Chemistry Class 12 NCERT Solutions Chapter 3 Electrochemistry – Important Questions

Chemistry is a crucial subject in the Class 12 curriculum, particularly for students preparing for competitive exams like JEE, NEET, and others. Chapter 3 of the Class 12 NCERT Chemistry textbook is titled "Electrochemistry." This chapter delves into the fundamental principles and applications of electrochemistry, a branch of chemistry that deals with the relationship between electrical energy and chemical changes. In this article, we'll explore some of the most important questions and concepts from this chapter, providing comprehensive solutions to aid students in their preparation.

Introduction to Electrochemistry

Electrochemistry is the study of chemical reactions that involve the transfer of electrons. These reactions are central to many processes in both nature and industry, including corrosion, batteries, fuel cells, and electroplating. Understanding electrochemistry is vital for students as it forms the basis for many practical applications in chemistry and engineering.

Key Concepts in Chapter 3

Before diving into important questions, let's review the essential concepts covered in Chapter 3:

1. Electrochemical Cells: These are devices that convert chemical energy into electrical energy or vice versa. The two main types are galvanic (voltaic) cells and electrolytic cells.

2. Cell Potential and EMF: The cell potential (or electromotive force, EMF) is the voltage developed by a cell. It is determined by the difference in electrode potentials of the two half-cells.

3. Standard Electrode Potentials: These are the potentials of electrodes measured under standard conditions (1 M concentration, 1 atm pressure, 25°C).

4. Nernst Equation: This equation relates the cell potential to the concentrations of the reactants and products in a cell.

5. Electrolytic Cells: These cells use electrical energy to drive non-spontaneous chemical reactions. They are used in processes like electrolysis.

6. Faraday's Laws of Electrolysis: These laws relate the amount of substance transformed at an electrode to the quantity of electricity passed through the cell.

7. Applications of Electrochemistry: Includes batteries, corrosion, electroplating, and fuel cells.

Important Questions and Solutions

1. Define and Explain the Terms: Electrochemical Cell, Galvanic Cell, and Electrolytic Cell.

Answer:

• Electrochemical Cell: An electrochemical cell is a device that converts chemical energy into electrical energy or vice versa through redox reactions. It consists of two electrodes (anode and cathode) and an electrolyte.

• Galvanic Cell: Also known as a voltaic cell, it is a type of electrochemical cell that converts chemical energy into electrical energy spontaneously. It generates electrical energy from spontaneous redox reactions occurring within the cell. For example, a common galvanic cell is the Daniel cell, which uses zinc and copper electrodes.

• Electrolytic Cell: This cell uses electrical energy to drive a non-spontaneous chemical reaction. Electrolytic cells are used in processes like electrolysis, where electrical current is used to decompose compounds. An example is the electrolysis of water to produce hydrogen and oxygen gases.

2. Derive the Nernst Equation and Explain Its Significance.

Answer:

The Nernst equation is used to calculate the cell potential (E) under non-standard conditions. It is derived from the relationship between the Gibbs free energy change and the cell potential.

The Nernst equation is given by:

$E = E^\circ - \frac{RT}{nF} \ln Q$

where:

• $E$ = cell potential under non-standard conditions
• $E^\circ$ = standard cell potential
• $R$ = gas constant (8.314 J/mol·K)
• $T$ = temperature in Kelvin
• $n$ = number of moles of electrons transferred
• $F$ = Faraday constant (96485 C/mol)
• $Q$ = reaction quotient

Significance:

The Nernst equation allows us to determine how the concentration of reactants and products affects the cell potential. It is essential for understanding and predicting the behavior of electrochemical cells under various conditions, which is crucial for applications such as batteries and corrosion prevention.

3. What Are the Applications of Electrochemical Cells in Everyday Life?

Answer:

Electrochemical cells have numerous applications in daily life, including:

• Batteries: They store and provide electrical energy for various devices, from small electronics like watches to larger applications like electric vehicles.

• Fuel Cells: These cells convert chemical energy directly into electrical energy with high efficiency. They are used in hydrogen fuel cells for vehicles and backup power systems.

• Corrosion Protection: Electrochemical principles are used in methods like cathodic protection to prevent metal corrosion.

• Electroplating: This process involves depositing a layer of metal onto a surface to enhance appearance, protect against corrosion, or increase thickness.

• Electrolysis: Electrochemical cells are used in the electrolysis of water for hydrogen production and in the extraction of metals from ores.

4. Explain Faraday’s Laws of Electrolysis with Examples.

Answer:

Faraday's Laws of Electrolysis describe the relationship between the amount of substance altered at an electrode and the quantity of electricity passed through the cell.

• First Law: The mass of a substance altered at an electrode is directly proportional to the quantity of electricity (charge) passed through the cell. Mathematically, it is expressed as:

  $m = \frac{Q}{F} \cdot \frac{M}{n}$

  where $m$ is the mass of the substance, $Q$ is the total electric charge, $F$ is Faraday's constant, $M$ is the molar mass, and $n$ is the number of electrons involved in the reaction.

• Second Law: The mass of substances altered at each electrode is directly proportional to their equivalent weights.

Example:

If 1 mole of copper is deposited from a copper sulfate solution using a current of 1 ampere for 1 hour, the mass of copper deposited can be calculated using Faraday's laws.

