4.3 Electrolysis of Halide Compound

Introduction

During the electrolysis of a halide compound (a compound containing halogen ions like chloride, bromide, or iodide), the products depend on whether the solution is dilute or concentrated.

This section explains the difference between the electrolysis of dilute and concentrated halide solutions.

Definition

Halide Compound: A compound containing halogen ions such as chloride (Cl^-), bromide (Br^-), or iodide (I^-).

In simple words: A halide compound is a compound that contains a halogen ion.

Main Idea

• The products of electrolysis of a halide compound depend on the concentration of the solution.
• In a dilute halide solution, oxygen gas is usually produced at the anode.
• In a concentrated halide solution, the halogen (chlorine, bromine, or iodine) is produced at the anode.
• In both cases, hydrogen gas is produced at the cathode.

Core Concept

Key point: The cathode always produces hydrogen gas in both cases. The anode product depends on the concentration of the halide solution.

Detailed Explanation

1. Electrolysis of Dilute Halide Solution

At the Anode (positive electrode):

• Oxygen gas is usually produced because water breaks down into oxygen and hydrogen ions.
• This happens because the water molecules compete with the halide ions for discharge.
• In dilute solution, the concentration of halide ions is low, so water is discharged instead.

At the Cathode (negative electrode):

- Hydrogen gas is produced as water molecules are reduced to form hydrogen gas and hydroxide ions.

Example:

If you electrolyse a dilute sodium chloride (NaCl) solution, the products are:

• Anode: Oxygen gas (O₂)
• Cathode: Hydrogen gas (H₂)

2. Electrolysis of Concentrated Halide Solution

At the Anode:

- The halogen (chlorine, bromine, or iodine) is produced because there are many halide ions, so they are discharged instead of water.

At the Cathode:

- Hydrogen gas is still produced because water is reduced.

Example:

If you electrolyse concentrated sodium chloride (NaCl) solution, the products are:

• Anode: Chlorine gas (Cl₂)
• Cathode: Hydrogen gas (H₂)

Summary Table

Intermediate Level

Why the anode product changes with concentration:

• In a dilute solution, the concentration of halide ions is low.
• Water molecules compete with halide ions for discharge.
• Water is discharged instead, producing oxygen gas.
• In a concentrated solution, the concentration of halide ions is high.
• Halide ions are discharged instead of water, producing the halogen.

Key point:

• Dilute solution: Oxygen at the anode.
• Concentrated solution: Halogen at the anode.
• Cathode: Hydrogen gas in both cases.

Example:

• Dilute NaCl → O₂ at anode.
• Concentrated NaCl → Cl₂ at anode.

Important Points

• - Halide compound = compound containing halogen ions (Cl^-, Br^-, I^-).
• - Dilute halide solution: Oxygen gas at the anode, hydrogen gas at the cathode.
• - Concentrated halide solution: Halogen at the anode, hydrogen gas at the cathode.
• - The cathode always produces hydrogen gas in both cases.
• - The anode product depends on the concentration of the halide solution.
• - Example: Dilute NaCl → O₂ at anode; concentrated NaCl → Cl₂ at anode.

Remember

• - Halide compound = contains Cl^-, Br^-, or I^-.
• - Dilute solution → O₂ at anode.
• - Concentrated solution → halogen at anode.
• - Cathode → H₂ in both cases.
• - Concentration affects the anode product.

Common Mistakes

Quick Revision

Electrolysis of Halide Compound:

During the electrolysis of a halide compound (a compound containing halogen ions like chloride, bromide, or iodide), the products depend on whether the solution is dilute or concentrated.

1. Electrolysis of Dilute Halide Solution:

• At the Anode (positive electrode): Oxygen gas is usually produced because water breaks down into oxygen and hydrogen ions. This happens because the water molecules compete with the halide ions for discharge.
• At the Cathode (negative electrode): Hydrogen gas is produced as water molecules are reduced to form hydrogen gas and hydroxide ions.

• Example:
• If you electrolyse a dilute sodium chloride (NaCl) solution, the products are:

• Anode: Oxygen gas (O₂)
• Cathode: Hydrogen gas (H₂)

2. Electrolysis of Concentrated Halide Solution:

• At the Anode: The halogen (chlorine, bromine, or iodine) is produced because there are many halide ions, so they are discharged instead of water.
• At the Cathode: Hydrogen gas is still produced because water is reduced.

• Example:
• If you electrolyse concentrated sodium chloride (NaCl) solution, the products are:

• Anode: Chlorine gas (Cl₂)
• Cathode: Hydrogen gas (H₂)

60-Second Revision

• Halide compound = contains Cl^-, Br^-, or I^-.
• Dilute solution: O₂ at anode, H₂ at cathode.
• Concentrated solution: Halogen at anode, H₂ at cathode.
• Cathode: H₂ in both cases.
• Anode: O₂ (dilute) or halogen (concentrated).
• ⚠️ Warning: the anode product changes with concentration, but the cathode product remains hydrogen gas.