2.8.1 Electrolytic Cells

In the previous section, we learned that electrochemical cells are divided into two main types: galvanic cells (which generate electricity from spontaneous reactions) and electrolytic cells. An electrolytic cell is a device that uses electrical energy from an external source to drive a non-spontaneous chemical reaction. This process is called electrolysis.

Definition

An electrolytic cell is a device that converts electrical energy to chemical energy by electrolysis. Electrolysis is the process by which chemical reactions are stimulated by an external electrical current.

Core Concept

The core concept is that in an electrolytic cell, an external power source (like a battery) forces electrons to flow in a direction that is thermodynamically unfavorable. This drives a non-spontaneous redox reaction. The key components are the electrolyte (which provides ions for conduction), two electrodes (where the reactions occur), and an external power source.

Detailed Explanation

The Apparatus for Electrolysis

The apparatus for electrolysis consists of an electrolytic cell containing the electrolyte (either in a molten state or in an aqueous state). Two electrodes are placed in the solution, and the electrodes are connected to a battery.

How the Cell Works

• Conduction: The current is carried from the battery through the wires by means of electrons (metallic conduction). Within the cell, the current is carried by the anions and cations of the electrolyte (electrolytic conduction). The electrodes serve as a point where conduction changes from metallic to electrolytic or vice versa.
• Electrode Reactions: At each electrode, a chemical reaction takes place in which electrons are gained by the ions in solution at one electrode. Simultaneously, electrons are released by some substance at the other electrode. These electrons are returned to the battery through the connecting wire.
• Oxidation and Reduction: Thus, oxidation-reduction reactions occur at the electrodes.
• The electrode at which oxidation occurs is called the anode.
• The electrode at which reduction occurs is called the cathode.
• Choice of Products: The changes that occur at the electrodes depend on the relative oxidation-reduction tendencies of the substances involved.

Key Characteristics of Electrolytic Cells

• Energy Conversion: Converts electrical energy into chemical energy.
• Reaction Type: Drives a non-spontaneous redox reaction.
• Anode: The electrode where oxidation occurs. It is the positive electrode (connected to the positive terminal of the power source).
• Cathode: The electrode where reduction occurs. It is the negative electrode (connected to the negative terminal of the power source).

Units Related to Electrolysis

• Charge: The SI unit of charge is the coulomb (C). It is the charge on 6.25 × 10¹⁸ electrons. Although the coulomb is the usual unit for measuring charge, the chemist finds that a more convenient unit is the Faraday (F). It corresponds to the charge carried by one mole of electrons and amounts to 96487 C.
• Current: The SI unit of current is the ampere, which is the amount of current flowing when one coulomb passes a specific point in one second. Frequently, an ampere is referred to as "a coulomb per second".

Intermediate Level

An electrolytic cell is characterized by an external power source that drives a non-spontaneous reaction. The positive terminal of the power source is connected to the anode, where oxidation occurs (electrons are removed from the species in solution). The negative terminal is connected to the cathode, where reduction occurs (electrons are supplied to the species in solution). The amount of chemical change is proportional to the quantity of electricity passed, which is measured in coulombs. The Faraday constant (F = 96487 C/mol) is the charge of one mole of electrons. This relationship is fundamental for calculating the mass of products formed during electrolysis.

Important Points

• - Electrolytic Cell: Converts electrical energy to chemical energy.
• - Electrolysis: The process of driving a non-spontaneous reaction with an electric current.
• - Anode: The electrode where oxidation occurs. It is the positive electrode.
• - Cathode: The electrode where reduction occurs. It is the negative electrode.
• - Faraday (F): The charge of one mole of electrons, equal to 96487 C.
• - Current (I) × Time (t) = Charge (Q).
• - Products of electrolysis depend on the substances involved, their concentrations, and the electrode potentials.

Remember

• - Electrolytic Cell = Electrical → Chemical.
• - Reaction is non-spontaneous.
• - Anode = Oxidation = Positive electrode.
• - Cathode = Reduction = Negative electrode.
• - **Faraday = 96487 C/mol of electrons.**
• - Charge (C) = Current (A) × Time (s).
• - Examples: Electrolysis of molten NaCl produces Na metal and Cl_2 gas.

Common Mistakes

• - Confusing the signs of the electrodes in electrolytic vs. galvanic cells. In a galvanic cell, the anode is negative and the cathode is positive. In an electrolytic cell, the anode is positive and the cathode is negative.
• - Thinking that the reaction is spontaneous. Electrolysis requires an external power source to drive a non-spontaneous reaction.
• - Forgetting that water can be electrolyzed. In aqueous solutions, water can be oxidized or reduced instead of the ions from the electrolyte.
• - Using the wrong charge for the electron. The Faraday constant (96487 C) is for one mole of electrons.
• - Assuming the anode is always the positive electrode. This is true only for electrolytic cells. In a galvanic cell, the anode is the negative electrode.

