4.1.3 Electrolytic Cells

Introduction

In the previous section, we learned about electrolytes — substances that conduct electricity in molten or aqueous state because they contain free-moving ions. But how do we actually use electricity to drive a chemical reaction? The answer is an electrolytic cell.

An electrolytic cell is a type of electrochemical cell in which electrical energy is used to drive a non-spontaneous chemical reaction. This process is called electrolysis.

Electrolytic cells are used in many important industrial processes:

• Electroplating — coating one metal with another
• Electrolysis of brine — producing chlorine and sodium hydroxide
• Electrolysis of molten salts — producing reactive metals like sodium and aluminium
• Electrorefining — purifying metals like copper

Definition

Electrolytic Cell: An electrochemical cell in which electrical energy is used to drive a chemical reaction is called an electrolytic cell.

In simple words: An electrolytic cell is a device that uses electricity to make a chemical reaction happen.

Main Idea

• An electrolytic cell uses electrical energy to drive a non-spontaneous chemical reaction.
• It consists of:
• Oxidation occurs at the anode.
• Reduction occurs at the cathode.
• Electrons flow from anode to cathode in the external circuit.
• In the solution, cations move towards the cathode and anions move towards the anode.

Core Concept

Key rule:

• Oxidation always occurs at the anode.
• Reduction always occurs at the cathode.

Detailed Explanation

Structure of an Electrolytic Cell

An electrolytic cell consists of:

(a) A vessel containing an electrolyte (MX)

• The electrolyte is an ionic compound in molten state or aqueous solution.
• It contains free-moving ions (cations M⁺ and anions X^-).
• Examples: molten NaCl, aqueous CuSO₄, concentrated NaCl solution.

(b) Two inert electrodes

• Inert electrodes do not react with the electrolyte or the products.
• Common inert electrodes: platinum or carbon (graphite).
• The anode is the positive electrode.
• The cathode is the negative electrode.

(c) A battery

• The battery provides the electrical energy needed to drive the non-spontaneous reaction.
• The battery pushes electrons from the anode to the cathode through the external circuit.

How an Electrolytic Cell Works

Step-by-step process:

• The battery pushes electrons to the cathode (negative electrode).
• The cathode becomes negatively charged.
• Cations (M⁺) in the electrolyte move towards the cathode.
• At the cathode, cations gain electrons and are reduced:
• The anode (positive electrode) attracts anions (X^-).
• At the anode, anions lose electrons and are oxidized:
• The electrons released by anions at the anode move through the external circuit towards the cathode.
• This completes the circuit and allows the reaction to continue.

Movement of Ions and Electrons

Key point:

• Electrons flow in the external circuit.
• Ions move in the electrolyte (solution or molten salt).

Electrode Reactions

At the anode (oxidation): X^- → X + 1e^-

• Anions lose electrons.
• Oxidation occurs.

At the cathode (reduction): M⁺ + 1e^- → M

• Cations gain electrons.
• Reduction occurs.

Key point:

• Oxidation = loss of electrons = at anode.
• Reduction = gain of electrons = at cathode.

Why Electrolytic Cells Are Important

Electrolytic cells are used in many industrial processes:

• Electroplating — coating steel with zinc, tin, or chromium for protection from corrosion.
• Electrolysis of brine — producing chlorine (used in disinfectants) and sodium hydroxide (used in soap manufacturing).
• Production of reactive metals — sodium, aluminium, and magnesium are produced by electrolysis of their molten salts.
• Electrorefining — purifying copper and other metals.
• Hydrogen production — hydrogen produced via electrolysis is used in fuel cells for clean energy.

Intermediate Level

Structure of an electrolytic cell:

• Vessel containing electrolyte (MX).
• Two inert electrodes (platinum or graphite).
• Battery to provide electrical energy.

Working:

1. Battery pushes electrons to the cathode. 2. Cathode becomes negatively charged. 3. Cations (M⁺) move towards the cathode and are reduced: M⁺ + 1e^- → M 4. Anions (X^-) move towards the anode and are oxidized: X^- → X + 1e^- 5. Electrons released at the anode flow through the external circuit to the cathode.

Key point:

• **Oxidation** = at anode.
• **Reduction** = at cathode.
• Electrons flow in the external circuit.
• Ions move in the electrolyte.

