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) \(\mathrm{X}>\mathrm{Y}>\mathrm{Z}\) (b) \(\mathrm{Z}>\mathrm{Y}>\mathrm{X}\) (c) \(\mathrm{Y}>\mathrm{Z}>\mathrm{X}\) (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 potential measures the tendency of a chemical species to acquire electrons and thereby be reduced. A more positive or less negative value indicates a greater tendency for the species to gain electrons.

Step 2: Correlation with Reducing Power

The reducing power of a metal is the ability to donate electrons or reduce another substance. Metals with a lower (more negative) standard reduction potential have a greater ability to donate electrons, indicating stronger reducing power.

Step 3: Assessing the Given Potentials

We have the following reduction potentials: - Metal X: +0.50 V - Metal Y: -3.03 V - Metal Z: -1.20 V The metal with the lowest reduction potential exhibits the highest reducing power.

Step 4: Ordering by Reducing Power

Since Metal Y has the most negative reduction potential (-3.03 V), it has the highest reducing power. Metal Z follows with -1.20 V, and Metal X, with the least reducing power, has the highest reduction potential at +0.50 V. Therefore, the order of reducing power is Y > Z > X.

Key Concepts

Electrolysis

In electrolysis, electricity is used to drive a non-spontaneous chemical reaction. This means that we are converting electrical energy into chemical energy, typically within an electrolytic cell. The key aspect is that an external electrical voltage is necessary to force the chemical changes. During this process, ions in the electrolyte are attracted to electrodes of opposite charge, resulting in chemical transformations.

For example, if you have a solution of sodium chloride ( NaCl ), applying a direct current will cause sodium ions to move to the cathode to gain electrons (reduction), while chloride ions move to the anode to lose electrons (oxidation). The overall process results in the decomposition of the compound, illustrating how electrical energy can cause chemical reactions.

Chemical Energy Conversion

Chemical energy conversion fundamentally underlies the operation of electrochemical cells. In electrochemical cells, the energy stored in chemical bonds is converted into electrical energy, and vice versa. For electrolytic cells, this conversion involves using electrical energy to drive chemical reactions, thus storing energy in chemical form.

The reverse process takes place in galvanic or voltaic cells, where spontaneous chemical reactions occur, generating electricity. This is the principle that powers batteries, where chemical energy from reactions inside the battery is converted to electrical energy you use in various devices.

• Electrolytic cells: Convert electrical to chemical energy.
• Galvanic cells: Convert chemical to electrical energy.

This fundamental concept is essential to understanding how batteries store energy and how electrolysis allows energy storage in chemical bonds.

Standard Reduction Potential

The standard reduction potential is a measure of the tendency of a chemical species to gain electrons and be reduced. It is measured in volts and is essential in predicting the direction of redox reactions within an electrochemical cell. A species with a more positive standard reduction potential is more likely to gain electrons compared to a species with a more negative potential.

Let's consider two species with different reduction potentials. The one with the higher potential will act as the cathode and accept electrons, while the other species with a lower potential acts as the anode, donating electrons. These potentials help determine which reactions can spontaneously occur and form the basis for understanding the principles of batteries and fuel cells.

Faraday's Laws

Faraday's Laws of Electrolysis relate the amount of substance transformed at an electrode to the quantity of electricity used. There are two main laws that Faraday formulated:

1. **First Law:** The amount of chemical change during electrolysis is directly proportional to the electricity passed through the electrolyte. This means if you increase the charge, more substance will be deposited or dissolved.
2. **Second Law:** The amounts of different substances transformed by the same electricity depend on their equivalent weights. In other words, for the same amount of electricity, substances with fewer moles will change more readily than those with more moles.

Using these laws, we can predict how much of a substance will be produced or consumed during electrolysis, given the amount of charge applied.