4.2.2 Electrolysis of Concentrated Aqueous Solution

Introduction

Electrolysis can be carried out not only on molten electrolytes but also on aqueous solutions of electrolytes. When an ionic compound is dissolved in water, the solution contains:

• Ions from the electrolyte itself
• Ions from water (H⁺ and OH^-)

This makes the electrolysis of aqueous solutions more complex than the electrolysis of molten electrolytes. The products depend on:

• The concentration of the electrolyte
• The nature of the ions present
• The position of the ions in the electrochemical series

This section describes the electrolysis of a concentrated aqueous solution of sodium chloride (brine), which is an important industrial process.

Definition

Electrolysis of Concentrated Aqueous Solution: The process of passing an electric current through a concentrated aqueous solution of an electrolyte, causing it to decompose into new substances.

In simple words: When you pass electricity through a concentrated salt solution, it breaks down into new substances.

Main Idea

• Electrolysis of a concentrated aqueous solution of sodium chloride (brine) is used for the industrial production of sodium hydroxide.
• The solution contains:
• Na⁺ and Cl^- from NaCl
• H⁺ and OH^- from water
• During electrolysis:
• Chloride ions (Cl^-) are oxidized at the anode to produce chlorine gas.
• Hydrogen ions (H⁺) from water are reduced at the cathode to produce hydrogen gas.
• Sodium ions (Na⁺) remain in solution and combine with OH^- to form sodium hydroxide (NaOH).
• The electrolyte becomes sodium hydroxide (NaOH).

Core Concept

Key point: In the electrolysis of concentrated aqueous NaCl:

• Chlorine gas is produced at the anode.
• Hydrogen gas is produced at the cathode.
• Sodium hydroxide remains in the solution.

Detailed Explanation

What is Brine?

Brine is a concentrated aqueous solution of sodium chloride (NaCl).

It contains:

• Na⁺ ions (from NaCl)
• Cl^- ions (from NaCl)
• H⁺ ions (from water)
• OH^- ions (from water)

Note: Water ionizes slightly:

H₂O ⇌ H⁺ + OH^-

The Electrolytic Cell

Apparatus:

• An electrolytic tank containing brine
• Inert electrodes such as platinum or carbon (graphite)
• A battery or DC power supply

Key features:

• Anode: Positive electrode
• Cathode: Negative electrode

During Electrolysis

What happens:

• Chloride ions (Cl^-) move towards the anode.
• At the anode, chloride ions are oxidized to produce chlorine gas.
• Sodium ions (Na⁺) move towards the cathode.
• However, sodium ions are not reduced to sodium metal in this process.
• Instead, hydrogen ions (H⁺) from water are discharged to produce hydrogen gas.
• More and more water molecules keep ionizing to replace H⁺ ions as soon as they are consumed.
• The electrolyte in the solution becomes sodium hydroxide (NaOH).

Why sodium is not produced:

• Sodium is a very reactive metal.
• In the electrochemical series, sodium is higher than hydrogen.
• This means sodium has a greater tendency to stay as ions (Na⁺) rather than being reduced to metal.
• Hydrogen ions (H⁺) are lower in the series and are more easily reduced.
• Therefore, hydrogen gas is produced at the cathode instead of sodium metal.

Electrode Reactions

At the anode (oxidation): 2Cl^-_(aq) → Cl_2(g) + 2e^-

• Chloride ions lose electrons.
• Chlorine gas is produced.

At the cathode (reduction): 2H⁺_(aq) + 2e^- → H_2(g)

• Hydrogen ions gain electrons.
• Hydrogen gas is produced.

Overall reaction: 2NaCl_(aq) + 2H₂O_(l) → 2NaOH_(aq) + Cl_2(g) + H_2(g)

Products of Electrolysis

Key point:

• Chlorine gas at the anode.
• Hydrogen gas at the cathode.
• Sodium hydroxide in the solution.

Industrial Importance

Electrolysis of brine is used for the industrial production of:

• Sodium hydroxide (NaOH) — used in soap manufacturing, paper production, and water treatment.
• Chlorine gas (Cl₂) — used in disinfectants, PVC, and other chemicals.
• Hydrogen gas (H₂) — used in fuel cells and ammonia production.

Key point: This process is important because it produces three valuable products simultaneously.

