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) \(\mathrm{E}_{\mathrm{A}^{3+} / \mathrm{Al}}^{0}\) is much smaller than \(E_{\mathrm{H}}^{0}+\mathrm{H}_{2}\) (d) All of these

Step-by-Step Solution

Answer

All options are correct; (d) is the right choice.

Step 1: Understanding Electrochemical Series

The electrochemical series is a list of electrode systems arranged by increasing standard reduction potentials. Systems with lower standard reduction potentials are more likely to lose electrons and act as anodes, while those with higher potentials are more likely to gain electrons and act as cathodes.

Step 2: Identifying the Behavior of Aluminum

Aluminum's standard reduction potential, denoted as \(E_{Al^{3+}/Al}\), is negative compared to the standard hydrogen potential \(E^0_{H^+/H_2}\). This implies that aluminum is prone to oxidation.

Step 3: Interaction with HCl

Hydrochloric acid (HCl) can cause oxidation since it is acidic and can act as an oxidizing agent, reacting with metals like aluminum to produce aluminum chloride and hydrogen gas.

Step 4: Evaluating Each Option

(a) Aluminum's high reactivity explains why it cannot store HCl. (b) HCl as an acid can oxidize aluminum. (c) The significant difference in reduction potentials indicates aluminum will transfer electrons to HCl, causing the reaction.

Step 5: Conclusion on the Compatibility of Aluminum and HCl

Since options (a), (b), and (c) all describe aspects of why aluminum cannot store HCl, they combine to explain the overall reaction scenario.

Key Concepts

Standard Reduction Potential

The standard reduction potential of a substance is a measure of its ability to gain electrons in a redox reaction. This is expressed in volts and is typically compared against the standard hydrogen electrode, which is given a potential of zero volts. A higher standard reduction potential means a greater likelihood of the substance being reduced or gaining electrons.
In the electrochemical series, which arranges substances in order of increasing standard reduction potentials, those at the bottom have greater affinities for electrons. These substances act as strong oxidizing agents. Conversely, substances with low or negative standard reduction potentials, found at the top, tend to lose electrons and are more likely to act as reducing agents.
When considering a reaction like that between aluminum and hydrochloric acid, recognizing their reduction potentials can predict the outcome of the interaction. Aluminum, with its negative potential, suggests its propensity to lose electrons, making it unable to store acidic substances without reacting.

Anode and Cathode

In the context of an electrochemical cell, the anode and cathode are critical components where oxidation and reduction reactions occur. The anode is the electrode where oxidation takes place; it gives up electrons and usually has a negative reduction potential. In contrast, the cathode is where reduction occurs, accepting electrons, and typically possesses a positive standard reduction potential.
In a simple setup when connecting the aluminum system with a standard hydrogen electrode, aluminum acts as the anode due to its lower reduction potential. Hydrogen's standard potential is higher, making it the cathode, where reduction will preferentially happen.
This understanding helps interpret why an aluminum vessel cannot store HCl. The aluminum will oxidize, acting as the anode, and deteriorate in presence of an acidic medium that might reduce, like the hydrogen in HCl.

Reactivity of Metals

The reactivity of metals in the electrochemical series is closely related to their position based on standard reduction potentials. Metals higher up often act as strong reducing agents because they willingly give up electrons. The opposite is true for metals lower in the series; they are more likely to gain electrons.
For example, aluminum, which resides near the top due to its negative reduction potential, indicates its readiness to oxidize. This property explains its high reactivity and why it cannot contain solutions like HCl without undergoing a chemical reaction.
In practical applications, understanding the reactivity of metals assists in predicting their behavior with various substances, including acids. By examining where a metal falls in the series, one can anticipate its interactions and the feasibility of using it in industrial or laboratory settings.

Oxidizing Agents

Oxidizing agents are substances that can accept electrons during a chemical reaction. They effectively enable other species to lose electrons, being reduced themselves in the process. The standard electrochemical series highlights that substances with high reduction potentials are good oxidizing agents.
A classic example is the hydrogen ion in HCl, which can oxidize metals like aluminum, which tends to act as a reducing agent due to its low standard reduction potential. This means HCl easily accepts electrons from aluminum, driving the oxidation process of the metal.
This characteristic is fundamental in determining which metals can or cannot be safely used as containers for different chemicals. Knowing that an acid like HCl can serve as a potent oxidizing agent, it becomes clear why it should not be stored in metals prone to oxidization like aluminum.