5. Calculate the Standard Cell Potential for a Galvanic Cell with Given Half-Cell Reactions.

Answer:

To calculate the standard cell potential (E^\circ) for a galvanic cell, use the following steps:

1. Identify the Half-Cell Reactions: Write the reduction reactions for both half-cells. For instance:

   • Anode (oxidation): $\text{Zn} \rightarrow \text{Zn}^{2+} + 2e^-$
   • Cathode (reduction): $\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}$

2. Find the Standard Electrode Potentials: Look up the standard reduction potentials ($E^\circ$) for each half-cell reaction. For example:

   • $E^\circ_{\text{Zn}^{2+}/\text{Zn}} = -0.76 \text{ V}$
   • $E^\circ_{\text{Cu}^{2+}/\text{Cu}} = +0.34 \text{ V}$

3. Calculate the Cell Potential: Use the formula:

   $E^\circ_{\text{cell}} = E^\circ_{\text{cathode}} - E^\circ_{\text{anode}}$

   Substituting the values:

   $E^\circ_{\text{cell}} = 0.34 \text{ V} - (-0.76 \text{ V}) = 1.10 \text{ V}$

   The standard cell potential is 1.10 V.

6. Describe the Process of Electrolysis and Its Applications.

Answer:

Electrolysis is a process that uses electrical energy to drive a non-spontaneous chemical reaction. It involves passing an electric current through an electrolyte to cause a chemical change.

Process:

1. Electrolyte Preparation: The electrolyte is a substance that conducts electricity when dissolved or molten. Examples include water with dissolved salt or molten sodium chloride.

2. Electrolytic Cell Setup: An electrolytic cell has two electrodes (anode and cathode) immersed in the electrolyte. The anode is connected to the positive terminal of the power supply, while the cathode is connected to the negative terminal.

3. Current Flow and Chemical Reactions: When current flows through the cell, oxidation occurs at the anode and reduction occurs at the cathode. For example, in the electrolysis of water, water is split into hydrogen and oxygen gases.

Applications:

• Electrolysis of Water: Produces hydrogen and oxygen gases used in fuel cells and industrial processes.

• Electroplating: Deposits a layer of metal onto objects to improve appearance and resistance to corrosion.

• Extraction of Metals: Extracts metals from their ores, such as aluminum from bauxite ore.

• Production of Chlorine and Sodium Hydroxide: Electrolysis of brine produces chlorine gas and sodium hydroxide.

7. Discuss the Role of Electrochemical Cells in Batteries and Fuel Cells.

Answer:

Batteries: Batteries are electrochemical cells that store and provide electrical energy through chemical reactions. They consist of one or more electrochemical cells arranged in series or parallel. Common types include:

• Dry Cell: Used in everyday items like flashlights and remote controls. It contains a zinc anode and a carbon cathode.

• Lead-Acid Battery: Used in automobiles. It comprises lead dioxide (PbO₂) and sponge lead (Pb) electrodes immersed in sulfuric acid.

Fuel Cells: Fuel cells convert the chemical energy of a fuel (often hydrogen) directly into electrical energy through electrochemical reactions. Unlike batteries, fuel cells require a continuous supply of fuel and oxygen. They are used in various applications, including:

• Hydrogen Fuel Cells: Powering vehicles and backup power systems.

• Methanol Fuel Cells: Used in portable power applications.

8. Solve a Numerical Problem Involving the Calculation of Cell Potential Using Standard Electrode Potentials.

Answer:

Consider the following half-cell reactions and their standard electrode potentials:

• $\text{Fe}^{3+} + 3e^- \rightarrow \text{Fe}$ $E^\circ = -0.77 \text{ V}$
• $\text{Cl}_2 + 2e^- \rightarrow 2\text{Cl}^-$ $E^\circ = +1.36 \text{ V}$

To calculate the cell potential for a cell with these half-reactions:

1. Write the Overall Reaction:

   To balance the electrons, multiply the first reaction by 2 and the second reaction by 3:

   $2\text{Fe}^{3+} + 3\text{Cl}_2 \rightarrow 2\text{Fe} + 6\text{Cl}^-$

2. Determine the Cell Potential:

   Calculate the cell potential using the formula:

   $E^\circ_{\text{cell}} = E^\circ_{\text{cathode}} - E^\circ_{\text{anode}}$

   In this case:

   $E^\circ_{\text{cell}} = 1.36 \text{ V} - (-0.77 \text{ V}) = 2.13 \text{ V}$

   The cell potential is 2.13 V.

9. What is the Importance of Standard Electrode Potentials in Electrochemical Cells?

Answer:

Standard electrode potentials are crucial for several reasons:

1. Prediction of Cell Behavior: They help predict whether a given electrochemical reaction will occur spontaneously or not. A positive cell potential indicates a spontaneous reaction.

2. Determination of Reaction Feasibility: By comparing standard electrode potentials, one can determine the feasibility of different reactions in electrochemical cells.

3. Design of Electrochemical Cells: They guide the design and optimization of batteries, fuel cells, and other electrochemical devices by selecting appropriate materials for electrodes.

4. Quantification of Electrochemical Performance: They allow for the calculation of cell potential and the overall efficiency of electrochemical processes.

Conclusion

Chapter 3 of the Class 12 NCERT Chemistry textbook covers fundamental concepts and applications of electrochemistry. Understanding these principles is essential for students aiming to excel in their exams and grasp the practical applications of chemistry in technology and industry. By studying the important questions and solutions provided in this article, students can enhance their knowledge and prepare effectively for their examinations.