0\)" - This is also correct as it explains the ability of HCl to oxidize aluminum.- (d) "All of these" - Given all explanations are correct, this should be the answer.

Step 5: Conclusion

The correct answer must account for the reactivity of aluminum with HCl and the comparative potentials. Therefore, option (d) "All of these" best explains why HCl cannot be stored in an aluminum vessel.

Key Concepts

Standard Reduction Potential

The concept of **standard reduction potential** can initially seem confusing, but it's essentially a measure of a substance's tendency to gain electrons and undergo reduction. This parameter is measured in volts (V) and tells us how likely an electrode is to gain electrons in comparison to the standard hydrogen electrode, which is set at 0 V by definition. The more positive the standard reduction potential of an electrode system, the more likely it is to act as a cathode and gain electrons.
An electrode system with a high standard reduction potential indicates that it easily accepts electrons, which means it's a strong oxidizing agent. Conversely, a negative value implies that the substance is more prone to losing electrons and acts as a reducing agent.
Let's consider aluminum (Al), which has a relatively negative standard reduction potential compared to hydrogen. In the presence of acids like HCl, this position in the electrochemical series indicates that aluminum can easily release electrons, thus fueling its high reactivity.

Electrode Systems

**Electrode systems** are fundamental to understanding electrochemical reactions because they involve two electrodes - anode and cathode - where oxidation and reduction occur, respectively. In an electrochemical cell, these systems consist of electrodes submerged in solutions containing ions.
The electrochemical series helps predict the behavior of these systems by ranking electrode potentials, which reflect their tendency to gain or lose electrons. Depending on the standard reduction potential, one can determine if the electrode will act as an anode or a cathode.
Consider the aluminum-hydrogen electrode system scenario: Aluminum serves as the anode with its negative reduction potential, suggesting its readiness to donate electrons. On the other hand, a standard hydrogen electrode acts as a cathode, receiving electrons. This electrode setup facilitates a redox reaction, aligning with the concept of electron exchange dictated by their positions in the electrochemical series.

Oxidizing Acid

An **oxidizing acid** is a type of acid capable of acting as an oxidizing agent that can readily accept electrons from other substances. In simple terms, these acids can facilitate oxidation by attracting electrons away from other materials.
Within contexts like the electrochemical series, understanding the role of oxidizing acids becomes crucial when examining how metals react with these acids. For instance, hydrochloric acid (HCl) serves as an oxidizing acid. It can react with metals like aluminum, which have lower standard reduction potential than hydrogen.
Because HCl is an oxidizing acid, it facilitates the dissolution of aluminum. As the aluminum reacts and oxidizes, it loses electrons, demonstrating the acid's capability to effect change based on standard reduction potentials. This reaction highlights why storing HCl in aluminum is problematic, showcasing the metal's reactivity and susceptibility to oxidation.