Quick Revision

An electrolytic cell is a device that uses an external electrical energy source to drive a non-spontaneous redox reaction, converting electrical energy into chemical energy. This process is called electrolysis. In an electrolytic cell, oxidation occurs at the anode (the positive electrode), and reduction occurs at the cathode (the negative electrode). The current is carried through the electrolyte by ions. The quantity of electricity passed is measured in coulombs, with one mole of electrons carrying a charge of one Faraday (96487 C). The products of electrolysis depend on the substances involved, their concentrations, and their relative oxidation-reduction tendencies.

60-Second Revision

An electrolytic cell uses an external power source to drive a non-spontaneous redox reaction, converting electrical energy into chemical energy (electrolysis). Oxidation occurs at the anode (positive electrode), and reduction occurs at the cathode (negative electrode). The charge of one mole of electrons is one Faraday (96487 C), and charge is calculated as Q = I × t. This process is used to produce substances like sodium and chlorine from molten NaCl.

Frequently Asked Questions (FAQs)

Q1. What is an electrolytic cell? Answer: An electrolytic cell is a device that converts electrical energy to chemical energy by driving a non-spontaneous redox reaction using an external electrical current. This process is called electrolysis.

Q2. Explain the process of electrolysis. Answer: Electrolysis is a process in which a chemical reaction occurs by means of an electric current in a molten or aqueous state. The apparatus consists of an electrolytic cell containing the electrolyte, two electrodes, and an external power source. Oxidation occurs at the anode and reduction at the cathode. The changes at the electrodes depend on the relative oxidation-reduction tendencies of the substances involved.

• Electrolyte: A molten or aqueous solution that contains ions and conducts electricity.
• Electrodes: Two conductors (anode and cathode) placed in the electrolyte.
• External Power Source: A battery or power supply that provides the electrical energy to drive the non-spontaneous reaction.

Q4. How does an electrolytic cell differ from a galvanic cell in terms of energy conversion? Answer: A galvanic cell converts chemical energy from a spontaneous reaction into electrical energy. An electrolytic cell converts electrical energy from an external source into chemical energy to drive a non-spontaneous reaction.

Q5. What is the difference between the anode and cathode in an electrolytic cell? Answer: In an electrolytic cell, the anode is the positive electrode where oxidation occurs. The cathode is the negative electrode where reduction occurs.

Q6. What is the Faraday constant and why is it important? Answer: The Faraday constant (F) is the charge carried by one mole of electrons, approximately 96487 C. It is important because it allows chemists to relate the amount of a substance produced during electrolysis to the quantity of electricity passed (charge) using Faraday's laws of electrolysis.

Q7. How is the quantity of charge passed during electrolysis calculated? Answer: The quantity of charge (Q) is calculated using the formula Q = I × t, where I is the current in amperes and t is the time in seconds. The result is in coulombs (C).

Q8. What factors determine the products of electrolysis? Answer: The products of electrolysis depend on the state of the electrolyte (molten or aqueous), the position of the ions in the electrochemical series (their tendency to be oxidized or reduced), and the concentration of the electrolyte.

Q9. Give an example of an industrial application of electrolysis. Answer: Electrolysis is used in the extraction of reactive metals, such as aluminum from bauxite (aluminum oxide). It is also used in the production of chlorine gas and sodium hydroxide from brine, and in electroplating.

Multiple Choice Questions (MCQs)

Q1. What does an electrolytic cell convert? A. Chemical energy into electrical energy. B. Electrical energy into chemical energy. C. Mechanical energy into electrical energy. D. Electrical energy into mechanical energy. Correct Answer: B Explanation: An electrolytic cell uses an external electrical current to drive a non-spontaneous chemical reaction, converting electrical energy into chemical energy.

Q2. In an electrolytic cell, which electrode is the anode? A. The negative electrode, where reduction occurs. B. The positive electrode, where oxidation occurs. C. The positive electrode, where reduction occurs. D. The negative electrode, where oxidation occurs. Correct Answer: B Explanation: Oxidation occurs at the anode. In an electrolytic cell, the anode is connected to the positive terminal of the power source, making it the positive electrode.

• Q3. What is the charge of one mole of electrons?
• A. 96487 C
• B. 6.022 × 10²³ C
• C. 1.602 × 10⁻¹⁹ C
• D. 8.314 C
• Correct Answer: A
• Explanation: The charge of one mole of electrons is one Faraday, equal to approximately 96487 C.

• Q4. What is the unit of current?
• A. Coulomb
• B. Volt
• C. Ampere
• D. Faraday
• Correct Answer: C
• Explanation: The SI unit of current is the ampere (A), which is equivalent to one coulomb per second.