Formula / Mathematical Form

Anode (oxidation): X^- → X + 1e^-

Cathode (reduction): M⁺ + 1e^- → M

General cell reaction: MX → M + X

Solved Example

Example: Electrolysis of Molten NaCl

Electrolyte: Molten NaCl (contains Na⁺ and Cl^- ions)

At the anode (oxidation): 2Cl^- → Cl₂ + 2e^-

At the cathode (reduction): Na⁺ + 1e^- → Na

Overall reaction: 2NaCl → 2Na + Cl₂

Products:

• Sodium metal at the cathode
• Chlorine gas at the anode

Example: Electrolysis of Molten KCl

Electrolyte: Molten KCl (contains K⁺ and Cl^- ions)

At the anode (oxidation): 2Cl^- → Cl₂ + 2e^-

At the cathode (reduction): K⁺ + 1e^- → K

Overall reaction: 2KCl → 2K + Cl₂

Products:

• Potassium metal at the cathode
• Chlorine gas at the anode

Concept Assessment Exercise 4.1

1. Sketch an electrolytic cell for the electrolysis of fused KCl.

Answer:

• Electrolyte: Molten KCl
• Anode: Positive electrode (carbon or platinum)
• Cathode: Negative electrode (carbon or platinum)
• Battery: Connected to the electrodes
• Direction of electron flow: From anode to cathode through the external circuit
• Reactions:
• At anode: 2Cl^- → Cl₂ + 2e^-
• At cathode: K⁺ + 1e^- → K

2. Sketch a simple representation of electrolytic cell for the electrolysis of fused sodium chloride. Indicate the direction of the flow of electrons. Identify anode and cathode.

Answer:

• Electrolyte: Molten NaCl
• Anode: Positive electrode
• Cathode: Negative electrode
• Battery: Connected to the electrodes
• Direction of electron flow: From anode to cathode through the external circuit
• Reactions:
• At anode: 2Cl^- → Cl₂ + 2e^-
• At cathode: Na⁺ + 1e^- → Na

Summary Table

Important Points

• - Electrolytic cell = electrochemical cell that uses electrical energy to drive a chemical reaction.
• - Components:
• 1. Electrolyte (MX)
• 2. Two inert electrodes
• 3. Battery
• - Anode = positive electrode; oxidation occurs; anions move here.
• - Cathode = negative electrode; reduction occurs; cations move here.
• - Electrons flow from anode to cathode in the external circuit.
• - Cations move towards the cathode.
• - Anions move towards the anode.
• - Anode reaction: X^- → X + 1e^- (oxidation).
• - Cathode reaction: M⁺ + 1e^- → M (reduction).
• - Examples: Electrolysis of molten NaCl, KCl; electroplating; electrolysis of brine.

Remember

• - Electrolytic cell = uses electricity to drive a chemical reaction.
• - Anode = positive; oxidation occurs.
• - Cathode = negative; reduction occurs.
• - Electrons flow from anode to cathode.
• - Cations move towards cathode.
• - Anions move towards anode.
• - Anode reaction: X^- → X + 1e^-.
• - Cathode reaction: M⁺ + 1e^- → M.
• - Examples: molten NaCl, KCl; electroplating.

Common Mistakes

Quick Revision

• Electrolytic Cell:
• An electrochemical cell in which electrical energy is used to drive a chemical reaction is called an electrolytic cell.

Components:

• A vessel containing an electrolyte (MX)
• Two inert electrodes
• A battery

Working:

• Electrons move from anode to cathode in the outer circuit.
• In the solution, the cations move towards the cathode and anions towards anode.
• At the anode, anions oxidize by losing electrons.
• At the cathode, cations reduce by gaining electrons.
• Oxidation occurs at the anode and reduction at the cathode.
• The electrons released by anions at the anode move in the external circuit towards the cathode.

Electrode Reactions:

• At Anode: X^- → X + 1e^-
• At Cathode: M⁺ + 1e^- → M

Concept Assessment Exercise 4.1:

• Sketch an electrolytic cell for the electrolysis of fused KCl.
• Sketch a simple representation of electrolytic cell for the electrolysis of fused sodium chloride. Indicate the direction of the flow of electrons. Identify anode and cathode.

60-Second Revision

• Electrolytic cell = uses electricity to drive a chemical reaction.
• Anode = positive; oxidation occurs.
• Cathode = negative; reduction occurs.
• Electrons flow from anode to cathode.
• Cations move towards cathode.
• Anions move towards anode.
• Anode reaction: X^- → X + 1e^-.
• Cathode reaction: M⁺ + 1e^- → M.
• Examples: molten NaCl, KCl; electroplating; electrolysis of brine.
• ⚠️ Warning: oxidation always occurs at the anode, and reduction always occurs at the cathode — do not mix them up.