Problem 156 - A Complete Resource Book in Chemistry for JEE Main

Question

In an electrolytic cell, electrolysis is carried out. Electrical energy is converted into chemical energy. In an electrochemical cell, chemical reaction, i.e., redox reaction occurs and electricity is generated. So chemical energy is converted into electrical energy. Electrolysis is governed by Faraday's laws. The potential difference between the electrodes which is called electromotive force is responsible for the generation of electric energy in the electrochemical cells. The standard reduction potential values of three metallic cations \(\mathrm{X}, \mathrm{Y}\) and \(\mathrm{Z}\) are \(0.50 \mathrm{~V},-3.03 \mathrm{~V}\) and \(-1.2 \mathrm{~V}\) respectively. The order of reducing power of the corresponding metals is (a) \(X>Y>Z\) (b) \(\mathrm{Z}>\mathrm{Y}>\mathrm{X}\) (c) \(\mathrm{Y}>Z>\mathrm{X}\) (d) \(\mathrm{X}>\mathrm{Z}>\mathrm{Y}\)

Step-by-Step Solution

Answer

The order of reducing power is (c) \(Y > Z > X\).

Step 1: Understanding Reduction Potential

The standard reduction potentials indicate the tendency of a species to gain electrons. A more negative reduction potential means a greater tendency to donate electrons, which corresponds to stronger reducing power.

Step 2: Identifying Reduction Potentials

The given reduction potentials are: for \(X\), \(0.50\, ext{V}\); for \(Y\), \(-3.03\, ext{V}\); and for \(Z\), \(-1.2\, ext{V}\). These values indicate how readily each cation gains electrons.

Step 3: Ranking by Reduction Potential

Since a lower (more negative) standard reduction potential means higher reducing power, compare the reduction potentials: \(Y\) has the most negative value, \(-3.03\, ext{V}\), followed by \(Z\) at \(-1.2\, ext{V}\), and \(X\) at \(0.50\, ext{V}\).

Step 4: Conclusion on Reducing Power

Based on their reduction potentials, the order of reducing power from strongest to weakest is: \(Y\) (strongest), \(Z\), and then \(X\) (weakest).

Key Concepts

Electrochemical Cell

An electrochemical cell is a device that uses chemical reactions to produce electrical energy. It essentially converts chemical energy into electrical energy. The core process within this cell is a type of chemical reaction known as a redox reaction. "Redox" stands for reduction-oxidation, where one species gains electrons (reduction) and another loses electrons (oxidation).

In an electrochemical cell, two electrodes are placed in an electrolyte solution. These electrodes are the sites of these redox reactions. The electrode where oxidation occurs is called the anode, and the electrode where reduction occurs is called the cathode.

Subsequently, the movement of electrons from the anode to the cathode through an external circuit creates an electric current. This current can be harnessed to perform work, just like how batteries power devices. Notably, the potential difference between the electrodes, also known as the electromotive force (EMF), drives the flow of electrons, thus enabling the cell to generate electricity.

• Oxidation occurs at the anode.
• Reduction occurs at the cathode.
• Generates electrical energy through redox reactions.

Faraday's Laws

Faraday's laws provide a critical insight into the process of electrolysis, which is a key part of electrochemical reactions. His laws help relate the amount of material modified at an electrode during electrolysis to the quantity of electricity used.

The first law states that the mass of a substance altered at an electrode during electrolysis is directly proportional to the total electric charge passed through the electrode. In simpler terms, more electricity results in more material changing.

The second law states that the quantity of different substances that are transformed at the electrodes upon sending the same electric charge is proportional to their equivalent weights. Equivalent weight is a concept that links the atomic or molecular weight with the change in oxidation number in a given redox reaction.

• First Law: Mass changes proportional to electric charge.
• Second Law: Multiple substances transformed depend on equivalent weights.

These laws are essential when calculating how much substance forms or dissolves during electrolysis processes.

Standard Reduction Potential

The standard reduction potential (3°) is a measure of the tendency of a chemical species to be reduced, which means to gain electrons. It is measured in volts (V) under standard conditions, typically at 25°C, 1 molar concentration, and 1 atm pressure. The higher or more positive the electrode potential, the more likely it is to gain electrons and undergo reduction.

Conversely, a more negative reduction potential indicates that the species is more inclined to lose electrons - or in other words, acts as a better reducing agent. For example, among the metallic cations X, Y, and Z, if X has a potential of 0.50 V, it indicates it is less likely to give up electrons compared to Y with −3.03 V, which is more eager to donate electrons and thus is a stronger reducing agent.

This understanding of reduction potentials is crucial for predicting the flow of electrons in redox reactions and designing electrochemical cells accordingly. When arranging metals by their ability to act as reducing agents, they will be ranked by their potential values from more negative to less negative.

• Higher (positive) potential: better oxidizing agent.
• More negative potential: better reducing agent.
• 3° is measured in volts (V) under standard conditions.