The Solution Contains NaOH

After electrolysis, the solution contains:

• Na⁺ ions
• OH^- ions

Evaporation of water from this solution produces pure solid sodium hydroxide (NaOH).

Key point: The electrolyte becomes sodium hydroxide (NaOH).

Summary Table

Intermediate Level

Electrolysis of concentrated aqueous NaCl:

Ions present:

• Na⁺ and Cl^- from NaCl
• H⁺ and OH^- from water

At anode:

• Cl^- is oxidized to Cl₂.
• Reaction: 2Cl^- → Cl₂ + 2e^-

At cathode:

• H⁺ is reduced to H₂.
• Reaction: 2H⁺ + 2e^- → H₂

In solution:

• Na⁺ and OH^- remain.
• Solution becomes NaOH.

Overall reaction: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂

Key point:

• Chlorine gas at anode.
• Hydrogen gas at cathode.
• Sodium hydroxide in solution.

Solved Example

Example: Electrolysis of Concentrated Aqueous NaCl

Given: Concentrated aqueous NaCl (brine) contains Na⁺, Cl^-, H⁺, and OH^- ions.

Required: Products at cathode and anode.

Solution:

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

At cathode (reduction): 2H⁺ + 2e^- → H₂

Final Answer:

• Cathode: Hydrogen gas (H₂)
• Anode: Chlorine gas (Cl₂)
• Solution: Sodium hydroxide (NaOH)

Important Points

• - Brine = concentrated aqueous solution of NaCl.
• - Brine contains Na⁺, Cl^-, H⁺, and OH^- ions.
• - Anode product: Chlorine gas (Cl₂).
• - Cathode product: Hydrogen gas (H₂).
• - Solution product: Sodium hydroxide (NaOH).
• - Anode reaction: 2Cl^- → Cl₂ + 2e^-.
• - Cathode reaction: 2H⁺ + 2e^- → H₂.
• - Overall reaction: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂.
• - Sodium is not reduced to sodium metal because it is higher in the electrochemical series than hydrogen.
• - This process is used for the industrial production of NaOH, Cl₂, and H₂.

Remember

• - Brine = concentrated NaCl solution.
• - Anode = chlorine gas.
• - Cathode = hydrogen gas.
• - Solution = sodium hydroxide.
• - Anode reaction: 2Cl^- → Cl₂ + 2e^-.
• - Cathode reaction: 2H⁺ + 2e^- → H₂.
• - Overall: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂.
• - Sodium is not reduced because it is more reactive than hydrogen.
• - Industrial production of NaOH, Cl₂, and H₂.

Common Mistakes

Quick Revision

Electrolysis of Concentrated Aqueous Solution:

Electrolysis of a concentrated aqueous solution of sodium chloride called brine is used for the industrial production of sodium hydroxide. Electrolysis is carried out by using inert electrodes such as platinum, or carbon (graphite).

The brine contains:

• Sodium ions (Na⁺) and chloride ions (Cl^-) from NaCl
• Hydrogen ions (H⁺) and hydroxide ions (OH^-) from water

During electrolysis:

• Chloride ions move towards the anode. At anode chloride ions are oxidized to produce chlorine gas.
• Sodium ions move towards the cathode. However, sodium ions are not reduced to sodium metals in this process. Instead, hydrogen ions from water are discharged to produce hydrogen gas. More and more water molecules keep ionizing to replace H⁺ ions as soon as they are consumed.
• The electrolyte in the solution becomes sodium hydroxide (NaOH).

Reactions:

• At anode:
• 2Cl^-_(aq) → Cl_2(g) + 2e^-

• At cathode:
• 2H⁺_(aq) + 2e^- → H_2(g)

The solution contains Na⁺ and OH^- ions. Evaporation of water from this solution produces pure solid sodium hydroxide.

60-Second Revision

• Brine = concentrated NaCl solution.
• Anode = chlorine gas (Cl₂).
• Cathode = hydrogen gas (H₂).
• Solution = sodium hydroxide (NaOH).
• Anode reaction: 2Cl^- → Cl₂ + 2e^-.
• Cathode reaction: 2H⁺ + 2e^- → H₂.
• Overall: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂.
• Sodium is not reduced because it is more reactive than hydrogen.
• Industrial production of NaOH, Cl₂, and H₂.
• ⚠️ Warning: sodium metal is NOT produced at the cathode; hydrogen gas is produced instead.