• Q5. Which of the following is a characteristic of an electrolytic cell?
• A. The reaction is spontaneous.
• B. The reaction is non-spontaneous.
• C. It converts chemical energy to electrical energy.
• D. It does not require an external power source.
• Correct Answer: B
• Explanation: An electrolytic cell uses an external power source to drive a non-spontaneous reaction. This is the key difference from a galvanic cell.

• Q6. What is the process of driving a chemical reaction with an electric current called?
• A. Galvanization
• B. Electrolysis
• C. Hydrolysis
• D. Neutralization
• Correct Answer: B
• Explanation: Electrolysis is the process by which chemical reactions are stimulated by an external electrical current.

• Q7. In an electrolytic cell, the electrode where reduction occurs is called the:
• A. Anode
• B. Cathode
• C. Salt bridge
• D. Voltmeter
• Correct Answer: B
• Explanation: Reduction always occurs at the cathode (Red Cat). In an electrolytic cell, the cathode is the negative electrode.

• Q8. What is the charge of one mole of electrons called?
• A. Coulomb
• B. Ampere
• C. Faraday
• D. Volt
• Correct Answer: C
• Explanation: The charge of one mole of electrons is one Faraday, which is approximately 96487 C.

• Q9. How is the quantity of charge calculated?
• A. Q = It
• B. Q = I × t
• C. Q = tI
• D. Q = I + t
• Correct Answer: B
• Explanation: The quantity of charge (Q) is the product of current (I) and time (t), Q = I × t.

• Q10. In an electrolytic cell, the anode is the:
• A. Negative electrode
• B. Positive electrode
• C. Salt bridge
• D. Cathode
• Correct Answer: B
• Explanation: In an electrolytic cell, the anode is the positive electrode, where oxidation occurs.

Multiple Choice Questions (MCQs)

• Q1. What is an electrolytic cell used for?
• A. To generate electricity from a spontaneous reaction.
• B. To drive a non-spontaneous reaction using electrical energy.
• C. To measure the pH of a solution.
• D. To store electrical energy.
• Correct Answer: B
• Explanation: An electrolytic cell uses an external electrical current to drive a non-spontaneous chemical reaction, a process called electrolysis.

• Q2. In an electrolytic cell, the electrode at which oxidation occurs is called the:
• A. Cathode
• B. Anode
• C. Salt bridge
• D. Voltmeter
• Correct Answer: B
• Explanation: Oxidation always occurs at the anode (An Ox). In an electrolytic cell, the anode is the positive electrode.

• Q3. Which of the following is a characteristic of an electrolytic cell?
• A. The anode is negative.
• B. The cathode is positive.
• C. The reaction is spontaneous.
• D. An external power source is required.
• Correct Answer: D
• Explanation: An electrolytic cell requires an external power source to drive a non-spontaneous reaction.

• Q4. What is the SI unit of charge?
• A. Ampere
• B. Volt
• C. Coulomb
• D. Faraday
• Correct Answer: C
• Explanation: The SI unit of charge is the coulomb (C).

• Q5. What is the Faraday constant approximately equal to?
• A. 6.022 × 10²³
• B. 1.602 × 10⁻¹⁹
• C. 96487 C
• D. 8.314 J/mol·K
• Correct Answer: C
• Explanation: The Faraday constant (F) is the charge of one mole of electrons, approximately 96487 C.

• Q6. In an electrolytic cell, the cathode is the:
• A. Positive electrode
• B. Negative electrode
• C. Salt bridge
• D. Anode
• Correct Answer: B
• Explanation: In an electrolytic cell, the cathode is the negative electrode, where reduction occurs.

• Q7. What is the relationship between charge, current, and time?
• A. Q = I × t
• B. Q = It
• C. Q = tI
• D. Q = I + t
• Correct Answer: A
• Explanation: The quantity of charge (Q) is the product of current (I) and time (t), Q = I × t.

• Q8. Which of the following is an example of electrolysis?
• A. A battery producing electricity.
• B. Electroplating a metal object.
• C. A fuel cell generating power.
• D. Rusting of iron.
• Correct Answer: B
• Explanation: Electroplating is an industrial application of electrolysis, where an electric current is used to deposit a metal coating on an object.

• Q9. What happens at the anode during electrolysis?
• A. Reduction occurs.
• B. Oxidation occurs.
• C. Electrons are gained.
• D. The electrode is negative.
• Correct Answer: B
• Explanation: Oxidation always occurs at the anode. In an electrolytic cell, the anode is the positive electrode.

• Q10. What is the purpose of the external power source in an electrolytic cell?
• A. To provide ions for the electrolyte.
• B. To drive a non-spontaneous chemical reaction.
• C. To measure the cell potential.
• D. To act as a salt bridge.
• Correct Answer: B
• Explanation: The external power source provides the electrical energy needed to drive a non-spontaneous redox reaction in an electrolytic cell.